LIBRARY 

UNIVERSITY  OF  CALIFORNIA 
RIVERSIDE 


.  ^     '.v  v 


BIED'S-EYE  VIEW  or  MARBLE  CxSoN  FBOM  THE  VERMILION  CLIFFS,  NEAR  THE  MOUTII  OF  THE  PABIA. 
In  the  distance  the  Colorado  River  is  seen  to  turn  to  the  west,  where  its  gorge  divides  the  Twin 
Plateaus.  On  the  right  are  seen  the  Eastern  Karbab  Displacements  appearing  as  folds,  and  farther 
in  the  distance  as  faults. 


ELEMENTS 


GEOLOGY 


A   TEXT-BOOK 

FOE 

COLLEGES  AND  FOR  THE  GENERAL  READER. 


BY 


JOSEPH   LE    CONTE, 

•'/' 

MirnoE  OF  "BELIGIOX  AND  SCIENCE,"  ETC.,  AXD  PBOFESSP*  OF  GEOLOGY  ASD  NATTOEAL 

IN   TUB  UXIVEBSITY   OF  CAilFOESIA. 


NEW    YORK : 
D.    APPLETON    AND    COMPANY, 

549    AND    551    BROADWAY. 
1878. 


COPYRIGHT   BY 

D.   APPLETON   AND   COMPANY, 

1877. 


P  E  E  F  A  O  E . 


IN  preparing  the  following  work  I  have  not  attempted  to  make  an 
exhaustive  manual  to  be  thumbed  by  the  special  student ;  for,  even  if 
I  felt  able  to  write  such  a  work,  Prof.  Dana's  is  already  in  the  field, 
and  is  all  that  can  be  desired  in  this  respect.  I  have  endeavored  only  to 
present  clearly  to  the  thoroughly  cultured  and  intelligent  student  and 
reader  whatever  is  best  and  most  interesting  in  Geological  Science. 
I  have  attempted  to  realize  what  I  conceive  to  be  comprised  in  the 
word  elements,  as  contradistinguished  from  manual.  I  have  attempted 
to  give  a  really  scientific  presentation  of  all  the  departments  of  the 
wide  field  of  geology,  at  the  same  time  avoiding  too  great  multiplica- 
tion of  detail.  I  have  desired  to  make  a  work  which  shall  be  both 
interesting  and  profitable  to  the  intelligent  general  reader,  and  at  the 
same  time  a  suitable  text-book  for  the  higher  classes  of  our  colleges. 
In  the  selection  of  material  and  mode  of  presentation  I  have  been 
guided  by  long  experience,  as  to  what  it  is  possible  to  make  interest- 
ing to  a  class  of  young  men,  somewhat  advanced  in  general  culture 
and  eager  for  knowledge,  but  not  expecting  to  become  special  geolo- 
gists. In  a  word,  I  have  tried  to  give  such  knowledge  as  every  thor- 
oughly cultured  man  ought  to  have,  and  at  the  same  time  is  a  suitable 
foundation  for  the  further  prosecution  of  the  subject  to  those  who  so 
desire.  The  work  is  the  substance  of  a  course  of  lectures  to  a  senior 
class,  organized,  compacted,  and  disencumbered  of  too  much  detail,  by 
re-presentation  for  many  successive  years,  and  now  for  the  first  time 
reduced  to  writing. 

Most  text-books  now  in  use  in  this  country  are,  in  my  opinion, 
either  too  elementary  on  the  one  hand,  or  else  adapted  as  manuals  for 
the  specialists  on  the  other.  I  wish  to  fill  this  gap— to  supply  a  want 


iv  PREFACE. 

felt  by  many  intelligent  students  and  general  readers,  who  desire  a 
really  scientific  general  knowledge  of  geology.  Lyell's  "  Elements  " 
comes  nearest  to  supplying  this  want ;  but  there  are  two  objections  to 
this  admirable  work :  1.  The  principles  (dynamical  geology)  are  sepa- 
rated from  the  elements  (structural  and  historical  geology),  and  treated 
in  a  different  work ;  2.  Its  treatment  of  American  geology  is  of  course 
meagre. 

I  have  treated  several  subjects  in  dynamical  and  structural  geology 
— e.  g.,  rivers,  glaciers,  volcanoes,  geysers,  earthquakes,  coral-reefs, 
slaty  cleavage,  metamorphism,  mineral  veins,  mountain-chains,  etc. 
— more  fully  than  is  common.  I  feel  hopeful  that  many  geologists  and 
physicists  will  thank  me  for  so  doing.  I  am  confident  that  I  give 
somewhat  fairly  the  present  condition  of  science  on  these  subjects. 

In  the  historical  part  I  have  found  much  more  difficulty  in  being 
scientific  without  being  tiresome,  and  in  being  interesting  without 
being  superficial  and  wordy.  I  have  attempted  to  accomplish  this  diffi- 
cult task  by  making  evolution  the  central  idea,  about  which  many  of 
the  facts  are  grouped.  I  have  tried  to  keep  this  idea  in  view,  as  a 
thread  running  through  the  whole  history,  often  very  slender — some- 
times, indeed,  invisible — but  reappearing  from  time  to  time  to  give 
consistency  and  meaning  to  the  history. 

If  this  work  have  any  advantage  over  others  already  before  the 
public,  it  is  chiefly  in  the  two  points  mentioned  above,  viz.,  in  a  fuller 
presentation  of  some  subjects  in  dynamical  and  structural  geology,  and 
in  the  attempt  to  keep  evolution  in  view,  and  to  make  it  the  central 
idea  of  the  history.  Another  advantage,  I  believe,  is,  that  it  does  not 
seek  to  compete  with  the  best  works  now  before  the  public,  but  occu- 
pies a  distinct  field  and  supplies  a  distinct  want. 

I  have  confined  myself  mostly,  though  not  entirely,  to  American 
geology,  especially  in  giving  the  distribution  of  the  rocks  and  the 
physical  geographv  of  the  different  periods.  In  only  one  case  have  I 
made  American  geology  subordinate,  viz.,  in  the  Jura-Trias  period,  and 
that  only  because  of  the  meagreness  of  the  record  of  this  period  in  this 
country. 

In  a  science  so  comprehensive  and  many-sided  as  geology,  it  is 
simply  impossible,  as  every  teacher  knows,  to  avoid  anticipations  in 
one  part  of  what  strictly  belongs  to  a  subsequent  part.  It  is  for  this 
reason  that  the  order  of  presentation  of  the  different  departments,  and 


PREFACE.  v 

of  the  various  subjects  under  each  department,  is  so  different  in  the 
hands  of  different  writers.  The  order  which  I  have  adopted  I  know  is 
not  free  from  objection  on  this  score,  but  it  seemed  to  me,  on  the 
whole,  the  best. 

In  preparing  the  work  I  have,  of  course,  drawn  largely  from  many 
sources,  both  text-books  and  works  of  original  research ;  for  whatever 
of  merit  there  be  in  a  work  of  this  kind  must  consist  not  so  much  in 
the  novelty  of  the  matter  as  in  the  selecting,  grouping,  and  presenta- 
tion. Such  obligations  are  acknowledged  in  the  pages  of  the  work. 
I  cannot  forbear,  however,  making  here  a  special  acknowledgment  of 
my  indebtedness,  in  the  historical  part,  to  the  invaluable  manual  of 
Prof.  Dana.  I  must  also  acknowledge  especial  indebtedness  to  Profs. 
Marsh  and  Newberry,  and  the  geologists  and  paleontologists  of  the 
United  States  Surveys,  in  charge  of  Prof.  Hayden  and  Lieutenant 
Wheeler,  not  only  for  valuable  materials,  but  also  for  much  personal 
aid. 


CONTENTS. 


INTRODUCTORY. 

PAGE 

DEFINITION  OF  GEOLOGY,  AND  OF  ITS  DEPAKTNENTS 1-2 

GEOLOGY,  1 ;  Principal  Departments,  2  ;  Order  of  Treatment,  2. 


PART  I. 

DYNAMICAL     GEOLOGY. 

CHAPTER  I. 
ATMOSPHERIC  AGENCIES 3-8 

Soils,  4 ;  General  Explanation,  6  ;  Granite,  Gneiss,  Volcanic  Rocks,  etc.,  7  ;  Lime- 
stone, 1 ;  Sandstones,  7 ;  Slate,  7.  MECHANICAL  AGENCIES  OF  THE  ATMOSPHERE. 
—Frost,  8 ;  Winds,  8. 

CHAPTER  II. 
AQUEOUS  AGENCIES 9-V6 

SECTION  1.  EIVERS,  9.  Erosion  of  Rain  and  Rivers,  9  ;  Hydrographical  Basin,  10 ; 
Kate  of  Erosion  of  Continents,  10  ;  Law  of  Variation  of  Erosive  Power,  11.  Ex- 
amples of  Great  Erosion  now  going  on  :  Waterfalls,  12 ;  Niagara,  General 
Description,  12 ;  Recession  of  the  Falls,  12 ;  Other  Falls,  13 ;  Time  necessary 
to  excavate  Niagara  Gorge,  14 ;  Ravines,  Gorges  Canons,  15 ;  Time,  17.  Trans- 
portation and  Distribution  of  Sediments,  18  ;  Experiments,  18  ;  Law  of  Vari- 
ation, 18.  1.  Stratification,  20.  2.  Winding  Course  of  Rivers,  21.  3.  Flood- 
Plain  Deposits,  22 ;  River-Swamp,  22  ;  Natural  Levies,  23 ;  Artificial  Levees, 
23.  4.  Deltas,  24 ;  Process  of  Formation,  26 ;  Rate  of  Growth,  27 ;  Age  of  River- 
Deposits,  27.  5.  Estuaries,  29  ;  Mode  of  Formation,  29  ;  Deposits  in  Estuaries, 
30.  6.  Bars,  30. 

SECTION  2.  OCEAN.—  Waves  and  Tides.— Waves,  31 ;  Tides,  32 ;  Examples  of  the 
Action  of  Waves  and  Tides,  33  ;  Transporting  Power,  36  ;  Deposits,  36. 
Oceanic  Currents,  37  ;  Theory  of  Oceanic  Currents,  37  ;  Application,  38  ;  Geo- 
logical Agency  of  Oceanic  Currents,  39  ;  Submarine  Banks,  40 ;  Land  formed 
by  Ocean  Agencies,  42. 

SECTION  3.  ICE,  43.  Glaciers.—  Definition,  43  ;  Necessary  Conditions,  43 ;  Rami- 
fications of  Glaciers,  44 ;  Motion  of  Glaciers,  44 ;  Graphic  Illustration,  46  ;  Line 
of  the  Lower  Limit  of  Glaciers,  46  ;  General  Description,  47 ;  Earth  and  Stones, 
etc.,  49.  Moraines,  50.  Glaciers  as  a  Geological  Agent,  51 ;  Erosion,  51 ;  Trans- 
portation, 52 ;  Deposit— Balanced  Stones,  52 ;  Material  of  the  Terminal  Moraine, 


i  CONTENTS. 

PAG! 

53 ;  Evidences  of  Former  Extension  of  Glaciers,  53  ;  Glacial  Lakes,  54.  Mo- 
tion of  Glaciers  and  its  Laws. — Evidences  of  Motion,  54;  Laws  of  Glacier- 
Motion,  55.  Theories  of  Glacier-Motion,  57.  Viscosity  Theory  of  Forbes.— 
Statement  of  the  Theory,  57 ;  Argument,  57.  Regelation  Theory  of  Tyndall, 
58 ;  Eegelation,  59 ;  Application  to  Glaciers,  59  ;  Comparison  of  the  Two 
Theories,  60 ;  Conclusion,  60.  Structure  of  Glaciers,  60 ;  Veined  Structure, 
60  ;  Fissures,  61.  Theories  of  Structure.—  Fissures,  62  ;  Veined  Structure,  62  ; 
Physical  Theory  of  Veins,  63.  Floating  Ice — Icebergs,  64 ;  General  Descrip- 
tion, 65 ;  Icebergs  as  a  Geological  Agent — Erosion,  66  ;  Deposits,  66.  Shore- 
Ice,  67.  Comparison  of  the  Different  Forms  of  the  Mechanical  Agencies  of 
Water,  67. 

SECTION  4.  CHEMICAL  AGENCIES  OF  WATER. — Subterranean  Waters,  Springs,  etc., 
68 ;  Springs,  68 ;  Artesian  Wells,  69 ;  Chemical  Effects  of  Subterranean  Waters, 
70 ;  Limestone  Caves,  70.  Chemical  Deposits  in  Springs. — Deposits  of  Carbon- 
ate of  Lime,  71 ;  Explanation,  71 ;  Kinds  of  Materials,  71 ;  Deposits  of  Iron,  72 ; 
Deposits  of  Silica,  72  :  Deposits  of  Sulphur  and  Gypsum,  73.  Chemical  De- 
posits in  Lakes.— Salt  Lakes  and  Alkaline  Lakes,  73 ;  Conditions  of  Salt-Lake 
Formation,  74 ;  Deposits  in  Salt  Lakes,  75.  Chemical  Deposits  in  Seas,  76. 


CHAPTER  III. 
IGNEOUS  AGENCIES 76-132 

SECTION  1.  INTERIOR  HEAT  OF  THE  EAETH.— Stratum  of  Invariable  Temperature, 
76 ;  Increasing  Temperature  of  the  Interior  of  the  Earth,  77 ;  Constitution  of 
the  Earth's  Interior,  78  ;  1.  Rate  of  Increase  not  uniform,  78  ;  2.  Fusing-Point 
not  the  same  for  all  Depths,  79  ;  Astronomical  Reasons,  80. 

SECTION  2.  VOLCANOES. — Definition,  81 ;  Size,  Number,  and  Distribution,  81 ;  Phe- 
nomena of  an  Eruption,  82 ;  Monticules,  83  ;  Materials  erupted,  83  ;  Stones,  83  ; 
Lava,  83  ;  Liquid  Lava,  84 ;  Hardened  Lava,  85 ;  Gas,  Smoke,  and  Flame,  85  ; 
Kinds  of  Volcanic  Cones,  86 ;  Mode  of  Formation  of  a  Volcanic  Cone,  86 ; 
Comparison  between  a  Volcanic  Cone  and  an  Exogenous  Tree,  89  ;  Estimate  of 
the  Age  of  Volcanoes,  89.  Thtory  of  Volcanoes,  90 ;  Force,  90 ;  The  Heat, 
91 ;  Internal  Fluidity  Theory,  91  ;  Objections,  91 ;  Chemical  Theory,  92 ;  Re- 
cent Theories,  92.  Subordinate  Volcanic  Phenomena,  94 ;  General  Explanation, 
94.  Geysers,  94 ;  Description,  94 ;  Phenomena  of  an  Eruption,  94;  Yellowstone 
Geysers,  96  ;  Theories  of  Geyser-Eruption,  99  ;  Mackenzie's  Theory,  99  ;  Bun- 
sen's  Investigations,  100  ;  Theory  of  Geyser-Eruption—Principles,  101 ;  Appli- 
cation to  Geysers,  101 ;  Bunsen's  Theory  of  Geyser-Formation,  103. 

SECTION  3.  EARTHQUAKES,  104 ;  Frequency,  104 ;  Connection  with  other  Forms  of 
Igneous  Agency,  104 ;  Ultimate  Cause  of  Earthquakes,  106  ;  Proximate  Cause, 
106 ;  Waves— their  Kinds  and  Properties,  106  ;  Definition  of  Terms,  107  ;  Ap- 
plication to  Earthquakes,  109  ;  Experimental  Determination  of  the  Velocity  of 
the  Spherical  Wave,  111 ;  Explanation  of  Earth  quake- Phenomena,  111 ;  Vorti- 
cose Earthquakes,  114 ;  Explanation,  114.  Earthquakes  originating  beneath  the 
Ocean,  119  ;  Great  Sea-Wave,  119  ;  Examples  of  the  Sea-Wave,  120.  Depth  of 
Earthquake- Focus,  122;  Seismometers,  122;  The  Determination  of  the  Epi- 
centrum,  124 ;  Determination  of  the  Focus,  124 ;  Effect  of  the  Moon  on  Earth- 
quake-Occurrence, 126  ;  Relation  of  Earthquake-Occurrence  to  Seasons  and  At- 
mospheric Conditions,  126. 

SECTION  4.  GRADUAL  ELEVATION  AND  DEPRESSION  OF  THE  EARTH'S  CRUST,  127 ; 
Elevation  or  Depression  during  Earthquakes,  127  ;  Movements  not  connected 
with  Earthquakes— South  America,  127  ;  Italy,  127  ;  Scandinavia,  129  ;  Green- 
land, 129  ;  Deltas  of  Large  Rivers,  129 ;  Southern  Atlantic  States,  130  ;  Pacific 
Ocean,  130.  Theories  of  Elevation  and  Depression,  131 ;  Babbage's  Theory, 
131 ;  Herschel's  Theory,  132 ;  General  Theory,  132. 


CONTEXTS.  ix 

CHAPTER   IV. 

PAGE 

ORGANIC  AGENCIES 133-163 

SECTION  1.  VEGETABLE  ACCUMULATIONS. — Peat-Bogs  and  Peat-Swamps. — Descrip- 
tion, 133 ;  Composition  and  Properties  of  Peat,  133  ;  Mode  of  Growth,  134 ; 
Rate  of  Growth,  135 ;  Conditions  of  Growth,  135 ;  Alternation  of  Peat  with 
Sediments,  136.  Drift- Timber,  136. 

SECTION  2.  BOG-!RON  ORE,  136. 

SECTION  3.  LIME  ACCUMULATIONS. — Coral  Reefs  and  Islands. — Interest  and  Im- 
portance, 138 ;  Coral  Polyp,  138 ;  Compound  Coral,  or  Corallum,  138 ;  Coral 
Forests,  138  ;  Coral  Eeef,  139  ;  Coral  Islands,  139  ;  Conditions  of  Coral-Growth, 
140 ;  Pacific  Eeefs,  140 ;  Fringing  Eeefs,  140  ;  Barrier  Eeefs,  141 ;  Circular 
Reefs,  or  Atolls,  142  ;  Small  Atolls  and  Lagoonless  Islands,  143.  Theories  of 
Barrier  and  Circular  Reefs,  143 ;  Crater  Theory,  143 ;  Objections,  144 ;  Sub- 
sidence Theory,  144 ;  Proofs,  145 ;  Area  of  Land  lost,  146  ;  Amount  of  Vertical 
Subsidence,  146  ;  Eate  of  Subsidence,  147 ;  Time  involved,  148  ;  Geological  Ap- 
plication, 148.  Rtefs  of  Florida,  149  ;  Description  of  Florida,  149  ;  General 
Process  of  Formation,  150  ;  History  of  Changes,  150  ;  Mangrove  Islands,  151 ; 
Florida  Eeefs  compared  with  other  Eeefs,  152 ;  Probable  Agency  of  the  Gulf 
Stream,  152.  Shell-Deposits,  153  ;  Molluscous  Shells,  153 ;  Microscopic  Shells, 
154. 

SECTION  4.  GEOGRAPHICAL  DISTRIBUTION  OF  ORGANISMS.— Fauna  and  Flora,  155 ; 
Kinds  of  Distribution,  155  ;  Vertical  Botanical  Temperature-Eegions,  156  ;  Bo- 
tanical Temperature-Regions  in  Latitude,  156  ;  Further  Definition  of  Eegions, 
157 ;  Zoological  Temperature-Eegions,  153  ;  Continental  Fauna  and  Flora,  159 ; 
Subdivisions,  160 ;  Special  Cases,  161 ;  Marine  Fauna— Distribution  in  Lati- 
tude, 162 ;  Distribution  in  Longitude,  162 ;  Depth  and  Bottom,  162 ;  Special 
Cases,  162. 


PART  II. 

STRUCTURAL    GEOLOGY. 

CHAPTER  I. 

GENERAL  FORM  AND  STRUCTURE  OF  THE  EARTH          ....     164-170 
1.  Form  of  the  Earth,  164.     2.  Density  of  the  Earth,  165.     3.  The  Crust  of  the 
Earth,  166 ;  Means  of  Geological  Observation,  166.    4.  General  Surface  Con- 
figuration of  the  Earth,  167  ;   Cause  of  Land-Surfaces  and  Sea-Bottoms,  167  ; 
Laws  of  Continental  Form,  168.    Rocks,  169  ;  CLsses  of  Eocks,  170. 

CHAPTER  II. 

STRATIFIED  OR  SEDIMENTARY  ROCKS 170-202 

SECTION  1.  STRUCTURE  AND  POSITION.— Stratification,  170  ;  Extent  and  Thickness, 
170 ;  Kinds  of  Stratified  Eocks,  171 ;  I.  Stratified  Eocks  are  more  or  less  Con- 
solidated Sediments,  171 ;  Cause  of  Consolidation,  172 ;  II.  Stratified  Eocks 
have  been  gradually  deposited,  172  ;  III.  Stratified  Eocks  were  originally  nearly 
horizontal,  173 ;  Elevated,  Inclined,  and  Folded  Strata,  174 ;  Dip  and  Strike, 
175  ;  Anticlines  and  Synclines,  177  ;  Monoclinal  Axes,  178  ;  Unconformity,  178 ; 
Formation,  179.  Cleavage  Structure,  179  ;  Sharpe's  Mechanical  Theory,  181 ; 
Physical  Theory,  185  ;  Sorby's  Theory,  185  ;  Tyndall's  Theory,  186  ;  Geolog- 
ical Application,  187.  Nodular  or  Concretionary  Structure,  188 ;  Cause,  188 ; 
Forms  of  Nodules,  189  ;  Kinds  of  Nodules  found  in  Different  Strata,  189.  Fos- 
sils :  their  Origin  and  Distribution,  190  ;  The  Degrees  of  Preservation  are  very 
various,  191 ;  Theory  of  Petrifaction,  192.  Distribution  of  Fossils  in  the  Strata, 


x  CONTENTS. 

PAGE 

195 ;  1.  Kind  of  Eock,  195.     2.  The  Country  where  found,  195  ;    3.  The  Age, 
196. 

SECTION  2.  CLASSIFICATION  OF  STRATIFIED  BOCKS,  197 ;  1.  Order  of  Superposition, 
198 ;  2.  Lithological  Character,  198 ;  3.  Comparison  of  Fossils,  199 ;  Manner 
of  constructing  a  Geological  Chronology,  200. 

CHAPTER  III. 

UNSTRATIFIED  OR  IGNEOUS  ROCKS 202-212 

Characteristics,  202 ;  Origin,  202 ;  Mode  of  Occurrence,  202 ;  Extent  on  the  Sur- 
face, 203  ;  Classification  of  Igneous  Rocks,  203.  1.  GRANITIC  ROCKS,  203  ;  Chem- 
ical Composition  and  Kinds,  204 ;  Mode  of  Occurrence,  204.  2.  TRAPPEAN  OR 
FISSURE-ERUPTION  ROCKS,  205 ;  General  Characteristics,  205 ;  Varieties,  205 ; 
Mode  of  Occurrence,  206  ;  Effect  of  Dikes  on  the  Intersected  Strata,  207  ;  Age 
—how  determined,  208.  3.  VOLCANIC  ROCKS.— Characteristics,  208  ;  Varieties, 
208.  Of  Certain  Structures  found  in  many  Eruptive  Rocks.— Columnar  Struct- 
ure, 209  ;  Direction  of  the  Columns,  210 ;  Cause  of  Columnar  Structure,  210 ; 
Volcanic  Conglomerate  and  Breccia,  210;  Amygdaloid,  211.  Other  Nodes  of 
Classification  of  Igneous  Bocks,  211. 

CHAPTER  IV. 

METAMORPHIC  ROCKS 213-219 

Origin,  213  ;  Position,  213  ;  Extent  on  the  Earth-Surface,  213 ;  Principal  Kinds, 
214.  Theory  of  Metamorphism,  215  ;  Water,  215  ;  Alkali,  216  ;  Pressure,  216  ; 
Application,  216  ;  Crushing,  216  ;  Explanation  of  Associated  Phenomena,  217. 
Origin  of  Granite,  217. 

CHAPTER  V. 
STRUCTURE  COMMON  TO  ALL  ROCKS 220-260 

SECTION  1.  JOINTS  AND  FISSURES.—  Joints,  220.  Fissures,  or  Fractures,  221 ;  Cause, 
221 ;  Faults,  222  ;  Law  of  Slip,  224. 

SECTION  2.  MINERAL  VEINS.— Kinds,  225  ;  Characteristics,  226  ;  Irregularities,  226 ; 
Metalliferous  Veins,  227 ;  Contents,  227  ;  Ribboned  Structure,  228  ;  Age,  229  ; 
Surface-Changes,  230  ;  Cupriferous  Veins,  230  ;  Plumbiferous  Veins,  231 ;  Au- 
riferous Quartz-Veins,  231  ;  Placer-Mines,  232.  Some  Important  Lazes  affecting 
the.  Occurrence  and  the  Richness  of  Metalliferous  Veins,  232.  Theory  of  Metal- 
liferous Veins,  234 ;  Outline  of  the  most  Probable  Theory,  235 ;  Vein-Stuffs,  235 ; 
Metallic  Ores,  236 ;  Auriferous  Veins  of  California,  237  ;  Nuggets,  239 ;  Illustra- 
tions of  the  Law  of  Circulation,  239. 

SECTION  3.  MOUNTAIN-CHAINS:  THEIR  STRUCTURE  AND  ORIGIN,  240.  Mountain- 
Origin,  240.  General  Form,  and  how  produced,  240.  Mountain-Structure,  241 ; 
Rate  of  Mountain-Formation,  244;  Thickness  of  Mountain-Sediments,  244; 
Foldings  and  Metamorphism,  245.  Mountain-Sculpture,  245 ;  Resulting  Forms, 
246 ;  The  Age  of  Mountain-Chains,  251.  Theory  of  the  Origin  of  Mountain- 
Chains,  252  ;  1.  Thick  Sediments  of  Mountain-Chains,  254 ;  Appalachian  Chain, 
254 ;  Sierras,  256 ;  Coast  Range,  256 ;  Alps,  256 ;  2.  Position  of  Mountain- 
Chains  along  the  Borders  of  Continents,  256  ;  3.  Parallel  Ranges,  257  ;  4.  Meta- 
morphism of  Mountain-Chains,  257 ;  5.  Fissures  and  Slips,  and  Earthquakes, 
258  ;  6.  Fissure-Eruptions,  258 ;  7.  Volcanoes,  258. 

CHAPTER  VI. 

DENUDATION,  OR  GENERAL  EROSION 260-265 

Agents  of  Denudation,  260  ;  Amount  of  Denudation,  261 ;  Average  Erosion,  263 ; 
Estimate  of  Geological  Times,  264. 


CONTENTS.  xi 

PART  III. 

HISTORICAL  GEOLOGY;    OR,   THE  HISTORY  OF  THE  EVOLUTION  OF 
EARTH-STRUCTURE  AND   OF  THE  ORGANIC  KINGDOM. 

CHAPTER  I. 

PAGE 

GENERAL  PKINCIPLES 266-271 

Great  Divisions  and  Subdivisions  of  Time-Eras,  268 ;  Ages,  269 ;  Subdivisions, 
270  ;  Order  of  Discussion,  270  ;  Prehistoric  Eras,  271. 

CHAPTER  II. 

LAUREXTIAX  SYSTEM  OF  ROCKS  AND  AECH^AX  ERA  ....    272-276 
Rocks,  273 ;  Area  in  North  America,  274 ;  Physical  Geography  of  Archaean  Times, 
274 ;  Time  represented,  274 ;  Evidences  of  Life,  274. 

CHAPTER  III. 
PRIMARY  OR  PALEOZOIC  SYSTEM  OF  ROCKS  AND  PALEOZOIC  ERA  .        .    276-404 

General  Description,  276  ;  Eocks— Thickness,  etc.,  276;  Area  in  the  United  States, 
277  ;  Physical  Geography  of  the  American  Continent,  279 ;  Subdivisions,  280 ; 
The  Interval,  230. 

SECTION  1.  SILURIAN  SYSTEM  :  AGE  OF  INVERTEBRATES.— The  Eock  System,  282 ; 
Subdivisions,  282 ;  Character  of  the  Eocks,  282 ;  Area  in  America,  282 ;  Phys- 
ical Geography,  283 ;  Primordial  Beach  and  its  Fossils,  283  ;  General  Eemarks 
on  First  Distinct  Fauna,  287.  General  Life-System  of  the  Silurian  Age,  288. 
Plants,  290.  Animals.— Protozoans,  290  ;  Eadiates,  Corals,  290  ;  Hydrozoa,  293 ; 
Polyzoa,  296 ;  Echinoderms,  296 ;  Mollusks,  801 ;  General  Description  of  a 
Brachiopod,  301 ;  Lamellibranchs,  304 ;  Gasteropods,  305  ;  Cephalopods,  305  ; 
Articulates,  308 ;  Crustacea,  309 ;  General  Description,  309 ;  Affinities  of  Trilo- 
bites,  312 ;  Eurypterids,  313  ;  Anticipations  of  the  Next  Age,  314. 

SECTION  2.  DEVONIAN  SYSTEM  AND  AGE  OF  FISHES,  314 ;  Area  in  United  States,  315 ; 
Physical  Geography,  315  ;  Subdivision  into  Periods,  315.  Life-System  of  De- 
vonian Age — Plants,  315;  General  Eemarks  on  Devonian  Land-Plants,  316. 
Animals,  319  ;  Eadiates,  319  ;  Brachiopods,  320  ;  Cephalopods,  320  ;  Crustacea, 
322 ;  Insects,  322  ;  Fishes,  322 ;  Affinities  of  Devonian  Fishes,  327 ;  General 
Characteristics  of  Devonian  Fishes,  328  ;  Bank  of  Devonian  Fishes,  331 ;  Bear- 
ing of  these  Facts  on  the  Question  of  Evolutiori,  332  ;  Suddenness  of  Appear- 
ance, 332. 

SECTION  3.  CARBONIFEROUS  SYSTEM  :  AGE  OF  ACROGENS  AND  AMPHIBIANS. — Eetro- 
spect,  333 ;  Subdivisions  of  the  Carboniferous  System  and  Age,  334.  Car- 
boniferous Proper— Rock-System  or  Coal-Measures. — The  Name,  334 ;  Thickness 
of  Strata,  335  ;  Mode  of  Occurrence  of  Coal,  335  ;  Plication  and  Denudation, 
336 ;  Faults,  337 ;  Thickness  of  Seams,  338  ;  Number  and  Aggregate  Thickness, 
338 ;  Coal  Areas  of  the  United  States,  338 ;  Extra-Carboniferous  Coal,  339 ; 
Coal,  Areas  of  Different  Countries  compared,  339  ;  Eelative  Production  of  Coal, 
340.  Origin  of  Coal  and  of  its  Varieties,  340  ;  Varieties  of  Coal,  341 ;  Varieties 
depending  upon  Purity,  341 ;  Varieties  of  Coal  depending  on  the  Degree  of  Bitu- 
minization,  342 ;  Varieties  depending  upon  the  Proportion  of  Fixed  and  Vola- 
tile Matter,  342  ;  Origin  of  these  Varieties,  343  ;  In  Contact  with  Air,  344 ;  Out 
of  Contact  with  Air,  344 ;  Metamorphic  Coal,  345.  Plants  of  the  Coal— their 
Structure  and  Affinities,  346  ;  Where  found,  347  ;  Principal  Orders,  347 ;  1.  Con- 
ifers, 347  ;  Affinities  of  Carboniferous  Conifers,  349  ;  2.  Ferns,  349  ;  3.  Lepido- 
dendrids,  355  ;  4.  Sigillarids,  358 ;  5.  Calamites,  361 ;  General  Conclusion,  362. 


xii  CONTENTS. 

PAGE 

Theory  of  the  Accumulation  of  Coal,  363 ;  Presence  of  Water,  363 ;  Application 
of  the  Theory  to  the  American  Coal-Fields  :  a.  Appalachian  Coal-Field,  366 ;  b. 
Western  Coal-Fields,  367 ;  Appalachian  Revolution,  367.  Estimate  of  Time, 
367  ;  1.  From  Aggregate  Amount  of  Coal,  367  ;  2.  From  Amount  of  Sediment, 
368.  Physical  Geography  and  Climate  of  the  Coal  Period. — Physical  Geog- 
raphy, 369  ;  Climate,  369  ;  Cause  of  the  Climate,  370.  Iron- Ore  of  the  Coal- 
Measures,  373  ;  Mode  of  Occurrence,  373  ;  Kinds  of  Ore,  373  ;  Theory  of  the  Ac- 
cumulation of  the  Iron-Ore  of  the  Coal-Measures,  374.  Bitumen  and  Petro- 
leum, 376  ;  Geological  Eolations,  376 ;  Oil-Formations,  377 ;  Principal  Oil- 
Horizons  of  the  United  States,  377  ;  Laws  of  Interior  Distribution,  377  ;  Kinds 
of  Rocks  which  bear  Petroleum,  378.  Origin  of  Petroleum  and  Bitumen,  379  ; 
Origin  of  Varieties,  380 ;  Area  of  Oil-bearing  Strata  in  the  Eastern  United 
States,  380.  Fauna  of  the  Carboniferous  Age,  381 ;  Vertebrates  (Fishes),  388 ; 
Reptiles— Amphibians,  390;  1.  Reptilian  Footprints,  390;  2.  Dendrerpeton, 
391 ;  3.  Archegosaurus,  393 ;  4.  Eosaurus,  393 ;  Some  General  Observations 
on  the  Earliest  Reptiles,  395.  Some  General  Observations  on  the  Whole  Palaeo- 
zoic, 396  ;  Physical  Changes,  396 ;  Chemical  Changes,  396 ;  Progressive  Change 
in  Organisms,  397 ;  General  Comparison  of  the  Fauna  of  Paleozoic  with  that 
of  Neozoic  Times,  397.  General  Picture  of  Palceozoic  Times,  398.  Transmis- 
sion from  the  Palceozoic  to  the  Mesozoic— Permian  Period.— The  Permian  a  Tran- 
sition Period,  400. 

CHAPTER  IV. 
MESOZOIO  ERA— AGE  OF  EEPTII.ES 404-475 

General  Characteristics,  404 ;  Subdivisions,  404. 

SECTION  1.  TEIASSIC  PERIOD,  405  ;  Subdivisions,  405  ;  Animals,  406  ;  Fishes,  407  ; 
Reptiles,  409  ;  Birds,  411 ;  Mammals,  411.  Origin  of  Rock-Salt,  412  ;  Age  of 
Rock-Salt,  412  ;  Mode  of  Occurrence,  412  ;  Theory  of  Accumulation,  413. 

SECTION  2.  JURASSIC  PERIOD,  414  ;  Origin  of  Oolitic  Limestones,  415 ;  Jurassic  Coal- 
Measures,  415  ;  Dirt-Beds—Fossil  Forest-Grounds,  415.  Plants,  417.  Animals, 
419;  Corals,  419  ;  Brachiopods,  419  ;  Lamellibranchs,  419  ;  Cephalopods,  419'; 
Ammonites,  421 ;  Belemnites,  423  ;  Crustacea,  425  ;  Insects,  425 ;  Fishes,  425  ; 
Reptiles,  428  ;  Birds,  436  ;  Mammals,  438  ;  Affinities  of  the  First  Mammals,  438. 

SECTION  3.  JURA-TRIAS  IN  AMERICA,  439  ;  Distribution  of  Strata,  439.  Life-System, 
440 ;  Connecticut  River  Valley  Sandstone— the  Strata,  440  ;  The  Record,  441 ; 
Reptilian  Tracks,  442  ;  Bird-Tracks,  443 ;  Richmond  and  North  Carolina  Coal- 
Fields,  445  ;  Other  Patches,  447  ;  Interior  Plains  and  Pacific  Slope,  447  ;  Phys- 
ical Geography  of  the  American  Continent  during  the  Jura-Trias  Period,  449 ; 
Disturbances  which  closed  the  Period,  450. 

SECTION  4.  CRETACEOUS  PERIOD,  450  ;  Rock-System—Area  in  America,  451 ;  Phys- 
ical Geography  in  America,  451 ;  Rocks,  452 ;  Chalk,  452 ;  Origin  of  Chalk, 
453 ;  Extent  of  Chalk  Seas  of  Cretaceous  Times  in  Europe,  454 ;  Cretaceous 
Coal,  455 ;  Subdivisions  of  the  Cretaceous,  456.  Life-System :  Plants,  456. 
Animals.— Protozoa,  459  ;  Echinoderms,  460  ;  Mollusks,  461 ;  Vertebrates — 
Fishes,  465  ;  Reptiles,  467  ;  Birds,  470  ;  Mammals,  472.  Continuity  of  the 
Chalk,  473.  General  Observations  on  the  Mesozoic,  474.  Disturbance  which 
closed  the  Mesozoic,  475. 

CHAPTER  V. 

CENOZOIO  ERA — AGE  OF  MAMMALS 475-557 

General  Characteristics  of  the  Cenozoic  Era,  476  ;  Divisions,  476. 
SECTION  1.  TERTIARY  PERIOD.— Subdivisions,  477 ;  Rock-System—Area  in  the  United 
States,  477  ;  Physical  Geography,  478 ;  Character  of  the  Rocks,  480 ;  Coal,  480 ; 
Life-System.— General  Remarks,  480.    Plants,  481 ;  Diatoms,  483 ;  Origin  of  In- 
fusorial Earths,  483.    Animals,  485  ;  Insects,  488  ;  Fishes,  490  ;   Reptiles,  492 ; 


CONTENTS.  xiii 

PAGE 

Birds,  494 ;  Mammals— General  Remarks,  495  ;  1.  Eocene  Basin  of  Paris,  496  ; 
2.  Siwalik  Hills,  India— Miocene,  498  ;  American  Localities — 3.  Marine  Eocene 
of  Alabama,  500  ;  4.  Green-River  Basin— "Wahsatch  Beds— Lower  Eocene,  501 ; 
5.  Green-River  Basin— Bridger  Beds — Middle  Eocene,  502  ;  6.  Mauvaises  Terres 
of  Nebraska — White  River  Basin— Miocene,  505 ;  7.  Mauvaises  Terres— Nio- 
brara  Basin — Pliocene,  506  ;  Some  General  Observations  on  the  Tertiary  Mam- 
malian Fauna,  506 ;  Genesis  of  the  Horse,  509.  General  Observations  on  the 
Tertiary  Period,  511. 

SECTION  2.  QUATERNARY  PERIOD. — Characteristics,  513 ;  Subdivisions,  513.  Qua- 
ternary Period  in  Eastern  North  America— I.  Glacial  Epoch.— The  Materials — 
Drift,  514 ;  The  Bowlders,  515 ;  Surface-Rock  underlying  Drift,  516 ;  Extent, 
516;  Marine  Deposits,  517.  Theory  of  the  Origin  of  the  Drift,  517;  State- 
ment of  the  most  Probable  View,  518 ;  Objections  answered,  518 ;  Probable 
Condition  during  Glacial  Times  in  America,  519.  //.  Champlain  Epoch,  520  ; 
Evidences  of  Subsidence,  521 ;  Sea-Margins,  521 ;  Flooded  Lakes,  521 ;  River 
Terraces  and  Old  Flood-Plain  Deposits,  522.  ///.  Terrace  Epoch,  524 ;  Evi- 
dences—Sea, 524 ;  Lakes,  524 ;  Rivers,  524 ;  History  of  the  Mississippi  River, 
525.  Quaternary  Period  on  the  Western  Side  of  the  Continent,  526 ;  Glaciers, 
526 ;  Lake-Margins,  527 ;  Rivers,  529 ;  Seas,  530.  The  Quaternary  Period  in  Eu- 
rope, 530 ;  1.  Epoch  of  Elevation— First  Glacial  Epoch,  531 ;  2.  Epoch  of  Sub- 
mergence— Champlain,  532  ;  3.  Epoch  of  Reelevation — Second  Glacial  Epoch — 
Terrace  Epoch,  534 ;  4.  Modern  Epoch,  534.  Home  General  Results  of  Glacial 
Erosion. — 1.  Fiords,  534;  2.  Glacial  Lakes,  535.  Life  of  the  Quaternary  Period. 
— Plants  and  Invertebrates,  535  ;  Mammals,  536;  1.  Bone-Caverns,  536;  Origin 
of  Cave  Bone-Rubbish,  538 ;  Origin  of  Bone-Caverns,  539  ;  2.  Beaches  and  Ter- 
races, 539 ;  3.  Marshes  and  Bogs,  539  ;  4.  Frozen  Soils  and  Ice  Cliffs,  539 ;  Qua- 
ternary Mammalian  Fauna  of  England,  541.  Mammalian  fauna  in  North 
America,  542 ;  Bone-Caves,  542  ;  Marshes  and  Bogs,  542 ;  River-Gravels,  544 ; 
Quaternary  in  South  America,  544 ;  Australia,  547 ;  Geographical  Fauna  of 
Quaternary  Times,  547.  Some  General  Observations  on  the  Whole  Quaternary. — 
1.  Cause  of  the  Climate,  548 ;  2.  Time  involved  in  the  Quaternary  Period, 
650 ;  3.  The  Quaternary  a  Period  of  Revolution— a  Transition  between  the  Ceno- 
zoic  and  the  Modem  Eras,  550 ;  4.  Drift  in  Relation  to  Gold,  554 ;  Age  of  the 
River-Gravels,  556. 

CHAPTER   VI. 

PsTcnozoic  EBA — AGE  OF  MAX — RECENT  EPOCH  ....  557-570 
Characteristics,  557 ;  Distinctness  of  this  Era,  557 ;  The  Change  still  in  Progress 
—Examples  of  Recently-Extinct  Species,  558.  I.  ANTIQUITY  OF  MAN,  560.  Pri- 
meval Man  in  Europe.— Supposed  Miocene  Man— Evidence  unreliable,  561 ; 
Supposed  Pliocene  Man,  562 ;  Quaternary  Man— Mammoth  Age,  562 ;  a.  In 
River-Terraces,  562;  b.  Bone-Caves— Engis  Skull,  563;  Neanderthal  Skull, 
563 ;  Mentone  Skeleton,  564 ;  Reindeer  Age  or  Later  Palaeolithic,  564 ;  Auri- 
gnac  Cave,  5b5 ;  Perigord  Caves,  565 ;  Conclusions,  566.  Neolithic  Man — Refuse- 
Heaps— Shell- Mounds — Kitchen-Middens,  506  ;  Transition  to  the  Bronze  Age — 
Lake  Dwellings,  566.  Primeval  Man  in  America,  567 ;  Supposed  Pliocene  Man, 
567  ;  Quaternary  Man,  567  ;  Quaternary  Man  in  Other  Countries,  568 ;  Time 
since  Man  appeared,  569.  II.  CHAEACTER  OF  PBIMEVAL  MAN,  570. 


INTKODUCTOKY. 


DEFINITION  OF  GEOLOGY,  AND   OF  ITS  DEPARTMENTS. 

GEOLOGY  is  the  physical  history  of  the  earth  and  its  inhabitants, 
as  recorded  in  its  structure.  It  includes  an  account  of  the  changes 
through  which  they  have  passed,  the  laws  of  these  changes,  and  their 
causes.  In  a  word,  it  is  the  history  of  the  evolution  of  the  earth  and 
its  inhabitants. 

The  fundamental  idea  of  geology,  as  well  as  its  principal  sub- 
divisions and  its  objects,  may  be  most  clearly  brought  out  by  compar- 
ing it  with  organic  science.  We  may  study  an  organism  from  three 
distinct  points  of  view :  1.  We  may  study  its  general  form,  the  parts 
of  which  it  is  composed,  and  its  minute  internal  structure.  This  is 
anatomy.  It  is  best  studied  in  the  dead  body.  2.  We  may  study  the 
living  body  in  action,  the  function  of  each  organ,  the  circulation  of  the 
fluids,  and  the  manner  in  which  all  contribute  to  the  complex  phenom- 
ena of  life.  This  is  physiology.  3.  We  may  study  the  living  and 
growing  body,  by  watching  the  process  of  development  from  the  egg 
to  the  adult  state,  and  striving  to  determine  its  laws.  This  is  embry- 
ology. 

So,  looking  upon  the  earth  as  an  organic  unit,  we  may  study  its 
form,  the  rocks  and  minerals  of  which  it  is  composed,  and  the  manner 
in  which  these  are  arranged  ;  in  other  words,  its  external  form  and  in- 
ternal structure.  This  is  the  anatomy  of  the  earth,  and  is  called  struct- 
ural geology.  Or,  we  may  study  the  earth  under  the  action  of  physical 
and  chemical  forces,  the  action  and  reaction  of  land  and  water,  of  earth 
and  air,  and  the  effects  of  these  upon  the  form  and  structure.  This  is 
the  physiology  of  the  earth,  and  is  called  dynamical  geology.  Finally, 
we  may  study  the  earth  in  the  progress  of  its  development,  from  the 
earliest  chaotic  condition  to  its  present  condition,  as  the  abode  of  man, 
and  attempt  to  determine  the  laws  of  this  development.  This  is  the 
embryology  of  the  earth,  or  historical  geology. 
1 


2  INTRODUCTORY. 

Principal  Departments. — The  science  of  geology,  therefore,  nat- 
urally divides  itself  into  three  parts,  viz.:  1.  Structural  geology,  or 
geognosy.  2.  Dynamical  geology,  or  physical  and  chemical  geology. 
3.  Historical  geology,  or  the  history  of  the  earth. 

But  there  are  two  important  points  of  difference  between  geology 
and  organic  science.  The  central  department  of  organic  science  is 
physiology,  and  both  anatomy  and  embryology  are  chiefly  studied  to 
throw  light  on  this.  But  the  central  department  of  geology,  to  which 
the  others  are  subservient,  is  history.  Again :  in  case  of  organisms 
— especiallv  animal  organisms — the  nature  of  the  changes  producing 
development  is  such  that  the  record  of  each  previous  condition  is  suc- 
cessively and  entirely  obliterated ;  so  that  the  science  of  embryology  is 
possible  only  by  direct  observation  of  each  successive  stage.  If  this 
were  true  also  of  the  earth,  a  history  of  the  earth  would,  of  course,  be 
impossible.  But,  fortunately,  we  find  that  each  previous  condition  of 
the  earth  has  left  its  record  indelibly  impressed  on  its  structure. 

Order  of  Treatment. — The  prime  object  of  geology  is  to  determine 
the  history  of  the  earth,  and  of  the  organisms  which  have  successively 
inhabited  its  surface.  The  structure  and  constitution  of  the  earth  are 
the  materials  of  this  history,  and  the  physical  and  chemical  changes 
now  going  on  around  us  are  the  means  of  interpreting  this  structure 
and  constitution.  Evidently,  therefore,  the  only  logical  order  of  pre- 
senting the  facts  of  geology  is  to  study,  first,  the  causes,  physical  and 
chemical,  now  in  operation  and  producing  structure ;  then  the  structure 
and  constitution  of  the  earth  which,  from  the  beginning,  have  been 
produced  by  similar  causes;  and,  lastly,  from  the  two  preceding  to  un- 
fold the  history  of  the  earth. 

Geology  may  be  defined,  therefore,  as  the  history  of  the  earth  and 
its  inhabitants,  as  revealed  in  its  structure,  and  as  interpreted  by 
causes  still  in  operation. 

There  is  no  other  science  which  requires  for  its  full  comprehension 
a  general  knowledge  of  so  many  other  departments  of  science.  A 
knowledge  of  mathematics,  physics,  and  chemistry,  is  required  to  under- 
stand dynamical  geology ;  a  knowledge  of  mineralogy  and  lithology  is 
required  to  understand  structural  geology  ;  and  a  knowledge  of  zoology 
and  botany  is  required  to  understand  the  affinities  of  the  animals  and 
plants  which  have  successively  inhabited  the  earth,  and  the  laws  which 
have  controlled  their  distribution  in  time. 


PAET  I. 
DYNAMICAL    GEOLOGY. 


THE  agencies  now  in  operation,  modifying  the  structure  of  the  sur- 
face of  the  earth,  may  be  classed  under  four  heads,  viz.,  atmospheric 
agencies,  aqueous  agencies,  igneous  agencies,  and  organic  agencies. 
These  agencies  have  operated  from  the  beginning,  and  are  still  in 
operation.  We  study  their  operation  now,  in  order  that  we  may  un- 
derstand their  effects  in  previous  epochs  of  the  earth's  history — i.  e.,  the 
structure  of  the  earth. 

While  all  geologists  agree  that  the  nature  of  the  agencies  which 
have  operated  in  modifying  the  earth's  surface  has  remained  the  same 
from  the  beginning,  they  differ  in  their  views  as  to  the  energy  of  their 
operation  in  different  periods.  Some  believe  that  their  energy  has  been 
much  the  same  throughout  the  whole  history  of  the  earth,  while  others 
believe  that  many  facts  in  the  structure  of  the  earth  require  much 
greater  operative  energy  than  now  exists.  We  will  attempt  to  show 
hereafter  that  neither  of  these  extreme  opinions  is  probably  true,  but 
that  some  of  these  agencies  have  been  decreasing,  while  others  have 
been  increasing,  with  the  progress  of  time.  It  is  the  constant  change 
of  balance  between  these  which  determines  the  development  of  the 
earth. 


CHAPTER  I. 

ATMOSPHERIC  AGENCIES. 


THE  general  effect  of  atmospheric  agencies  is  the  disintegration  of 
rocks  and  the  formation  of  soils.  The  atmosphere  is  composed  of  nitro- 
gen and  oxygen,  with  small  quantities  of  watery  vapor  and  of  carbonic 
acid  There  are  but  few  rocks  which  are  not  gradually  disintegrated 


4  ATMOSPHERIC  AGENCIES. 

under  the  constant  chemical  action  of  the  atmosphere.  The  chemical 
agents  of  these  changes  are  oxygen,  carbonic  acid,  and  watery  vapor, 
the  nitrogen  being  inert.  To  these  must  be  added,  where  vegeta- 
tion is  present,  the  products  of  vegetable  decomposition,  especially 
ammonia. 

Atmospheric  agencies  graduate  so  insensibly  into  aqueous  agencies 
that  it  is  difficult  to  define  their  limits.  Water,  holding  in  solution 
carbonic  acid  and  oxygen,  may  exist  as  invisible  vapor;  or,  partially 
condensed,  as  fogs  ;  or,  completely  condensed,  as  rain  falling  upon  and 
percolating  the  earth.  In  all  these  forms  its  chemical  action  is  the 
same,  and,  therefore,  cannot  be  separated  and  treated  under  different 
classes :  and  yet  the  same  rain  runs  off  from  and  erodes  the  surface  of 
the  earth,  comes  out  from  the  strata  and  forms  springs,  rivers,  etc.,  all 
of  which  naturally  fall  under  aqueous  agencies.  We  shall,  therefore, 
treat  of  the  chemical  effects  of  atmospheric  water  in  the  disintegration 
of  rocks,  and  the  formation  of  soils,  under  the  head  of  atmospheric 
agencies ;  and  the  mechanical  effects  of  the  same,  in  eroding  the  surface 
and  carrying  away  the  soil  thus  formed,  under  the  head  of  aqueous 
agencies.  In  moist  climates  vegetation  clothes  and  protects  soil  from 
erosion,  but  favors  decomposition  of  rocks  and  formation  of  soil. 

Atmospheric  agencies  are  obscure  in  their  operation,  and,  therefore, 
imperfectly  understood.  Yet  these  are  not  less  important  than  aqueous 
agencies,  since  they  are  the  necessary  condition  of  the  operation  of  the 
latter.  Unless  rocks  were  first  disintegrated  into  soils  by  the  action 
of  the  atmosphere,  they  would  not  be  carried  away  and  deposited  as 
sediments  by  the  agency  of  water.  These  two  agencies  are,  therefore, 
of  equal  power  and  importance  in  geology,  but  they  differ  very  much  in 
the  conspicuousness  of  their  effects.  Atmospheric  agencies  act  almost 
equally  at  all  times  and  at  all  places,  and  their  effects,  at  any  one  place 
or  time,  are  almost  imperceptible.  Aqueous  agencies,  on  the  contra- 
ry, in  their  operation  are  occasional,  and  to  a  great  extent  local,  and 
their  effects  are,  therefore,  more  striking  and  easily  studied.  Never- 
theless, the  aggregate  effects  of  the  former  are  equal  to  those  of  the 
latter. 

Soils. — All  soils  (with  the  trifling  exception  of  the  thin  stratum  of 
vegetable  mould  which  covers  the  ground  in  certain  localities)  are 
formed  from  the  disintegration  of  rocks.  Sometimes  the  soil  is  formed 
in  situ,  and,  therefore,  rests  on  its  parent  rock.  Sometimes  it  is  re- 
moved as  fast  as  formed,  and  deposited  at  a  distance  more  or  less  remote 
from  the  parent  rock.  The  evidence  of  this  origin  of  soils  is  clearest 
when  the  soil  is  formed  in  situ.  In  such  cases  it  is  often  easy  to  trace 
every  stage  of  gradation  between  perfect  rock  and  perfect  soil.  This 
is  well  seen  in  railroad  cuttings,  and  in  wells  in  the  gneissic  or  so-called 
primary  region  of  our  southern  Atlantic  slope.  On  examining  such  a 


ATMOSPHERIC  AGENCIES. 


section,  we  find  near  the  surface  perfect  soil,  generally  red  clay ;  beneath 
this  we  find  the  same  material,  but  lighter  colored,  coarser,  and  more 
distinctly  stratified;  beneath  this,  but  shading  into  it  by  imperceptible 
gradations,  we  have  what  seems  to  be  stratified  rock,  but  it  crumbles 
into  coarse  dust  in  the  hand ;  this  passes  by  imperceptible  gradations 
into  rotten  rock,  and  finally  into  perfect  rock.  There  can  be  no  doubt 
that  these  are  all  different  stages  of  a  gradual  decomposition.  But  closer 
observation  will  make  the  proof  still  clearer.  In  gneissic  and  other 
metamorphic  regions  it  is  not  uncommon  to  find  the  rock  traversed,  in 
various  directions,  by  veins  of  quartz  OT  flint.  Now,  in  sections  such  as 
those  mentioned  above,  it  is  common  to  find  such  a  flint-vein  running 
through  the  rock  and  upward  through  the  superincumbent  soil,  until  it 
emerges  on  the  surface.  In  the  slow  decomposition  of  the  rock  into 
soil,  the  flint-vein  has  remained  unchanged,  because  flint  is  not  affected 
by  atmospheric  agencies.  Chemical  analysis,  also,  always  shows  an 
evident  relation  between  the  soil  and  the  subjacent  or  country  rock, 
except  in  cases  in  which  the  soil  has  been  brought  from  a  considerable 
distance. 

The  depth  to  which  soil  will  thus  accumulate  depends  partly  on  the 
nature  of  the  rock  and  the  rapidity  of  decomposition,  partly  on  the  slope 
of  the  ground,  and  partly 
on  climate.  When  the 
slope  is  considerable,  as 
at  d  (Fig.  1),  the  rocks  are 
bare,  not  because  no  soil 
is  formed,  but  because  it  is 
removed  as  fast  as  formed, 
while  at  a  the  soil  is  deep, 
being  formed  partly  by  de- 
composition of  rock  in  situ,  and  partly  of  soil  brought  down  from  d. 
Wherever  perfect  soil  is  found  resting  on  sound  rock,  the  soil  has 
been  shifted. 

If  rocks  were  solid  and  impervious  to  water,  this  process  would  be 
almost  inconceivably  slow ;  but  we  find  that  all  rocks,  for  reasons  to  be 
discussed  hereafter,  are  broken  by  fissures  into  irregular  prismatic  blocks, 
so  that  a  perpendicular  cliff  of  rock  usually  presents  the  appearance  of 
rude  gigantic  masonry.  These  fissures,  or  joints,  increase  immensely 
the  surface  exposed  to  the  action  of  atmospheric  water.  Again,  on 
closer  inspection,  we  find  even  the  most  solid  parts  of  rocks,  i.  e.,  the 
blocks  themselves,  penetrated  with  capillary  fissures  which  allow  water 
to  reach  every  part.  Thus  the  rock  is  decomposed,  or  becomes  rotten, 
to  a  great  depth  below  the  surface.  But,  while  the  rock  is  gradually 
changed  into  soil,  the  soil  is  also  slowly  carried  away  by  agencies  to  be 
hereafter  considered;  and  these  changes,  taking  place  more  rapidly  in 


FIG.  1.— Ideal  Section,  showing  Eock  passing  into  Soil. 


Q  ATMOSPHERIC  AGENCIES. 

some  places  than  in  others,  give  rise  to  a  great  variety  of  forms,  some 
of  which  are  represented  in  the  accompanying  figure  (Fig.  2). 


In  the  process  of  disintegration  the  original  blocks  lose  their  pris- 
matic form,  and  become  more  or 

less  rounded,  and  are  then  called    tt 

bowlders  of  disintegration.    These 

may  lie  on  the.  surface  (Fig.  2), 

or  may  be  buried  in  the  soil  (Fig. 

3).     When  of  great  size  and  very 

solid,  so  as  to  resist  decomposition 

to  a  greater  extent  than  the  surrounding  rocks,  they  often  form  huge 

rocMng-stones  (Fig.  4).  These 
must  not  be  confounded  with 
true  bowlders  and  rocking- 
stones  which  are  brought  from 


FIG.  8. 


a  distance,  by  agencies  which 
we  will  discuss  hereafter,  and 
which   are,  therefore,  entirely 
different  from  the  subjacent  or  country  rock. 

General  Explanation. — The  process  of  rock-disintegration  may  be 
explained,  in  a  general  way,  as  follows :  Almost  all  rocks  are  composed 
partly  of  insoluble  materials,  and  partly  of  materials  which  are  slowly 
dissolved  by  atmospheric  water.  In  the  process  of  time,  therefore, 
these  latter  are  dissolved  out,  and  the  rock  crumbles  into  an  insoluble 
dust,  more  or  less  saturated  with  water  holding  in  solution  the  soluble 
ingredients.  To  illustrate :  common  hardened  mortar  may  be  regarded 
as  artificial  stone ;  it  consists  of  carbonate  of  lime  and  sand ;  the  car- 
bonate of  lime  is  soluble  in  water  containing  carbonic  acid  (atmospheric 
water),  while  the  sand  is  quite  insoluble.  If,  therefore,  such  mortar  be 
exposed  to  the  air,  it  eventually  crumbles  into  sand,  moistened  with 
water  containing  lime  in  solution.  Again,  to  take  a  case  which  often 
occurs  in  Nature,  it  is  not  uncommon  to  find  rock  through  which  iron 
pyrites,  FeS2,  is  abundantly  disseminated.  This  mineral  is  insoluble ; 
but  under  the  influence  of  water  containing  oxygen  (atmospheric  water) 
it  is  slowly  oxidized  and  changed  into  sulphate  of  iron,  or  copperas, 
which,  being  soluble,  is  washed  out,  and  the  rock  crumbles  into  an 
insoluble  dust  or  soil,  saturated  with  a  solution  of  the  iron  salt.  We 


ATMOSPHERIC  AGENCIES.  7 

have  given  these  only  as  illustrative  examples.  We  now  proceed  to 
give  examples  of  the  principal  kinds  of  rocks,  and  of  the  soils  formed 
by  their  disintegration. 

Granite,  Gneiss,  Volcanic  Rocks,  etc. — Granite  and  gneiss  are 
mainly  composed  of  three  minerals,  quartz,  feldspar,  and  mica,  aggre- 
gated together  into  a  coherent  mass.  Quartz  is  unchangeable  and  in- 
soluble in  atmospheric  water.  Mica  is  also  very  slowly  affected.  Feld- 
spar is,  therefore,  the  decomposable  ingredient.  But  feldspar  is,  itself, 
a  complex  substance,  partly  soluble  and  partly  insoluble.  It  is  essen- 
tially a  silicate  of  alumina,  united  with  a  silicate  of  potash  or  soda, 
although  it  often  contains  also  small  quantities  of  iron  and  lime.  Now, 
while  the  silicate  of  alumina  is  perfectly  insoluble,  the  other  silicates 
are  slowly  dissolved  by  atmospheric  water,  with  the  formation  of  car- 
bonates, and  the  silicate  of  alumina  is  left  as  kaolin  or  clay.  But,  since 
we  may  regard  the  original  rock  as  made  up  of  quartz  and  mica,  bound 
together  by  a  cement  of  feldspar,  the  disintegration  of  the  latter  causes 
the  whole  rock  to  lose  its  coherence,  and  the  final  result  of  the  process 
is  a  mass  of  clay  containing  grains  of  sand  and  scales  of  mica,  and 
moistened  with  water  containing  a  potash  salt.  If  there  be  any  iron 
in  the  feldspar,  or  if  there  be  other  decomposable  ingredients  in  the 
rock  containing  iron,  such,  for  example,  as  hornblende,  the  clay  will  be 
red.  This  is  precisely  the  nature  of  the  soil  in  all  our  primary  regions. 
Volcanic  rocks  decompose  into  clay  soils  often,  though  not  always, 
deeply  colored  with  iron. 

Limestone.  —Pure  limestone  may  be  regarded  as  composed  of  gran- 
ules of  carbonate  of  lime,  cohering  by  a  cement  of  the  same.  The  dis- 
solving of  the  cement  by  atmospheric  water  forms  a  lime-soil,  moistened 
with  a  solution  of  carbonate  of  lime  (hard  water).  Impure  limestone 
is  a  carbonate  of  lime,  more  or  less  mixed  with  sand  or  clay ;  by  disin- 
tegration it  forms,  therefore,  a  marly  soil. 

Sandstones. — Sandstones  consist  of  grains  of  sand  cemented  togeth- 
er by  carbonate  of  lime  or  peroxide  of  iron.%  Where  peroxide  of  iron  is 
the  cementing  substance,  the  rock  is  almost  indestructible,  since  this 
substance  is  not  changed  by  atmospheric  water:  hence  the  great  value 
of  red  sandstone  as  a  building-material.  But  when  carbonate  of  lime 
is  the  cementing  material,  this  substance,  being  soluble  in  atmospheric 
water,  is  easily  washed  out,  and  the  rock  rapidly  disintegrates  into  a 
sandy  soil. 

Slate. — In  a  similar  manner  slate-rocks  disintegrate  into  a  pure 
clay  soil  by  the  solution  of  their  cementing  material,  which  is  often  a 
small  quantity  of  carbonate  of  lime. 

There  can  be  no  doubt  that  all  soils  are  formed  in  the  manner  above 
indicated.  We  have  given  examples  of  soils  formed  in  situ,  but,  as 
soils  are  often  shifted,  they  are  usually  composed  of  a  mixture  formed 


8  ATMOSPHERIC  AGENCIES. 

by  the  disintegration  of  several  kinds  of  rock.  In  some  cases  the 
soil  has  been  formed  in  situ  during  the  present  geological  epoch,  and 
the  process  is  still  going  on  before  our  eyes.  Such  are  the  soils  of  the 
hills  of  the  up-country  or  yrimary  region  of  our  Southern  Atlantic 
States.1  Sometimes  the  soil  formed  in  the  same  way  has  been  shifted 
to  a  greater  or  less  distance.  Such  are  the  soils  of  our  valleys  and 
river-bottoms.  In  still  other  cases  the  soil  has  been  formed  by  the  pro- 
cess already  described,  and  transported  during  some  previous  geologi- 
cal epoch  and  not  reconsolidated.  Such  are  many  of  the  soils  of  the 
Southern  low-country  or  tertiary  region. 

MECHANICAL  AGENCIES  OF  THE  ATMOSPHERE. 

Frost. — Water,  penetrating  rocks  and  freezing,  breaks  off  huge  frag- 
ments :  these  by  a  similar  process  are  again  broken  and  rebroken  until 
the  rock  is  reduced  to  dust.  These  effects  are  most  conspicuous  in  cold 

climates   and   in  mountain-regions. 
In  cold  climates  huge  piles  of  bowl- 
ders and  earth  are  always  seen  at  the 
*    base  of  steep  cliffs  (Fig.  5).     Such 


of  the 

cliff  above,  is  called  a  talus.  In 
mountainous  regions  frost  is  a  pow- 
erful agent  in  disintegrating  the 
rocks,  and  in  determining  the  out- 
lines of  mountain  -  peaks.  This  is 
well  seen  in  the  Alps  and  in  the  Sierras. 

Winds. — The  effect  of  winds  is  seen  in  the  phenomenon  of  shifting 
sands.  At  Cape  Cod,  for  instance,  the  sands  thrown  ashore  by  the  sea 
are  driven  by  the  winds  inland,  and  thus  advance  upon  the  cultivated 
lands,  burying  them  and  destroying  their  fertility.  The  sands  from 
the  beach  on  the  Pacific  coast  near  San  Francisco  are  driven  inland 
in  a  similar  manner,  and  are  now  regularly  encroaching  upon  the  better 
soil.  Large  areas  of  the  fertile  alluvial  soil  of  Egypt,  together  with 
their  cities  and  monuments,  have  been  buried  by  the  encroachments  of 
the  Sahara  Desert.  In  Brittany  and  in  other  parts  of  France  villages 
which  existed  during  the  middle  ages  have  been  overwhelmed  by  drift- 
ing sands.  The  same  phenomena  are  observed  on  various  parts  of  the 
coast  of  Holland  and  England.  The  rate  of  advance  has  been  measured 
in  some  instances.  Thus  on  the  coast  of  Suffolk  it  is  said  to  advance 
at  the  rate  of  about  five  miles  a  century;  at  Cape  Finisterre,  according 
;to  Ansted,  at  the  rate  of  thirty-two  miles  per  century,  or  560  yards  per 
annum.  The  Downs  of  England  and  Scotland  are  such  barrens  of  drifting 
sand.  Hills  may  be  formed  in  this  manner  thirty  to  forty  feet  in  height. 
1  In  the  Northern  States,  in  the  region  of  the  Drift,  nearly  all  the  soil  has  been  shifted. 


EROSION  OF  KAIN  AND  RIVERS. 


CHAPTER  IL 
AQUEOUS    AGENCIES. 

THE  agencies  of  water  are  either  mechanical  or  chemical.  The 
mechanical  agencies  of  water  may  be  treated  under  the  threefold  aspect 
of  erosion,  transportation,  and  sedimentary  deposit.  We  will  consider 
them  under  the  heads  of  Rivers,  Oceans,  and  Ice.  Under  chemical 
agencies  we  will  consider  the  phenomena  of  chemical  deposits  in 
Springs  and  Lakes. 


Aqueous  Agencies. 


Mechanical. 


I  Chemical. 


Rivers Erosion,  Transportation, 

Ocean 

Ice "  " 

Springs Deposit  in. 

Lakes...  "       " 


SECTION  1. — RIVEKS. 

Under  the  head  of  river  agencies  we  include  all  the  effects  of"  cir- 
culating meteoric  water  from  the  time  it  falls  as  rain  until  it  reaches 
the  ocean  :  i.  e.,  all  the  effects  of  Rain  and  Rivers. 

Water,  in  the  form  of  vapor,  fogs,  or  rain,  percolating  through  the 
earth,  slowly  disintegrates  the  hardest  rocks.  Much  of  these  percolat- 
ing waters,  after  accomplishing  the  work  of  soil-making,  already  treated 
in  the  preceding  chapter,  reappears  on  the  surface  in  the  form  of  springs, 
and  gives  rise  to  streamlets.  A  large  portion  of  rain-water,  however, 
never  soaks  into  the  earth,  but  runs  off  the  surface,  forming  rills,  which 
by  erosion  produce  furrows.  The  uniting  rills  form  rivulets,  which  exca- 
vate gullies.  The  rivulets,  uniting  with  one  another  and  with  the 
streamlets  issuing  from  springs,  form  torrents,  which  in  their  course 
excavate  ravines,  gorges,  and  canons.  The  vmiting  torrents,  finally  issu- 
ing from  their  mountain-home  upon  the  plains,  form  great  rivers,  which 
deposit  their  freight  partly  in  their  course  and  partly  in  the  sea.  Such 
is  a  condensed  history  of  rain-water  on  its  way  to  the  ocean  whence  it 
came.  Our  object  is  to  study  this  history  in  more  detail. 

Erosion  of  Rain  and  Rivers. 

The  whole  amount  of  water  falling  on  any  land-surface  may  be 
divided  into  three  parts :  1.  That  which  rushes  immediately  off  the 
surface,  and  causes  the  floods  of  the  rivers,  especially  the  smaller 
streams;  2.  That  which  sinks  into  the  earth,  and,  after  doing  its 
chemical  work  of  soil-making,  reappears  as  springs,  and  forms  the  regu- 


10  AQUEOUS  AGENCIES. 

lar  supply  of  streams  and  rivers  ;  and,  3.  That  which  reaches  the  sea 
wholly  by  subterranean  channels.  Of  these,  the  first  two  are  the  grand 
erosive  agents,  and  these  only  concern  us  at  present.  Of  these,  the  for- 
mer predominate  in  proportion  as  the  land-surface  is  bare  ;  the  latter  in 
proportion  as  it  is  covered  with  vegetation. 

Hydrographical  Basin. — An  hydrographical  basin  of  a  river,  lake, 
or  gulf,  is  the  whole  area  of  land  the  rainfall  of  which  drains  into  that 
river,  lake,  or  gulf.  Thus  the  hydrographical  basin  of  the  Mississippi 
River  is  the  whole  area  drained  by  that  river.  It  is  bounded  on  the 
east  and  west  by  the  Alleghany  and  Rocky  Mountains,  and  on  the 
north  by  a  low  ridge  running  from  Lake  Superior  westward.  The 
whole  area  of  continents,  with  the  exception  of  rainless  deserts,  may  be 
'regarded  as  made  up  of  hydrographical  basins.  The  ridge  which  sepa- 
rates contiguous  basins  is  called  a  water-shed.  It  is  evident  that  every 
portion  of  the  land,  with  the  exception  of  the  rainless  tracts  already 
mentioned,  is  subject  to  the  erosive  agency  of  water,  and  is  being  worn 
away  and  carried  into  the  sea.  There  have  been  various  attempts  to 
estimate  the  rate  of  this  general  erosion. 

Rate  of  Erosion  of  Continents. — This  is  usually  estimated  as  follows : 
Some  great  river,  such  as  the  Mississippi,  is  taken  as  the  subject  of 
experiment.  By  accurate  measurement  during  every  portion  of  the 
year,  the  average  amount  of  water  discharged  into  the  sea  per  second, 
per  hour,  per  day,  per  year,  is  determined.  This  is  a  matter  of  no  small 
difficulty,  as  it  involves  the  previous  determination  of  the  average  cross- 
section  of  the  river  and  the  average  velocity  of  the  current.  The  aver- 
age cross-section  X  average  velocity  =  the  average  discharge  per  sec- 
ond :  from  which  may  be  easily  obtained  the  annual  discharge.  Next, 
by  experiment  during  every  month  of  the  year,  the  average  quantity  of 
mud  contained  in  a  given  quantity  of  water  is  also  determined.  By  an 
easy  calculation  this  gives  us  the  annual  discharge  of  mud,  or  the  whole 
quantity  of  insoluble  matter  removed  from  the  hydrographical  basin  in 
one  year.  This  amount,  divided  by  the  area  of  the  river-basin,  will  give 
the  average  thickness  of  the  layer  of  insoluble  matter  removed  from  the 
basin  in  one  year.  To  this  must  be  added  the  soluble  matters,  which 
are  about  $  as  much  as  the  insoluble. 

Estimates  of  this  kind  have  been  made  for  two  great  rivers,  viz., 
the  Ganges  and  the  Mississippi.  The  whole  amount  of  sediment 
annually  carried  to  the  sea  by  the  Ganges  has  been  estimated  as 
6,368,000,000  cubic  feet.  This  amount,  spread  over  the  whole  basin  of 
the  Ganges  (400,000  square  miles),  would  make  a  layer  T-71-gT  of  a  foot 
thick.  The  Ganges,  therefore,  erodes  its  basin  one  foot  in  1,751  years.1 
The  area  of  the  Mississippi  Basin  is  1,244,000  square  miles.  The 
annual  discharge  of  sediment,  according  to  the  recent  and  accurate 
1  Philosophical  Magazine,  vol.  v.,  p.  261. 


EROSION  OF  RAIN  AND   RIVERS.  u 

experiments  of  Humphrey  and  Abbot,  is  7,471,411,200  cubic  feet,  a 
mass  sufficient  to  cover  an  area  of  one  square  mile,  268  feet  deep.1  This 
spread  over  the  whole  basin  would  cover  it  4-5^5-  of  a  foot.  Therefore, 
this  river  removes  from  its  basin  a  thickness  of  one  foot  in  4,640  years. 
The  cause  of  the  great  difference  in  favor  of  the  Ganges  is,  that  this 
river  is  situated  in  a  country  subject  to  very  great  annual  fall  of  water, 
the  whole  of  which  falls  during  a  rainy  season  of  six  months.  The  rains 
are  therefore  very  heavy,  and  the  floods  and  consequent  erosion  pro- 
portionately great.  The  erosive  power  of  this  river  is  still  further  in- 
creased by  the  great  slope  of  the  basin,  as  it  takes  its  rise  in  the  Him- 
alaya, the  highest  mountains  in  the  world. 

Now,  since  continents  may  be  regarded  as  made  up  of  hydrographi- 
cal  basins,  the  average  rate  of  their  erosion  may  be  determined  either  by 
making  similar  experiments  on  all  the  rivers  of  the  world,  or,  since  this 
is  impracticable,  by  taking  some  river  as  an  average.  We  believe  the 
Mississippi  is  much  nearer  an  average  river  than  the  Ganges.  It  can 
hardly  be  less  than  the  average,  for  a  considerable  portion  of  the  earth 
— as  rainless  deserts — is  not  subject  to  any  erosion.  It  is  probable, 
therefore,  that  the  whole  surface  of  continents  is  eroded  at  a  rate  not 
exceeding  one  foot  in  4,640  years.  For  convenience,  we  will  call  it  one 
foot  in  5,000  years.  We  will  use  this  estimate  when  we  come  to  speak 
of  the  actual  erosion  which  has  occurred  in  geological  times. 

Law  of  Variation  of  Erosive  Power.— The  erosive  power  of  water,  or 
itspoicer  of  overcoming  cohesion,  varies  as  the  square  of  the  velocity 
of  the  current  (p  a  v2).  The  velocity  depends  upon  the  slope  of  the 
bed,  the  depth  of  the  water,  and  many  other  circumstances,  so  numerous 
and  complicated  that  it  has  been  found  impossible  to  reduce  it  to  any 
simple  law.  The  angle  of  slope,  however,  is  evidently  the  most  im- 
portant circumstance  which  controls  velocity,  and  therefore  erosive 
power.  In  the  upper  portions  of  great  rivers,  like  the  Mississippi,  the 
erosion  is  very  great ;  while  in  the  plains  near  the  mouth  there  may  be 
no  erosion,  but,  on  the  contrary,  sedimentary  deposit.  The  high  lands 
therefore,  especially  mountain-chains,  are  the  great  theatres  of  erosion. 
•  The  general  effect  of  erosion  is  leveling.  If  unopposed,  the  final  effect 
would  be  to  cut  down  all  lands  to  the  level  of  the  sea,  at  an  average 
rate  of  about  one  foot  in  5,000  years.  But  the  immediate  local 
effect  is  to  increase  the  inequalities  of  land-surface,  deepening  the  fur- 
rows, gullies,  and  gorges,  and  increasing  the  intervening  ridges  and 
peaks.  The  effect,  therefore,  is  like  that  of  a  graver's  tool,  constantly 
cutting  at  every  elevation,  but  making  trenches  at  every  stroke. 

"  Thus  land-surfaces  everywhere,  especially  in  mountain-regions,  are 
cut  away  by  a  process  of  sculpturing,  and  the  debris  carried  to  the  low- 
lands and  to  the  sea.     The  smaller  lines  and  more  delicate  touches  are 
1  Humphrey  and  Abbot,  "Report  on  Mississippi  Riyer,"  pp.  148-150. 


12  AQUEOUS  AGENCIES. 

due  to  rain,  the  deeper  trenches  or  heavier  chiselings  to  rivers  proper. 
The  effects  of  the  former  are  more  universal  and  far  greater  in  the 
aggregate,  but  the  effects  of  the  latter  are  far  more  conspicuous.  It  is 
only  under  certain  conditions  that  rain-sculpture  becomes  conspicuous. 
These  conditions  seem  to  be  a  bare  soil  and  absence  of  frost.  Beautiful 
examples  are  found  in  the  arid  regions  of  Southern  Utah. 

We  now  proceed  to  discuss  the  more  conspicuous  effects  of  water 
concentrated  in  river-channels. 

EXAMPLES  OF  GREAT  EROSION  NOW  GOING  ON  :  WATERFALLS. 

The  erosive  power  of  water  is  most  easily  studied  in  ravines,  gorges, 
canons,  and  especially  in  great  waterfalls.  One  of  the  most  interesting 
of  these  is  Niagara. 

Niagara :  General  Description.  —  The  plateau  on  which  stands 
Lake  Erie  (P  JV,  Fig.  6)  is  elevated  about  300  feet  above  that  of 


L.ERIE. 


FIG- 6.— Ideal  Longitudinal  Section  through  Niagara  Elver  from  Lake  Erie  to  Lake  Ontario. 

Lake  Ontario,  and  is  terminated  abruptly  by  an  escarpment  about 
300  feet  high  (P)-  From  this  point  a  narrow  gorge  with  nearly 
perpendicular  sides,  and  200  to  300  feet  deep,  runs  backward  through 
the  higher  or  Erie  plateau  as  far  as  the  falls  (^V).  The  Niagara 
River  runs  out  of  Lake  Erie  and  upon  the  Erie  plateau  as  far  as 
the  falls,  then  pitches  167  feet  perpendicularly,  and  then  runs  in  the 
gorge  for  seven  miles  to  Queenstown  ( Q),  where  it  emerges  on  the  On- 
tario plateau.  Long  observation  has  proved  that  the  position  of  the 
fall  is  not  stationary,  but  slowly  recedes  at  a  rate  which  has  been  vari- 
ously estimated  from  one  to  three  feet  per  annum.  The  process  of  re- 
cession has  been  carefully  observed,  and  the  reason  why  it  maintains  its 
perpendicularity  is  very  elear.  The  surface-rock  of  Erie  plateau  is  a( 
firm  limestone  («).  Beneath  this  is  a  softer  shale  (5).  This  softer  rock 
is  rapidly  eroded  by  the  force  of  the  falling  water,  and  leaves  the  harder 
limestone  projecting  as  table-rocks.  From  time  to  time  these  project- 
ing tables  are  loosened  and  fall  into  the  chasm  below.  This  process  is 
facilitated  by  the  joint  structure  spoken  of  on  page  5. 

Recession  of  the  Falls.— Now,  there  is  every  reason  to  believe  that 
the  fall  was  originally  situated  at  Queenstown,  the  river  falling  over 
the  escarpment  at  that  place,  and  that  it  has  worked  its  way  backward 
seven  miles  to  its  present  position  by  the  process  we  have  just  described. 
These  reasons  are  as  follows:  1.  The  general  configuration  of  the  country 


EROSION  OF  RAIN  AND   RIVERS.  13 

as  already  described  forcibly  suggests  such  an  explanation  to  the  most 
casual  observer.  2.  A  closer  examination  confirms  it  by  showing  that  the 
gorge  is  truly  a  valley  of  erosion,  since  the  strata  on  the  two  sides  cor- 
respond accurately  (see  Fig.  7).  3.  As  already  seen,  the  falls  have 
receded  in  historic  times  at  a  rate,  according  to  Mr.  Lyell,  of  about  one 
foot  a  year.  The  portion  of  the  gorge  thus  formed  under  our  eyes  does 
not  differ  in  any  essential  respect  from  other  portions  farther  down  the 
stream.  The  evidence  thus  far  is  not  perfectly  conclusive  that  the  gorge 
was  formed  by  the  present  river  during  the  present  geologic  epoch,  since 
the  gorge  may  have  been  eroded  during  a  previous  epoch,  and  the  pres- 
ent river  found  it,  appropriated  it  as  its  channel,  and  continued  to  ex- 
tend it.  But  (4.)  certain  stratified  deposits  have  been  found  by  Mr. 
Lyell  and  others  on  the  upper  margin  of  the  ravine,  containing  shells, 
all  of  which  are  identical  with  the  shells  now  living  in  Niagara  River. 
On  the  margins  of  all  rivers  we  find  stratified  deposits  of  mud  and  sand 
containing  dead  shell.  The  stratified  deposits  found  by  Mr.  Lyell  were 
such  mud-banks  of  the  Niagara  River  before  the  falls  had  receded  so 
far,  and  therefore  when  the  river  still  ran  on  the  Erie  plateau  at  this 
point.  This  is  well  seen  in  the  subjoined  figure,  representing  an  ideal 


FIG.  7.— Ideal  Section  across  Chasm  below  the  Falls. 

cross-section  of  the  gorge  below  the  falls.  The  dotted  lines  represent 
the  former  bed  and  level  of  the  river ;  a  a  represent  the  banks  of  strati- 
fied mud  left  on  the  margin  of  the  gorge,  as  the  river  eroded  its  bed 
down  to  its  present  level. 

Other  Falls. — The  evidence  is  completed  by  examination  of  other 
great  falls.  In  almost  all  perpendicular  falls  we  find  a  similar  arrange- 
ment of  strata  followed  by  similar  results.  The  Falls  of  St.  Anthony, 
in  the  Mississippi  River,  are  a  very  beautiful  illustration.  Here  we  find 
a  configuration  of  surface  very  similar  to  that  in  the  neighborhood  of 
Niagara.  Above  the  falls  the  Mississippi  River  runs  on  a  plateau  which 
terminates  abruptly  at  the  mouth  of  Minnesota  River  by  an  escarpment 
some  fifty  feet  high.  From  this  escarpment,  backward  through  the 
upper  plateau,  runs  a  gorge  with  perpendicular  sides  fifty  feet  high  for 


14;  AQUEOUS  AGENCIES. 

ten  miles  to  the  foot  of  the  falls.  The  river  above  the  falls  runs  on  a 
hard,  silurian  limestone  rock,  only  a  few  feet  in  thickness.  Beneath 
this  is  a  white  sandstone,  so  soft  that  it  can  be  easily  excavated  with 
the  fingers.  This  sandstone  forms  the  walls  of  the  gorge  as  far  as  the 
escarpment.  The  recession  of  the  falls  by  the  undermining  and  falling 
of  the  limestone  is  even  more  evident  than  at  Niagara.  Tributaries 
running  into  the  Mississippi  just  below  the  falls  are,  of  course,  precipi- 
tated over  the  margin  of  the  gorge.  Here,  therefore,  the  same  condi- 
tions are  repeated,  and  hence  are  formed  subordinate  gorges,  headed  by 
perpendicular  falls.  Such  are  the  falls  and  gorge  of  Little  River  (Min- 
nehaha),  which  runs  into  the  Mississippi  about  three  miles  above  the 
mouth  of  the  Minnesota  River. 

Another  admirable  illustration  of  the  conditions  under  which  per- 
pendicular falls  recede  is  found  in  the  falls  of  the  numerous  tributaries 
of  Columbia  River  where  the  great  river  breaks  through  the  Cascade 
Range.  The  Columbia  River  gorge  is  2,500  to  3,000  feet  deep.  The 
walls  consist  of  columnar  basalt  underlaid  near  the  water-level  by  a 
softer  conglomerate.  Every  tributary  at  this  point  emerges  from  a  deep 
gorge,  headed  two  or  three  miles  back  by  a  perpendicular  wall,  over  which 
is  precipitated  the  water  of  the  tributary  as  a  fall  200  to  300  feet  high. 
The  falling  water  erodes  the  softer  conglomerate,  undermines  the  ver- 
tical-columned basalt,  which  tumbles  into  the  stream  and  is  carried 
away;  and  thus  the  fall  has  worked  back  in  each  case  about  two  or  three 
miles  to  its  present  position.1  All  of  this  has  taken  place  during  the 
present  geological  epoch.3 

The  wonderful  falls  of  the  Yosemite  Valley,  of  which  there  are  six 
in  a  radius  of  five  miles,  one  of  them  1,600  feet,  three  600  to  700  feet, 
and  two  over  400  feet  high,  seem  to  be  an  exception  to  the  law  given 
above.  Their  perpendicularity  seems  to  be  the  result  of  the  compara- 
tive recency  of  the  evacuation  of  the  valley  by  an  ancient  glacier,  and 
therefore  the  shortness  of  the  time  during  which  the  rivers  have  been 
falling,  combined  with  the  hardness  of  the  granite  rocks.  The  Yo- 
semite gorge  was  not  made  by  the  present  rivers. 

Time  necessary  to  excavate  Niagara  Gorge.— All  attempts  to  esti- 
mate accurately  the  time  consumed  in  excavating  Niagara  gorge  must 
be  unreliable,  since  we  do  not  yet  know  the  circumstances  which  con- 
trolled the  rate  of  recession  at  different  stages  of  its  progress.  Among 
these  circumstances,  the  most  important  are  the  volume  of  water,  and 
especially  the  hardness  of  the  rocks,  and  the  manner  in  which  hard  and 

1  Gilbert  has  shown  (American  Journal,  August,  1876)  that  comparative  freedom 
from  detritus  is  another  condition  of  the  formation  of  perpendicular  waterfalls.  In  mud- 
dy rivers  commencing  inequalities  are  filled  up  by  sediment,  and  waterfalls  cannot  be 
formed. 

a  American  Journal  of  Science  and  Art,  1874,  vol.  vii.,  pp.  167,  259. 


EROSION  OF  RAIN  AND   RIVERS.  15 

soft  are  superposed.  The  present  position  of  the  falls  is  apparently 
favorable  for  rapid  recession.  Mr.  Lyell  thinks,  from  personal  observa- 
tion, that  the  average  rate  could  not  have  been  more  than  one  foot  per 
annum,  and  probably  much  less.  At  this  rate,  it  would  require  about 
36,000  years.  But,  whether  more  or  less  than  this  amount,  this  period 
must  not  be  confounded  with  the  age  of  the  earth.  In  this  part  of 
geology  we  are  studying  causes  now  in  operation.  The  work  of  exca- 
vating the  Niagara  chasm  belongs  to  the  present  epoch,  and  the  time 
is  absolutely  insignificant  in  comparison  with'  the  inconceivable  ages  of 
which  we  will  speak  in  the  subsequent  parts  of  this  work. 

Ravines,  Gorges,  Canons. — We  have  already  seen  (page  11)  that 
ravines,  gorges,  etc.,  are  everywhere  produced  in  mountain- regions  by 
the  regular  operation  of  erosive  agents.  Nowhere  are  examples  more 
abundant  or  more  conspicuous  than  in  our  own  country,  and  especially 
in  the  Western  portion.  On  the  Pacific  slope,  the  most  remarkable  are 
the  gorges  of  the  Fraser  and  of  the  Columbia  Rivers,  fifty  miles  long 
and  several  thousand  feet  deep  ;  those  of  the  North  and  South  Forks  of 
the  American  River,  2,000  to  3,000  feet  deep  in  solid  slate ;  the  canon 
of  the  Tuolumne  River  with  its  Hetchhetchy  Valley ;  the  still  grander 
canon  of  the  Merced,  with  its  Yosemite  Valley,  with  nearly  vertical 
granite  cliffs,  3,000  to  nearly  5,000  feet  high  ;  and,  deepest  of  all, 
the  grand  canon  of  King's  River,  3,000  to  7,000  feet  deep,  in  hard 
granite. 

Some  of  these  great  canons  have  been  forming  ever  since  the  forma- 
tion of  the  Sierra  Range — i.  e.,  since  the  Jurassic  period.  It  is  possible, 
also,  that  in  some  of  them  the  erosive  agents  have  been  assisted  by 
antecedent  igneous  agencies,  producing  fissures,  which  have  been  en- 
larged and  deepened  by  water  and  by  ice.  But  there  are  some,  at  least, 
which  may  be  proved  to  have  been  produced  wholly  by  erosion,  and 
that  during  the  present  or  at  least  during  very  recent  geological  times. 
We  refer  especially  to  those  which  have  been  cut  through  lava-streams. 

In  Middle  and  Northern  California  are.Tound  lava-streams  which 
have  flowed  from  the  crest  of  the  Sierra.  By  means  of  the  strata  on 
which  they  lie,  these  streams  are  known  to  have  flowed  after  the  end  of 


FIG.  8.— Lava-Stream  cut  through  by  Rivers :  a  a,  Basalt ;  b  b.  Volcanic  Ashes  ;  c  c,  Tertiary ;  d  d, 
Cretaceous  Kocks.    (From  Whitney.) 


the  Tertiary  period.  Yet  the  present  rivers  have  since  that  time  cut 
great  canons  through  the  lava  and  into  the  underlying  rock,  in  some 
cases  at  least  2,000  feet  deep.  Such  facts  impress  us  with  the  immen- 


16 


AQUEOUS  AGENCIES. 


Sity  of  geological  times.     This  important  point  is  discussed  more  fully 
in  a  subsequent  part  of  this  work. 


FIG.  9.— Bnttes  of  the  Cross  (Powell). 

But  nowhere  in  this  country,  or  in  the  world,  are  the  phenomena  of 
canons  exhibited  on  so  grand  a  scale,  and  nowhere  are  they  so  obviously 


FIG.  10.— Canon  of  the  Colorado  and  its  Tributaries  (from  a  Drawing  by  Newberry). 


EROSION   OF  RAIN  AND   RIVERS. 


17 


the  result  of  pure  erosion,  as  in  the  region  of  the  Grand  Plateau  of  Utah, 
Arizona,  New  Mexico,  and  Colorado.  This  plateau  is  elevated  7,000 
to  8,000  feet  above  the  sea,  and  composed  entirely  of  nearly  horizontal 
strata,  comprising  nearly  the  whole  geological  series  from  the  Tertiary 
downward.  Through  this  series  all  the  streams  have  cut  their  way 
downward,  forming  narrow  canons  with  almost  perpendicular  walls  sev- 
eral thousand  feet  deep,  so  that  in  many  parts  we  have  the  singular 
phenomenon  of  a  whole  river-system  running  almost  hidden  far  below 
the  surface  of  the  country,  and  rendering  the  country  entirely  impass- 
able in  certain  directions  (see  Frontispiece).  Nor  is  the  erosion  confined 
to  canons  ;  for  the  rain-erosion  has  been  so  thorough  and  general  that 
much  of  the  upper  portion  of  the  plateau  has  been  wholly  carried  away, 
leaving  only  isolated  turrets  (buttes)  or  isolated  level  tables  with  cliff- 
like  walls  (mesas)  to  indicate  their  original  height.  All  these  facts  are 
well  shown  in  Fig.  10.  The  explanation  of  these  deep  and  narrow 
canons  is  probably  to  be  found  in  the  predominance  of  stream-erosion 
over  general  disintegration  and  rain-erosion,  which  is  characteristic  of 
an  arid  climate  (Gilbert). 

Chief  among  these  canons  is  the  Grand 
Canon  of  the  Colorado,  300  miles  long 
and  3,000  to  6,200  feet  deep,  forming  the 
grandest  natural  geological  section  known. 
Into  this  the  tributaries  enter  by  side-ca- 
nons of  nearly  equal  depth,  and  often  of 
extreme  narrowness.  Fig.  11  represents 
the  natural  proportions  of  such  a  canon. 

Time. — These  remarkable  canons  have 
evidently  been  cut  wholly  by  the  streams 
which  now  occupy  them,  and  which  are 
still  continuing  the  work.  The  work,  prob- 
ably commenced  in  the  early  Tertiary  with 
the  emergence  of  this  portion  of  the  con- 
tinent, became  more  rapid  in  the  latter 
portion  of  the  Tertiary  with  the  great  ele- 
vation of  the  plateau,  and  has  continued 
to  the  present  time.  Thus,  causes  now  in 
operation  are  identified  with  geological 
agencies. 

In  the  Appalachian  chain  gorges  and 
valleys  of  erosion  are  abundant,  but  the 
evidences  of  present  action  are  less  ob- 
vious, and  therefore  we  defer  their  treat- 
ment to  Part  II.,  for  we  are  now  discuss- 
ing agencies  still  in  operation.  Among 
2 


FIG.  11  —Section  of  the  Virgen  River 
(after  Gilbert). 


18  AQUEOUS  AGENCIES. 

the  more  remarkable  narrow  gorges  in  this  region,  we  may  mention, 
in  passing,  the  Tallulah  River  gorge,  several  miles  long  and  nearly 
1,000  feet  deep,  in  Rabun  Count}-,  Georgia,  and  the  gorge  of  the  French 
Broad  in  North  Carolina.  The  general  effects  of  erosion  will  be  more 
fully  treated  under  "  Mountain  Sculpture." 

Transportation  and  Distribution  of  Sediments. 

The  specific  gravity  of  most  rocks  is  about  2.5.  Immersed  in  water, 
they,  therefore,  lose  nearly  half  their  weight.  This  fact  greatly  in- 
creases the  transporting  power  of  water.  The  actual  transporting 
power  of  water  is  determined  partly  by  experiment  and  partly  by  reason- 
ing on  the  general  laws  of  force.  By  experiment  we  determine  the 
transporting  power  under  a  given  set  of  circumstances  :  by  general  rea- 
soning we  determine  its  law  of  variation,  and  apply  the  data  given  by 
experiment  to  every  possible  case. 

Experiments. — It  has  been  found  by  experiment  that  a  current, 
moving  at  the  rate  of  three  inches  per  second,  will  take  up  and  carry 
along  fine  clay  ;  moving  six  inches  per  second,  will  carry  fine  sand ; 
eight  inches  per  second,  coarse  sand,  the  siza  of  linseed ;  twelve  inches, 
gravel ;  twenty-four  inches,  pebbles  ;  three  feet,  angular  stones  of  the 
size  of  a  hen's-egg.1  It  will  be  readily  seen  from  the  above  that  the 
carrying-power  increases  much  more  rapidly  than  the  velocity.  For 
instance,  a  current  of  twelve  inches  per  second  carries  gravel,  while  a 
current  of  three  feet  per  second,  only  three  times  greater  velocity, 
carries  stones  many  hundred  times  as  large  as  grains  of  gravel.  Let  us 
investigate  the  law. 

Law  of  Variation. — If  the  surface  of  the  obstacle  is  constant,  the 
force  of  running  water  varies  as  the  velocity  squared:  f  oc  if  (1).  This 
may  be  easily  proved.  Suppose  we  have  an  obstacle  like  the  pier  of  a 
bridge,  standing  in  water  running  with  any  given  velocity.  Now, 
if  from  any  cause  the  velocity  of  the  current  be  doubled,  since  mo- 
.mentum  or  force  is  equal  to  quantity  of  matter  multiplied  by  velocity 
(M '  =  Q  X  V),  the  force  of  the  current  will  be  quadrupled,  for  there 
will  be  double  the  quantity  of  water  strike  the  pier  in  a  given  time  with 
double  the  velocity.  If  the  velocity  of  the  current  be  trebled,  there 
will  be  three  times  the  quantity  of  matter  striking  with  three  times  the 
velocity,  and  the  force  will  be  increased  nine  times.  If  the  velocity  be 
quadrupled,  the  force  is  increased  sixteen  times,  and  so  on. 

Next,  if  the  velocity  of  the  current  remains  constant,  while  the  size 
of  the  opposing  obstacle  varies,  then  evidently  the  force  of  the  current 
will  vary  as  the  opposing  surface :  if  the  opposing  surface  is  doubled, 
the  force  is  doubled ;  if  trebled,  the  force  is  trebled,  etc.  But  in  similar 
figures,  surfaces  vary  as  the  square  of  the  diameter.  Therefore,  in  this 
case,  force  varies  as  diameter  squared:  fl<xd*  (2).  Therefore,  when 
1  Page's  "  Geology,"  p.  28 — Rankiue. 


TRANSPORTATION  AND   DISTRIBUTION  OF  SEDIMENTS. 


10 


/l  «  d?  (2) 
F  oc  a2  x  d*  (3) 
TFcc  ds 
d3  a  «2  x  d? 
d  cc  v2 


both  the  velocity  of  the  current  and  the  size  of  the  stone  or  other 
obstacle  vary,  then  the  force  varies  as  the  square  of  the  velocity 
of  the  current  multiplied  by  the  square  of  the  diameter  of  the  stone : 
F  a  u2  x  d*  (3). 

This  last  equation  gives  the  law  of  variation  of  the  moving  force. 
But  the  resistance  to  be  overcome,  or  the  weight  of  the  stone,  varies 
as  the  cube  of  the  diameters :  W  cc  d3.  We  have,  therefore,  both  the 

law  of  the  moving  force  and  the  law  of  the  resistance:    ] 

'  W  <x.  d . 

Now  the  case  we  wish  to  consider  is  that  in  which  the  current  is  just 
able  to  move  the  stone,  or  when  F  a  W.  In  this  case  d*  oc  v*  X  d*, 
or  d  on  v2.  Substituting,  in  the  third  equation,  for  d  its  value, 
F<x  v*  X  v*  =  ve.  We  place  these  equations  together,  so  that  they 
may  be  better  understood  : 

When  surface  =  constant       .        .        .     f  oc  02  (1) 
When  velocity  =  constant . 
When  both  vary     .... 

But 

And  when  W  cc  F,  then . 

Dividing  by  d?     .... 

Substituting  in  3     . 

Or F  oc  *6 

That  is,  the  transporting  power  of  a  current  varies  as  the  sixth  power 
of  the  velocity.  This  seems  so  extraordinary  a  result  that,  before  ac- 
cepting it,  we  will  try  to  make  it  still  clearer  by  an  example. 

Let  a  (Fig.  12)  represent  a  cubic  inch  of  stone,  which  a  current  of 
a  certain  velocity  will  just  move.  Now,  the 
proposition  is  that,  if  the  velocity  of  the  cur- 
rent be  doubled,  it  will  move  the  stone  b, 
sixty-four  times  as  large.  That  it  would  do  6 
so  is  evident  from  the  fact  that  the  oppos- 
ing surface  of  b  is  sixteen  times  as  great  as 
that  of  a,  and  the  moving  force  would  be 
increased  sixteen  times  from  this  cause.  But 
the  velocity  being  double,  as  we  have  already 
seen,  the  force  against  every  square  inch  of 
b  will  be  four  times  that  against  a,  and, 
therefore,  the  whole  force  from  these  two 

causes  would  be  16  X  4  =  64  times  as  great.  But  the  weight  is  also 
sixty-four  times  as  great ;  therefore,  the  current  would  be  just  able  to 
move  it.  We  may  accept  it,  therefore,  as  a  law,  that  the  transporting 
power  varies  as  the  sixth  power  of  the  velocity.  If  the  velocity,  there- 
fore, be  increased  ten  times,  the  transporting  power  is  increased  1,000,- 
000  times. 


20  AQUEOUS  AGENCIES. 

"We  have  seen  that  a  current  running  three  feet  per  second,  or  about 
two  miles  per  hour,  will  move  fragments  of  stone  of  the  size  of  a  hen's- 
egg,  or  about  three  ounces'  weight.  It  follows  from  the  above  law  that 
a  current  of  ten  miles  an  hour  will  bear  fragments  of  one  and  a  half 
ton,  and  a  torrent  of  twenty  miles  an  hour  will  carry  fragments  of  100 
tons'  weight.  "We  can  thus  easily  understand  the  destructive  effects  of 
mountain-torrents  when  swollen  by  floods. 

The  transporting  power  of  water  must  not  be  confounded  with  its 
erosive  power.  The  resistance  to  be  overcome  in  the  one  case  is  weight, 
in  the  other  cohesion  ;  the  latter  varies  as  the  square,  the  former  as  the 
sixth  power  of  the  velocity.  In  many  cases  of  removal  of  slightly  coher- 
ing material  the  resistance  is  a  mixture  of  these  two  resistances,  and  the 
power  of  removing  material  will  vary  at  some  rate  between  v*  and  v*. 

There  are  certain  corollaries  which  follow  from  the  above  law: 

A.  If  a  current  bearing  sediment  have  its  velocity  checked  by  any 
cause,  even  in  a  slight  degree,  a  comparatively  large  portion  of  the  sedi- 
ment is  immediately  deposited.     But  if,  on  the  other  hand,  the  velocity 
of  a  current  be  increased  by  any  cause,  in  never  so  small  a  degree,  it 
will  again  take  up  and  carry  on  materials  which  it  had  deposited ;  in 
other  words,  it  will  erode  its  bed  and  banks  ;  and  these  effects  are  sur- 
prisingly large  on  account  of  the  great  change  in  erosive  and  transport- 
ing power,  with  even  slight  changes  of  velocity. 

B.  Water,  whether  still  or  running,  has  a  wonderful  power  of  sorting 
materials.     If  heterogeneous  material,  such  as  ordinary  earth,  consisting 
of  grains  of  all  sizes,  from  pebbles  to  the  finest   clay,  be  thrown  into 
still  water,  the  coarse  material  sinks  first  to  the  bottom,  and  then  the  next 
finer,  and  the  next,  and  so  on,  until  the  finest  clay,  falling  last,  covers 
the  whole.     In  running  water  the  same  sorting  takes  place  even  more 
perfectly,  only  the  different  kinds  of  materials  are  not  dropped  upon  one 
another,  but  successively  farther  and  farther  down  the  stream  in  the 
order  of  their  fineness.    This  property  we  will  call  the  sorting  power  of 
water.    Advantage  is  often  taken  of  this  property  in  the  arts  to  separate 
materials  of  different  sizes  or  specific  gravities.     By  this  means  grains 
of  gold  are  separated  from  the  gravel  with  which  it  is  mingled,  and 
emery  or  other  powders  are  separated  into  various  degrees  of  fineness. 

We  will  now  apply  the  foregoing  simple  principles  in  the  explana- 
tion of  all  the  phenomena  of  currents. 

I.— Stratification. 

We  have  seen  that  heterogeneous  material  thrown  into  still  water 
is  completely  sorted.  This  is  not  stratification,  since  the  various  degrees 
of  fineness  graduate  insensibly  into  one  another.  But  if  we  repeat  the 
experiment,  the  coarsest  material  will  fall  upon  the  finest  of  the  previ- 
ous experiment,  and  then  graduate  similarly  upward.  If  we  examine  the 


WINDING   COURSE   OF  RIVERS.  21 

deposit  thus  made,  we  observe  a  distinct  line  of  junction  between  the 
first  and  the  second  deposit.  This  is  stratification,  or  lamination.  For 
every  repetition  of  the  experiment  a  distinct  lamina  is  formed.  It  is 
evident,  therefore,  that  to  produce  stratification  two  conditions  are 
necessary,  namely :  1.  An  heterogeneous  material ;  and,  2.  An  inter- 
mittently-acting cause.  Now,  these  two  conditions  are  always  present 
in  Nature  where  sediments  are  depositing.  Into  every  body  of  still 
water,  as  a  lake  or  sea,  rivers  bring  heterogeneous  material  torn  from  the 
land ;  but  this  process  is  not  equable,  being  increased  in  the  case  of 
small  streams  by  every  rain,  and  in  large  rivers  by  the  annual  floods. 
Therefore,  sedimentary  deposits  in  still  water  are  always  stratified. 

In  running  water  the  case  is  somewhat  different.  If  the  stream  runs 
with  a  velocity  at  all  times  the  same,  then  with  every  repetition  of  the 
foregoing  experiment  the  same  kind  of  material  falls  on  the  same 
spot — gravel  on  gravel,  sand  on  sand,  and  mud  on  mud — and  there  will 
be  no  stratification.  In  running  water,  therefore,  another  condition  is 
necessary,  namely,  a  variable  current.  For,  when  the  velocity  increases, 
coarser  material  will  be  carried  and  deposited  where  finer  was  previ- 
ously deposited;  when  the  velocity  decreases,  finer  will  be  deposited  on 
coarser,  and  very  perfect  stratification  is  the  result.  Now,  these  three 
conditions  are  always  present  in  every  natural  current.  The  velocity 
of  every  river-current  varies  not  only  very  greatly  in  different  portions 
of  the  year,  as  in  seasons  of  low  water  and  seasons  of  flood,  but  also 
(from  the  constant  shifting  of  the  subordinate  currents  of  the  stream) 
from  day  to  day,  from  hour  to  hour,  and  even  from  moment  to  moment. 
It  follows,  therefore,  that  deposits  in  running  water  are  also  always 
stratified.  Sometimes  extreme  beauty  and  distinctness  of  stratification  in 
the  deposits  of  large  rivers  are  due  to  the  fact  that  the  different  branches 
flood  at  different  seasons,  and  bring  down  differently-colored  sediments. 

We  may,  therefore,  announce  it  as  a  law,  that  all  sedimentary  de- 
posits are  stratified  ;  and,  conversely,  that  all  stratified  masses  in  which 
the  stratification  is  the  result  of  sorted  material  are  sedimentary  in 
their  origin.  Upon  this  law  is  founded  almost  all  geological  reasoning. 

2. —  Winding  Course  of  Eivers 

The  winding  course  of  rivers  is  due  partly  to  erosion,  and  partly  to 
sedimentary  deposit.  It  is  most  conspicuous  and  most  easily  studied 
in  rivers  which  run  through  extensive  alluvial  deposit.  If  the  channel 
of  such  a  river  be  made  perfectly  straight  by  artificial  means,  very  soon 
some  portion  of  the  bank  a  little  softer  than  the  rest  will  be  excavated ; 
this  will  reflect  the  current  obliquelv  across  to  the  other  side,  which 
will  become  similarly  excavated.  Thus  the  current  is  reflected  from 
side  to  side,  increasing  the  excavations.  In  the  mean  time,  while  ero- 
sion is  progressing  on  the  outer  side  of  the  curves,  because  the  current 


AQUEOUS  AGENCIES. 


is  swiftest  there,  deposit  is  taking  place  on  the  inner  side,  because  there 
the  current  is  slowest ;  thus,  while  the  outer  curve  extends  by  erosion, 
the  inner  curve  extends, paripassu,  by  deposit  (Fig. 
13),  and  the  winding  continues  to  increase,  until, 
under  favorable  circumstances,  contiguous  curves 
on  the  same  side  run  into  each  other,  as  at  a  b, 
and  the  curve  c  on  the  other  side  is  thrown  out 
and  silted  up.  Thus  are  formed  the  crescentic 
lakes,  or  lagoons  (I  I),  so  common  in  the  swamps 
of  great  rivers.  They  are  abundant  in  the  swamps 
of  all  the  Gulf  rivers,  especially  the  Mississippi. 
They  are  old  beds  of  the  river,  thrown  out  and 
silted  up  in  the  manner  indicated  above. 

3. — Flood-Plain  Deposits. 

All  great  rivers  annually  flood  portions  of  level 
land  near  their  mouths,  and  cover  them  with  sedi- 
mentary deposits.  The  whole  area  thus  flooded 
is  called  the  flood-plain.  These  flood-plains  are 
very  extensive,  and  the  deposits  very  large,  in  the 
case  of  rivers  rising  in  lofty  mountains  and  flow- 
ing in  the  lower  portion  of  their  course  through 
extensive  tracts  of  flat  country.  In  the  lofty 
mountains  the  current  runs  with  great  velocity, 
and  gathers  abundant  sediment ;  on  reaching  the 
flat  country  the  velocity  is  checked,  the  river  over- 
flows, and  the  sediment  is  deposited.  The  flood- 

^^    °f    the    MississiPPi    River    is    30,000    Square 

Meandering  miles.  The  flood-plain  of  the  Nile  is  the  whole 
land  of  Egypt. 

The  flood-plain  of  a  river  may  be  divided  into  two  parts,  viz.,  the 
river-swamp  and  the  delta.  The  river-swamp  is  that  part  which  was 
originally  land-surface ;  the  delta  that  part  which  has  been  reclaimed 
from  the  sea  or  lake  by  the  river.  We  will  take  up  these  in  succession. 

River-Swamp. — We  have  already  seen  that,  with  every  recurrence 
of  the  rainy  season  or  of  the  melting  of  snows,  the  flooding  and  the 
deposition  of  sediment  are  repeated.  Thus  the  river-swamp  deposit 
increases  in  thickness,  and  the  level  of  the  whole  flood-plain  rises  con- 
tinually. Fig.  14  is  an  ideal  section  showing  the  manner  in  which  the 
flood-plain  is  successively  built  up ;  a  a  a  is  the  supposed  original  con- 
figuration of  the  surface,  b  b  the  successive  levels  of  deposit,  e  the  level 
of  the  river  at  low  water,  and  i  i  the  level  of  flood- water. 

The  extent  of  such  river-swamp  deposits  is  sometimes  very  great. 
The  river-swamp  of  the  Nile  constitutes  the  whole  fertile  land  of  Egypt 


FLOOD-PLAIN   DEPOSITS.  23 

above  the  delta.  The  river-swamp  of  the  Mississippi  River,  or  its 
flood-plain  exclusive  of  the  delta,  extends  from  fifty  miles  above  the 
mouth  of  the  Ohio  to  the  head  of  the  delta,  a  distance  of  about  700 
miles ;  its  width  is  ten  to  fifty  miles,  and  it  includes  an  area  of  16,000 


FIG.  14.— Ideal  Section  of  a  River  subject  to  Floods. 

square  miles.  It  is  bounded  on  either  side  by  high  bluffs  belonging  to 
a  previous  geological  period.  The  depth  of  this  deposit  at  the  head 
of  the  delta  is  assumed  by  Lyell  to  be  264  feet.1  But  Hilgard  has 
shown  that  but  a  small  portion  of  this  is  actually  river  deposit. 

Natural  Levies. — It  is  seen  by  the  cross-section  (Fig.  14)  that  the 
level  of  the  river-swamp  slopes  gently  from  the  river  outward,  so  that 
the  river  is  bounded  on  each  side  by  a  higher  ridge,  d  d.  The  material 
of  this  ridge  is  coarser  than  that  of  the  swamp  farther  back.  Such 
natural  levees  are  found  along  all  rivers  subject  to  regular  overflows. 
They  are  formed  as  follows :  In  times  of  flood  the  whcle  flood-plain  is 
covered  with  water  moving  slowly  seaward.  Through  the  midst  of  this 
wide  expanse  of  water  runs  the  rapid  current  of  the  river.  Now,  on 
either  side,  just  where  the  rapid  current  of  the  river  comes  in  contact 
with  the  comparatively  still  water  of  the  flood-plain,  and  is  checked  by 
it,  a  line  of  abundant  sediment  is  determined,  which  forms  the  natural 
levee.  Except  in  very  high  freshets,  these  natural  ridges  are  not  en- 
tirely covered,  so  that  the  river  in  ordinary  floods  is  often  divided  into 
three  streams,  viz.,  the  river  proper  and  the  river-swamp  water  on 
either  side.  They  cannot,  however,  confine  the  river  within  its  bank 
and  prevent  overflows,  since  the  river-bed  is  also  constantly  rising  by 
deposit.  Thus  the  river-bed,  the  natural  levee,  and  the  river-swamp, 
all  rise  together,  maintaining  a  certain  constant  relation  to  one  another. 

Artificial  Levies, — This  constant  relation  is  interfered  with  by  the 
construction  of  artificial  levees.  These  are  constructed  for  the  purpose 
of  confining  the  river  within  its  banks,  and  thus  reclaiming  the  fertile 
lands  of  the  river-swamp.  As  the  bed  of  the  river  continues  to  rise 
by  deposit,  the  levees  must  be  constantly  elevated  in  proportion ;  but 
the  river-swamp,  being  deprived  of  its  share  of  deposit,  does  not  rise. 
Thus,  under  the  combined  effect  of  human  and  river  agencies  contend- 
ing for  mastery,  an  ever-increasing  embankment  is  formed,  until  finally 
the  river  runs  in  an  aqueduct  elevated  far  above  the  surrounding  plain. 
1  Lyell,  "  Principles  of  Geology,"  vol.  i.,  p.  462. 


AQUEOUS   AGENCIES. 


This  is  very  remarkably  the  case  with  the  river  Po,  which  is  said  to  run 
in  a  channel  that  has  been  thus  elevated  above  the  tops  of  the  houses 
in  the  town  of  Ferrara.  Fig.  15  is  an  ideal  cross-section  of  a  river  and 


flood-plain,  left  at  first  to  the  action  of  natural  causes  for  a  time,  but 
afterward  interfered  with  by  the  construction  of  artificial  levees.  The 
dotted  strata  show  the  work  of  Nature,  and  the  undotted  the  work  of 
man.  It  is  easy  to  see  that  the  destructive  effects  of  overflow  from  acci- 
dental crevasses  become  greater  and  greater  with  the  elevation.  The 
Po  has  thus  several  times  broken  through  its  levees  and  deserted  its 
bed,  destroying  several  villages.  The  best  examples  of  rivers  success- 
fully levied  are  those  of  Italy  and  Holland.  The  Mississippi  has  never 
been  successfully  levied ;  but  if  it  should  be,  it  would  commence  to 
build  up  a  similar  aqueduct,  until  the  whole  bed  of  the  liver  would 
finally  rise  above  the  level  of  the  river-swamp.1 

4.— Deltas. 

Deltas  are  portions  of  land  situated  at  the  mouths  of  rivers,  and 
reclaimed  from  the  sea  by  their  agency.  Over  the  fiat  surface  of  the 
delta  the  river  runs  by  inverse  ramification,  and  empties  by  many 
mouths.  They  are  usually  of  irregular  triangular  form,  the  apex  of  the 
triangle  pointing  up  the  stream.  The  delta  of  the  Nile  (Fig.  16)  is 
perhaps  the  best  example  of  the  typical  form.  As  seen  in  the  figure, 
at  the  head  of  the  delta  the  river  divides  into  branches,  and  communi- 
cates with  the  sea  by  many  mouths.  The  area  of  land  thus  made  va- 
ries with  the  size  of  the  river,  the  proportion  of  sediment  in  its  waters, 
and  the  time  it  has  been  making  sedimentary  accumulations.  The 
delta  of  the  Nile  is  100  miles  long  and  200  miles  wide  at  its  base  ;  that 
of  the  Ganges  and  Brahmapootra  is  220  miles  long  and  200  miles  wide 
at  its  base,  comprising  an  area  of  20,000  square  miles.  The  delta  of 
the  Mississippi  (Fig.  17)  is  very  irregular  in  form,  and  is  an  admirable 
illustration  of  the  manner  in  which  each  mouth  pushes  its  way  into 
the  sea.  Its  area  is  estimated  at  12,300  square  miles.  The  materials 

1  It  is  probable  that  the  effect  of  levees  in  raising  the  river-bed  has  been  greatly  ex- 
aggerated. Recent  observations  on  the  Po  seem  to  show  that  the  elevation  is  confined  to 
the  upper  reaches  of  the  flood-plain  region,  being  prevented  in  the  lower  course  by  the 
increased  velocity  of  the  current  produced  by  levees. 


DELTAS. 


25 


FIG.  16.— Delta  of  the  Nile. 


of  which  deltas  are  composed  are  usually  the  finest  sands  and  clays,  all 
the  coarser  materials  having  been  deposited  higher  up  the  stream. 

Deltas  are  formed  only  in  lakes  and  tideless  or  nearly  tideless  seas. 


FIG.  17.— Delta  of  the  Mississippi. 


26 


AQUEOUS  AGENCIES. 


In  tidal  seas,  the  sediments  brought  down  by  the  rivers  are  swept 
away  and  carried  to  sea  by  the  retreating  tide  ;  and  instead  of  the  land 
encroaching  upon  the  domain  of  the  sea  by  the  formation  of  deltas, 
the  sea  encroaches  upon  the  land  by  the  erosive  action  of  the  tides,  and 
forms  bays  or  estuaries.  Thus  in  tideless  seas  or  lakes  the  rivers  empty 
by  many  slender  mouths,  while  in  tidal  seas  they  empty  by  wide  bays ; 
thus,  for  example,  all  the  rivers  emptying  into  the  great  Canadian 
lakes,  and  all  the  rivers  emptying  into  the  Gulf  of  Mexico,  form  deltas, 
while  all  the  rivers  emptying  into  the  Atlantic  in  both  North  and 
South  America  form  estuaries.  In  Europe  all  the  rivers  emptying  into 
the  Black,  the  Caspian,  the  Mediterranean,  and  the  Baltic,  form  deltas, 
while  those  emptying  into  the  Atlantic  form  estuaries. 

Process  of  Formation. — The  process  of  formation  of  a  delta  may  be 
best  studied  by  observing  it  on  a  small  scale,  in  the  case  of  streamlets 
running  into  ponds.  In  such  cases  we  observe  always  a  sand  or  mud 
flat  at  the  mouth  of  the  streamlet,  evidently  formed  by  the  sand  and 
clay  brought  down  by  the  current.  As  soon  as  the  current  strikes  the 

still  water  of  the  pond, 
its  velocity  is  checked, 
and  its  burden  of  secli- 
ment  is  deposited. 
Through  the  sand-flat 
thus  formed  the  stream- 
let ramifies,  as  seen  in 

Fig.  18.  The  ramification  seems  to  be  the  result  of  the  choking  of 
the  stream  by  its  own  deposit,  which  forces  it  to  seek  new  chan- 
nels. The  sand-flat  is  gradually  extended  farther  and  farther  into  the 
pond  by  successive  deposits,  as  shown  in  Fig.  18.  Fig.  19  shows 


the  irregular  stratified  appearance  of  the  deposit  as  seen  on  cross-sec- 
tion. In  all  such  cases  of  streams  flowing  into  ponds  or  lakes,  the 
stream  flows  in  at  a  muddy,  but  flows  out  at  b  perfectly  clear,  having 
deposited  all  its  sediment  in  the  pond  or  lake.  Evidently  if  this  pro- 
cess continues  without  interruption,  the  pond  will  eventually  be  filled 
up,  after  which,  of  course,  the  sediment  will  be  carried  farther  down 
the  stream.  In  this  manner  small  mountain-lakes  are  often  entirely 
filled  up.  The  Rhone  flows  into  Lake  Geneva  a  turbid  stream,  but 
flows  out  beautifully  transparent.  The  whole  of  its  sediment  is  de- 


TRANSPORTATION  AND   DISTRIBUTION   OF   SEDIMENTS.  2Y 

posited  where  it  enters  the  lake,  and  it  has  there  formed  a  delta  six 
miles  long.  We  may  confidently  look  forward  to  the  time,  though 
many  thousand  years  distant,  when  this  lake  will  be  entirely  filled  up. 
After  leaving  the  lake  the  Rhone  again  gathers  sediment  from  tribu- 
taries flowing  in  below  the  lake,  and  forms  another  delta  where  it  emp- 
ties into  the  Mediterranean.  Many  examples  of  lakelets  partially  filled, 
or  entirely  filled  and  converted  into  meadows,  are  found  among  the 
Sierra  Mountains. 

In  the  section  view  (Fig.  19),  we  have  represented  the  strata  as 
irregular  and  highly  inclined.  This  is  called  oblique  lamination.  This 
can  only  occur  when  a  rapid  stream,  bearing  abundant  coarse  material, 
rushes  into  still  water.  But  in  the  case  of  large  rivers  flowing  long 
distances  and  bearing  only  the  finest  sediment,  the  stratification  is 
much  more  regular  and  nearly  horizontal. 

Rate  of  Growth. — There  have  been  several  attempts  to  estimate  the 
rate  of  growth  of  deltas,  in  order  to  base  thereon  an  estimate  of  their 
age.  The  delta  of  the  Rhdne  in  Lake  Geneva  has  advanced  at  least 
one  and  a  half  mile  since  the  occupation  of  that  country  by  the  Romans ; 
for  the  ancient  town  Porta  Valesia  (now  Port  Valais),  which  stood  then 
on  the  margin  of  the  lake,  is  now  one  and  a  half  mile  inland.  The 
delta  of  the  same  river  at  its  mouth  in  the  Mediterranean  is  said  to 
have  advanced  twenty-six  kilometres,  or  sixteen  miles,  since  400  B.  c., 
or  thirteen  miles  during  the  Christian  era.1  The  delta  of  the  Po  has 
advanced  twenty  miles  since  the  time  of  Augustus  ;  for  the  town  Adria, 
a  seaport  at  that  time,  is  now  twenty  miles  inland.  But  the  most  elab- 
orate observations  have  been  made  on  the  Mississippi.  This  river,  as 
seen  in  Fig.  17,  has  pushed  its  way  into  the  Gulf  in  a  most  extra- 
ordinary manner.  According  to  Thomassy,2  and  also  Humphrey  and 
Abbot,  the  rate  of  advance  is  about  one  mile  in  sixteen  years.  The 
rate  of  progress  in  the  deltas  mentioned  has,  however,  probably  not 
been  uniform.  There  are  special  reasons  for  their  more  rapid  advance 
at  the  present  time.  In  the  case  of  the  Po,  the  successful  leveling  of 
this  river  has  transferred  to  the  sea  the  whole  of  the  sediment  which 
would  otherwise  have  been  spread  over  the  flood-plain.  In  the  case  of 
the  Mississippi,  for  many  centuries  the  principal  portion  of  the  deposit 
has  been  confined  to  a  narrow  strip  but  a  few  miles  wide,  and  the  ad- 
vance has  been  proportionately  rapid.  For  this  reason  the  river  has 
run  out  to  sea  for  more  than  fifty  miles,  confined  only  by  narrow  strips 
of  land,  the  continuation  of  the  natural  levees.  These  marginal  ridges 
are  continued  as  submarine  banks  even  much  beyond  the  present  mouths 
of  the  river.  The  rate  of  advance  of  the  Nile  delta  seems  to  be  much 
slower. 

Age  of  River-Deposits. — The  age  of  river-swamp  deposits  may  be 
1  "Archives  des  Sciences,"  vol.  li.,  p.  157.  2  "Geologie  pratique  de  la  Louisiana" 


28 


AQUEOUS  AGENCIES. 


estimated  by  determining  their  absolute  thickness  and  their  rate  of 
increase.  The  river  Nile  is  peculiarly  adapted  for  estimates  of  this 
kind,  because  we  have  on  its  alluvial  deposits  the  seat  of  the  oldest 
civilization  and  the  oldest  known  monuments  of  human  art.  These 
monuments,  the  ages  of  which  are  approximately  known,  are  many 
of  them  more  or  less  buried  in  the  river-deposit.  At  Memphis,  the 


FIG.  20.— Ideal  Section  of  Delta  and  Submarine  Bank. 

foundation  of  the  colossal  statue  of  Rameses  II.,  over  3,000  years 
old,  was  found  in  1854  buried  about  nine  feet  in  river-deposit.1  This 
makes  the  rate  of  increase  of  the  deposit  three  and  a  half  inches  per 
century.  Experiments  at  Heliopolis  bring  out  nearly  the  same  result. 
The  whole  depth  of  the  alluvial  deposit  at  Memphis  was  found  to  be 
about  forty  feet,  which,  at  the  above  rate,  would  make  the  age  of  the 
deposit  at  this  point  about  13,500  years.  The  alluvial  deposit  of  the 
Nile  is  much  thicker  at  some  points  than  forty  feet ;  but,  on  the  other 
hand,  the  rate  of  increase  for  different  places  is  probably  variable. 

The  age  of  a  delta  is  usually  estimated  by  determining  the  annual 
mud-discharge,  and  dividing  it  into  the  cubic  contents  of  the  delta.    The 

cubic  contents  of  the  delta  are 
estimated  by  multiplying  the 
superficial  area  by  the  mean 
depth.  The  mean  depth  of  the 
Mississippi  Delta,  as  determined 
by  borings,  is  taken  by  Mr. 
Lyell  as  528  feet,  the  superficial 
area  at  13,600  square  miles,  and 
the  annual  mud-discharge  at 
7,400,000,000  cubic  feet.  Upon 
these  data  he  makes  the  prob- 
able age  of  the  delta  33,500 
years.  To  this  he  adds  half  as 
much  for  the  age  of  the.  river- 
swamp,  making  in  all  50,000 
years. 

It  is  evident,  however,  that 
this  estimate  cannot  be  relied  on  as  even  approximately  accurate.     For 
there  is  no  reason  why  the  time  of  river-swamp  deposit  should  be  added 
to  that  of  the  delta,  for  they  were  both  probably  formed  at  the  same 
1  Philosophical  Magazine,  vol.  xvi.,  p.  225. 


FIG.  21.— Delta  and  Submarine  Bank. 


TRANSPORTATION  AND  DISTRIBUTION  OF  SEDIMENTS.  29 

time — one  by  deposits  higher  up  the  river,  the  other  by  deposits  at  the 
mouth.  Again,  on  the  other  hand,  the  estimate  takes  no  account  of 
the  submarine  extension  of  the  delta,  in  area  certainly,  and  in  cubic 
contents  probably,  much  greater  than  the  subaerial  delta.  Figs.  20 
and  21  are  an  ideal  section  and  a  map  of  a  delta,  in  which  a  is  the 
aerial  and  b  the  submarine  portion.  This  would  greatly  increase  the 
time. 

It  is  evident,  therefore,  that  although  the  problem  is  one  of  great 
interest,  we  are  not  yet  in  possession  of  data  to  make  a  reliable  esti- 
mate. Every  estimate,  however,  indicates  a  very  great  lapse  of  time. 

But  it  must  not  be  imagined,  as  all  estimators  seem  to  do,  that  this 
time,  be  it  greater  or  less  than  Mr.  Lyell's  estimate,  belongs  all  to  the 
present  geological  epoch.  Prof.  Hilgard  has  shown  that  the  true  allu- 
vial deposit  of  the  Mississippi  is  only  forty  or  fifty  feet  thick.  Beneath 
this  the  deposit  belongs  to  the  Quaternary  or  preceding  geological  epoch. 

5. — Estuaries. 

We  have  already  seen  that  rivers  which  empty  into  tideless  seas 
communicate  with  the  sea  by  numerous  branches  traversing  an  alluvial 
flat,  formed  by  the  deposits  of  the  river  ;  while  rivers  emptying  into 
tidal  seas  communicate  by  wide  mouths  or  bays,  formed  by  the  erosive 
action  of  the  flowing  and  ebbing  tide.  Such  bays  are  called  estuaries. 
We  have  fine  examples  of  estuaries  in  the  Amazon  and  La  Plata  Rivers, 
in  the  Delaware  and  Chesapeake  Bays,  in  the  friths  of  Scotland  and 
the  fiords  of  Norway  :  in  fact,  at  the  mouths  of  all  the  rivers  emptying 
into  the  Atlantic  on  our  own  coast  as  well  as  on  the  European  coast. 
The  mouth  of  the  Columbia  River  is  a  good  example  on  the  Pacific 
coast.  The  phenomena  of  a  delta  and  an  estuary  are  sometimes  com- 
bined in  the  same  river.  This  is  the  case  to  some  extent  in  the  Ganges. 

Mode  of  Formation. — Estuaries  are  evidently  formed  by  the  erosive 
action  of  the  inflowing  and  outflowing  tide.  Their  shape,  narrow  above 
and  widening  toward  the  sea,  gives  great  force  to  the  tidal  currents, 
which,  entering  below  and  concentrated  in  the  ever-narrowing  channel, 
rushes  along  with  prodigious  velocity  and  rises  to  an  immense  height. 
In  the  Bay  of  Fundy  the  tide  rises  seventy  feet,  and  at  Bristol,  England, 
it  rises  forty  feet,  in  Puget  Sound  twenty-five  feet.  Sometimes,  from  ob- 
structions at  the  mouth  of  the  river,  the  tide  enters  as  one  or  more  im- 
mense waves,  rushing  along  like  an  advancing  cataract.  This  is  called 
an  eagre  or  bore.  The  finest  examples  are  perhaps  in  the  Amazon  and 
Tsien-tang  Rivers.  In  the  eagre  of  the  Amazon  "  the  tide  passes  up  in 
the  form  of  five  or  six  waves  following  one  another  in  rapid  succession, 
and  each  twelve  to  fifteen  feet  high."  In  the  Tsien-tang,  a  single  wave 
plunges  along  at  the  rate  of  twenty-five  miles  an  hour,1  with  perpen- 
1  American  Journal  of  Science  and  Arts,  1855,  vol.  xx.,  p.  305. 


30  AQUEOUS  AGENCIES. 

dicular  front,  like  an  advancing  cataract,  four  or  five  miles  wide  and 
thirty  feet  high.  In  the  river  Severn  also  we  have  a  remarkable  exam- 
ple of  an  eagre.  According  to  the  laws  already  developed  (p.  19),  the 
erosive  and  transporting  power  of  such  currents  must  be  immense. 

Deposits  in  Estuaries. — The  larger  portion  of  the  materials  thus 
eroded  is  carried  out  to  sea  by  the  retreating  tide,  and  will  be  again 
spoken  of  under  "  Sea-deposits."  A  portion  of  these  materials,  however, 
is  always  deposited  in  the  estuary  in  sheltered  coves  and  bays  (Fig.  22, 
a  and  b),  and  often,  when  the  outflowing  tide  is  obstructed  by  sand- 
spits  and  islands  at  the  mouth,  over  every  portion  of  the  estuary.  In 
addition  to  this,  especially  in  rivers  subject  to  great  freshets,  there  are 
deposits  of  silt  from  the  river.  Thus  many  estuaries  are  occupied  alter- 
nately, during  the  wet  and  dry  seasons,  by  fresh  and  brackish  or  salt 
water,  and  the  deposits  in  them  are  therefore  alternately  fresh-water 
and  salt-water  deposits,  containing  fresh-water  and  salt  or  brackish 
water  shells.  These  alternations  are  highly  characteristic  of  estuary- 
deposits  in  all  geological  periods ;  in  fact,  of  all  deposits  at  the  mouths 
of  rivers  where  river  and  ocean  agencies  meet. 

6. — Bars. 

Bars  are  invariably  formed  in  accordance  with  the  law  already- 
enunciated  as  that  controlling  all  current-deposits,  viz.,  if  the  velocity 
of  a  current  bearing  sediment  be  checked,  the  sediment  is  deposited. 

There  are  two  positions  in  which  bars  are  formed :  1.  At  the  mouths 
of  rivers ;  and,  2.  At  the  head  of  the  estuaries.  In  the  first  position 


ft 

FIG.  22. — An  Estuary. 

(Fig.  22,  d  d}  the  bar  is  formed  by  the  contact  of  the  river-current 
with  the  still  water  of  the  ocean.  It  is  most  marked  in  the  case  of 
estuaries.  The  outfiWing  tide  scours  out  the  estuary,  carrying  with  it 
sediment  partly  brought  down  by  the  river,  and  partly  the  debris  of 
land  eroded  by  the  inflowing  tide.  The  larger  portion  of  this  is  dropped 
as  soon  as  the  tidal  current  comes  in  contact  with  the  open  sea  and  is 
checked  by  it.  They  are  usually  irregularly  crescentic  in  form.  Such 
are  the  bars  at  the  mouths  of  all  harbors.  In  the  second  position  they 


WAVES   AND  TIDES.  31 

are  found  just  where  the  upward  current  of  the  inflowing  tide  meets  the 
downward  current  of  the  river,  and  makes  still  water.  At  this  point 
we  have  not  only  a  bar,  but  usually  also  an  extensive  marsh  caused  by 
the  daily  overflow  of  the  river.  Through  this  marsh  the  river  winds 
in  a  very  devious  course,  as  is  common  in  all  rivers  whose  banks  are 
alluvial. 

Thus,  then,  in  rivers  like  the  Mississippi,  emptying  into  tideless  seas 
and  forming  deltas,  there  is  but  one  bar,  viz.,  that  at  the  mouth ;  while 
in  rivers  forming  estuaries  there  are  two  bars,  an  outer  and  an  inner. 
This  inner  bar  may  be  many  miles  up  the  river.  In  the  Hudson  River 
the  inner  bar  is  140  miles  up  the  river,  and  only  a  few  miles  below 
Albany.  This  is  really  the  head  of  the  estuary,  or  of  tide-water,  in  this 
river. 

It  is  evident  that  bars,  being  produced  by  natural  and  constantly- 
acting  causes,  cannot  usually  be  permanently  removed,  though  they  may 
be  sometimes  greatly  improved.  If  they  are  scraped  away  by  dredg- 
ing-machines,  they  are  speedily  reformed  on  the  same  spot.  If  we  cause 
the  river  itself  to  remove  them,  as  has  sometimes  been  done  by  narrow- 
ing the  channel  and  thus  increasing  the  erosive  power,  we  indeed 
remove  the  bar,  but  it  is  reformed  farther  down  the  stream  at  a  new 
point  of  equilibrium. 

We  have  thus  traced  river  agencies  from  their  source  to  the  sea. 
This  brings  us  naturally  to  ocean  agencies. 

SECTION  2. — OCEAN. 
Waves  and  Tides. 

Waves. — "Waves  produce  no  current,  and  therefore  no  geological 
effect  in  deep  water.  The  erosive  effect  of  this  agent  is  almost  en- 
tirely confined  to  the  coast-line,  but  at  this  point  is  incessant  and 
powerful.  The  average  force  of  waves  on  the  west  coast  of  Scotland 
for  the  summer  months  is  estimated  by  Stevenson  at  611  pounds  per 
square  foot,  and  for  the  winter  months  at  2,086  pounds  per  square 
foot.1  In  violent  storms  the  force  is  estimated  at  6,000  pounds 
per  square  foot,5  and  fragments  of  rock  of  many  hundred  tons'  weight 
are  often  hurled  to  a  considerable  distance  on  the  land.  These  frag- 
ments hurled  against  the  shore  are  the  principal  agent  of  wave-erosion. 
The  rapidity  of  the  erosion  of  a  coast-line  by  the  action  of  waves  is 
determined  partly  by  the  softness  and  partly  by  the  inclination  of  the 
strata.  If  the  strata  turn  their  faces  to  the  waves,  particularly  if  in- 
clined at  a  small  angle,  the  effect  of  the  waves  is  comparatively  slight 
(Fig.  23) ;  but  if  the  edges  of  the  strata  are  exposed  to  the  waves,  the 

1  Dana's  "  Manual,"  p.  654.  2  Herschel's  "  Physical  Geography,"  p.  75. 


32 


AQUEOUS  AGENCIES. 


erosion  is  much  greater.  For  instance,  if  the  strata  be  horizontal,  as 
in  Fig.  24,  then  the  strata  are  undermined  and  form  overhanging  table- 
rocks,  which  from  time  to  time  fall  into  the  sea  ;  if  the  strata  are  verti- 
cal or  highly  inclined 
and  their  edges 
turned  to  the  sea, 
then  an  exceedingly 
irregular  coast-line  is 
formed  and  the  ero- 
sion is  very  rapid,  as 
the  force  of  the 
waves  is  concen- 
trated upon  the  reen- 
tering  angles.  Fig. 
25  is  a  map  view  of  a 

coast,  in  which  from  a  to  b  the  waves  strike  the  edges,  while  from  a  to 
c  they  strike  the  faces  of  the  same  rocky  strata.  The  difference  in  the 
form  of  the  coast-line  is  seen  at  a  glance. 

Waves  cutting  ever  at  the  shore-line  only,  act  like  an  horizontal  saw. 
The  receding  shore-cliff,  therefore,  leaves  behind  it  an  ever-increasing 
subaqueous  platform  which  marks  the  amount  of  recession.  This  is 


"3 

. 

>£T         1 

FIG.  24.-Section  of  an  Exposed  Cliff. 

shown  in  the  section  (Fig.  26),  in  which  s  is  the  present  shore-line,  I  the 
water-level,  a  b  the  platform,  s'  the  original  shore-line,  and  s'  b  c  the 
original  slope  of  bottom.  The  recession  of  the  shore-line  and  the 
formation  of  the  shore  platform  have  been  accurately  observed  in  Lake 

Michigan       (Andrews). 
Level  platforms   termi- 
nated   by   cliffs,    there- 
fore,   when    found     in- 
|    land,    sometimes    indi- 
FlG  26  -    cate  the  position  of  old 

shore-lines. 

Tides. — The  tide  is  a  wave  of  immense  base,  and  three  or  four  feet 
in  height  in  the  open  ocean,  produced  by  the  attractive  force  of  the  moon 


WAVES  AND   TIDES.  33 

and  sun  on  the  waters  of  the  ocean.  The  velocity  of  this  wave  is  very 
great,  since  it  travels  around  the  earth  in  twenty-four  hours.  In  the 
open  ocean  it  produces  very  little  current,  only  a  slow  transfer  of  the 
water  back  and  forth,  too  slow  to  produce  any  geological  effect ;  *  but  in 
shallow  water,  where  the  progress  of  the  wave  is  impeded,  it  piles  up 
in  some  cases  forty  to  fifty  feet  in  height,  and  gives  rise  to  currents  of 
great  velocity  and  immense  erosive  power.  By  this  means  bays  and 
harbors  are  formed,  and  straits  and  channels  are  scoured  out  and 
deepened.  Tides  also  act  an  important  part  in  assisting  the  action  of 
waves  upon  the  whole  coast-line.  The  action  of  waves  on  exposed 
cliffs  quickly  forms  accumulations  of  d&bris  at  their  base,  composed  of 
sand,  mud,  shingle,  or  rocky  fragments  (Fig  24),  which  receive  first  and 
greatly  diminish  the  shock  of  the  waves  upon  the  cliff.  The  inces- 
sant beating  of  the  waves  upon  this  debris  reduces  it  to  a  finer  and 
finer  condition,  and  the  retreating  waves  bear  much  of  it  seaward; 
so  that,  even  without  the  assistance  of  any  other  agent,  the  protection 
is  incomplete,  and  the  erosion  therefore  progresses.  But  if  strong  tidal 
currents  run  along  the  coast,  these  effectually  remove  such  debris  and 
leave  the  cliff  exposed  to  the  direct  action  of  the  waves. 

Examples  of  the  Action  of  Waves  and  Tides. — The  coasts  of  the 
United  States  show  many  examples  of  the  erosive  action  of  waves  and 
tides.  The  form  of  the  whole  New  England  coast  is  largely  determined 
by  this  cause.  The  softer  parts  are  worn  away  into  harbors  by  the 
waves  and  scoured  out  by  the  tides,  while  the  harder  parts  reach  out 
like  rocky  arms  far  into  the  sea.  Sometimes  only  small  rocky  islands, 
stripped  of  every  vestige  of  earth,  mark  the  position  of  the  former  coast- 
line. Boston  Harbor  and  the  rocky  points  and  islands  in  its  vicinity 
are  good  examples.  The  process  is  still  going  on,  and  its  progress 
may  be  marked  from  year  to  year. 

On  the  Southern  coast  examples  of  a  similar  process  are  not  want- 
ing. At  Cape  May,  for  instance,  the  coast  is  wearing  away  at  a  rate 
of  about  nine  feet  per  annum.  The  more  exposed  portions  about 
Charleston  Harbor,  such  as  Sullivan's  Island,  are  said  to  be  wearing 
away  even  more  rapidly.  As  a  general  fact,  however,  the  low,  sandy  or 
muddy  shores  of  the  Southern  coasts  are  receiving  accessions  more 
rapidly  than  they  are  wearing ;  while,  on  the  contrary,  the  New  Eng- 
land coast,  as  proved  by  its  rocky  character,  is  losing  much  more  than 
it  gains.  The  shores  of  Lake  Superior  (Fig.  27)  furnish  many  beauti- 
ful examples  of  the  action  of  waves,  in  this  case,  of  course,  unassisted 
by  tides.  The  general  form  of  the  lake  along  its  south  shore  is  deter- 
mined by  the  vary  ing  hard  ness  of  the  rock ;  the  two  projecting  promon- 
tories La  Pointe  (a)  and  Keweenaw  Point  (c)  being  composed  of  hard, 
igneous  rocks,  while  the  intervening  bays  b  and  d  are  softer  sandstone. 

1  Herschel's  "  Physical  Geography,"  p.  64. 
3 


34.  AQUEOUS  AGENCIES. 

On  the  south  shore,  about  et  between  La  Pointe  and  Fond  du  Lac  (/),  the 
conditions  of  rapid  erosion  are  beautifully  seen.  The  shores  are  sand- 
stone cliffs,  with  nearly  horizontal  strata.  These  have  been  eroded 
beneath  by  the  waves,  in  some  places  for  hundreds  of  feet,  forming 


FIG.  27.— Lake  Superior. 


immense  overhanging  table-rocks,  supported  by  huge  sandstone  pillars 
of  every  conceivable  shape.  Among  these  huge  pillars,  and  along  these 
low  arches  and  gloomy  corridors,  the  waves  dash  with  a  sound  like  thun- 
der. From  time  to  time  these  overhanging  table-rocks,  with  their  load 
of  earth  and  primeval  forests,  fall  into  the  lake. 

The  coasts  of  Europe  furnish  examples  en  a  more  magnificent  scale, 
and  have  been  more  carefully  studied.  The  cliffs  of  Norfolk  are  carried 
away  at  a  rate  of  three  feet,  and  those  of  Yorkshire  six  feet,  annually. 
The  church  of  Reculver,  on  the  coast  of  Kent,  near  the  mouth  of  the 
Thames,  stood,  in  the  time  of  Henry  VIII.,  one  mile  inland.  Since  that 
time  the  sea  has  steadily  advanced  until,  in  1804,  a  portion  of  the  church- 
yard fell  in,  and  the  church  was  abandoned  as  a  place  of  worship.  The 
church  itself,  ere  this,  would  have  been  undermined  and  fallen  in,  had 
it  not  been  protected  by  artificial  means.  There  are  many  instances  in 
the  German  Ocean  of  islands  which  have  been  entirely  washed  away 
during  the  historic  period. 

The  tidal  currents  through  the  British  and  Irish  Channels,  along  the 
western  coasts  of  Ireland  and  Scotland,  among  the  Orkneys  and  Heb- 
rides, and  especially  along  the  coast  of  Norway,  are  very  powerful. 
Along  this  latter  coast  it  forms  the  celebrated  Maelstrom.  The  erosive 
effects  of  the  sea  are,  therefore,  very  conspicuous.  On  the  south  and 
east  coasts  of  England  the  erosion  is  now  progressing  rapidly.  On  the 
west  coasts  of  Ireland  and  Scotland  the  waste  is  not  now  so  great,  be- 
cause the  softer  material  is  all  removed,  but  the  configuration  of  the 
coast  shows  the  waste  which  it  has  suffered.  A  glance  at  a  good  map 


WAVES  AND  TIDES. 


35 


of  Ireland  shows  a  deeply-indented  western  coast,  composed  entirely 
of  alternating  rocky  promontories  and  deep  bays.  On  the  western  coast 
of  Scotland,  and  especially  on  the  Orkney,  Shetland,  and  Hebrides  Isl- 
ands, the  wasting  effect  of  the  sea  has  been  still  greater.  Not  only 


have  we  here  the  same  character  of  coast  as  already  described  (as  seen 
in  the  friths  of  Scotland),  but  many  small  islands  have  been  eroded, 
until  only  a  nucleus  of  the  hardest  rock  is  left ;  and  even  these  have 
been  worn  until  they  seem  but  the  ghastly  skeletons  of  once-fertile  isl- 
ands. Figs.  28  and  29  will  give  some  idea  of  the  appearance  of  these 
spectral  islands. 

The  coast  of  Norway  consists  entirely  of  deep  fiords  alternating 


with  jutting  headlands  of  hardest  rock  several  thousand  feet  high. 
Along  this  intricately -dissected  coast  there  runs  a  chain  of  high,  rocky 
islands,  which  in  an  accurate  map  is  scarcely  distinguishable  from  the 
coast  itself,  being  separated  only  by  narrow,  deep  fiords.  Toward  the 


36  AQUEOUS  AGENCIES. 

northern  part  of  the  coast  the  crest  of  the  Scandinavian  chain  seems  to 
run  directly  along  the  jutting  promontories  of  the  coast-line,  for  these 
headlands  are  the  most  elevated  part  of  the  country ;  in  fact,  in  some 
parts,  it  would  seem  that  the  original  crest  was  at  one  time  still  farther 
west,  along  the  line  of  coast-islands.  If  so,  then  the  sea  has  not  only 
carried  away  the  whole  western  slope,  but  has  broken  through  the  main 
axis,  leaving  only  these  isolated  rocky  islands  as  monuments  of  its 
former  position,  and  is  even  now  carrying  its  ravages  far  inland  on  the 
eastern  slope.  In  the  case  of  Norway,  however,  and  probably  in  case  of 
nearly  all  bold,  rocky  coasts,  the  intricacy  of  the  coast-line  is  not  due 
wholly  or  even  principally  to  the  action  of  waves  and  tides,  but  also  to 
other  causes  to  which  we  shall  refer  hereafter. 

Transporting  Power. — The  transporting  power  of  waves  is  immense- 
ly great,  often  taking  up  and  hurling  on  shore  masses  of  rock  hundreds 
of  tons  in  weight ;  but,  being  entirely  confined  to  the  coast-line,  the  dis- 
tance to  which  they  carry  is  necessarily  very  limited.  There  are  some 
instances,  however,  of  materials  carried  to  great  distances  by  the  inces- 
sant action  of  waves.  Thus,  according  to  Prof.  Bache,  coast-sand 
is  carried  slowly  farther  and  farther  south  by  the  action  of  waves,  and 
siliceous  sand  is  found  at  Cape  Sable  on  the  extreme  southern  point  of 
Florida,  although  the  whole  Florida  coast  as  far  as  St.  Augustine  is 
composed  of  coral  limestone  alone.  He  accounts  for  this  by  supposing 
that  the  trend  of  the  United  States  coast  is  such  that  waves  coming 
from  the  east  strike  the  coast  obliquely  and  fall  off  toward  the  south, 
carrying  each  time  a  little  sand  with  them.  A  similar  phenomenon  has 
been  observed  on  Lake  Michigan  :  the  sands  are  carried  steadily  toward 
the  south  end,  where  they  accumulate. 

Deposits. — The  invariable  effect  of  waves,  chafing  back  and  forth 
upon  coast  debris,  is  to  wear  off  their  angles  and  thus  to  form  rounded 
fragments  and  granules.  Thus  pebbles,  shingle,  and  round-grained 
sand,  though  produced  by  all  currents,  are  especially  characteristic  of 
wave-action.  Hippie-marks  are  also  characteristic  of  current-action  in 
shallow  water.  They  are,  therefore,  always  formed  on  shore  by  the 
action  of  waves  and  tides.  By  means  of  these  characteristics  of  shore 
deposit,  many  coast-lines  of  previous  geological  epochs  have  been  deter- 
mined. 

We  have  seen  that  waves  usually  destroy  land.  In  many  cases, 
however,  they  also  make  land.  This  is  the  case  whenever  other  agen- 
cies, such  as  river  or  tidal  currents,  drop  sediment  in  shallow  water,  and 
therefore  within  reach  of  wave-action.  We  shall  again  speak  of  these 
under  the  head  of  Land  formed  by  the  Ocean  Agencies. 


OCEANIC   CURRENTS.  3f 

Oceanic   Currents. 

The  ocean,  like  the  atmosphere,  is  in  constant  motion,  not  only  on 
its  surface,  but  throughout  its  whole  mass.  The  general  direction  of  the 
currents  in  the  two  cases  is  also  similar,  but  there  are  disturbing  and 
complicating  causes  peculiar  to  each,  which  interfere  with  the  regularity 
and  simplicity  of  the  phenomena.  If  the  currents  of  the  atmosphere  are 
more  variable  on  account  of  the  greater  levity  of  the  fluid,  oceanic  cur- 
rents have  also  their  peculiar  disturbing  causes  in  the  existence  of  im- 
passable barriers  in  the  form  of  continents.  In  both  atmosphere  and 
sea,  currents  may  also  be  deflected  by  submarine  banks,  for  mountain- 
chains  are  the  banks  of  the  aerial  ocean. 

Theory  of  Oceanic  Currents. — By  some  distinguished  physicists, 
oceanic  currents  have  been  attributed  entirely  to  the  action  of  the  trade- 
winds.1  There  can  be  no  doubt  that  this  is  a  real  cause;  yet  it  seems 
probable,  nay,  almost  certain,  that  the  great  and  controlling  cause  of 
currents  of  the  ocean,  as  of  the  air,  is  difference  of  temperature  between 
the  equatorial  and  polar  regions.8  "We  will,  therefore,  discuss  the  sub- 
ject from  this  point  of  view,  although  the  effect  would  be  much  the  same, 
whatever  be  our  view  of  the  theory.  For  the  sake  of  clearness,  we  will 
take  first  the  simplest  case,  and  then  introduce  disturbing  influences 
and  show  their  effects. 

Suppose,  first,  the  earth  covered  with  a  universal  ocean,  continually 
heated  at  the  equator,  and  cooling  at  the  poles  :  the  difference  of  den- 
sity of  the  equatorial  and  polar  seas  would  cause  exchange  or  circula- 
tion between  these  regions  by  means  of  north  and  south  currents  in  all 
longitudes,  the  equatorial  currents  being  superficial  because  warm,  and 
the  polar  currents  deep-seated  because  cold.  It  is  obviously  impossible, 
however,  that  the  principal  exchange  should  be  with  the  pole  itself, 
since  this  is  but  a  point,  but  with  the  northern  regions.  Observation 
shows  that  it  is  between  the  equator  and  the  polar  circle.  In  the  case 
we  are  now  considering,  the  exchange,  being  in  all  longitudes,  would  be 
scarcely,  if  at  all,  perceptible. 

Suppose,  second,  the  earth  be  set  a  rotating:  then  the  currents  pass- 
ing from  either  polar  to  the  equatorial  region  would  be  deflected  more 
and  more  to  the  westward  until,  uniting  at  the  equator,  they  would 
there  form  a  directly  westward  equatorial  current  running  around  the 
earth.  This  westward-moving  water  would  be  constantly  turning  north- 
ward and  southward  in  all  longitudes  as  a  superficial  current,  and  finally 
eastward  about  the  polar  circle,  to  join  again  the  deep-seated  polar  cur- 
rent going  to  the  equator ;  thus  forming  a  series  of  regular  ellipses 
lying  over  each  other  in  strata,  dipping  eastward  and  outcropping 

1  Herschel,  "  Physical  Geography,"  p.  13  ;  and  Croll,  "  Climate  and  Time." 
9  Guyot,  "  Earth  and  Man,"  p.  189. 


AQUEOUS  AGENCIES. 


westward — as  represented  in  Fig.  30.  As  the  north  and  south  currents 
a  a'  and  b  b'  would  take  place  in  all  longitudes,  they  would  be  scarcely, 
if  at  all,  perceptible ;  but  the  east  currents  d  d',  and  the  westward 
equatorial  current  c  c,  where  all  these  unite,  would  be  decided. 

In  the  third  place,  introduce  continents  passing  across  the  equator 
from  north  to  south,  forming  impassable  barriers  to  the  east  and  west 

currents  c  c  and  d  d.  Then 
—  many  of  the  lines  of  current 
aaa  would  be  crowded 
against  the  western  shore 
of  the  ocean,  and  of  the 
lines  bbb  against  the  east- 
ern shore,  forming  in  each 
case  by  concentration  very 
decided  currents,  while  in 
mid-ocean  these  currents 
would  be  still  impercepti- 
ble. Thus  the  perceptible 
situated  between  continents  would  be  repre- 


FIG.  80.— The  strong  lines  aaa  show  superficial, 
dotted  lines  b  b  b  deep-seated  currents. 

currents    of  an   ocean 

sented  by  the  figure  (Fig.  31)  taken  from  Dana. 

Besides  the  main  currents  above  mentioned  there  would  be  mi- 
nor exchanges  with  the  pole  itself.1 
A  portion  of  the  eastward  current 
d  and  d '  would  turn  north  and  south- 
ward, e  e',  and  circling  around  would 
return  toward  the  equator  as  a  deep- 
seated  current  under  «,  hugging  the 
shore  on  account  of  the  westward  ten- 
dency of  all  currents  moving  toward  the 
equator. 

The  effect  of  the  trade-winds  would 
be  to  conspire  with  the  cause  already 
discussed  in  the  formation  of  the  equa- 
torial current  c  c',  and  by  the  reflection 
of  this  from  continents,  the  other  cur- 


90" 


60 


30' 


60° 


DO 


FIG.  81.— Ideal  Diagram,  showing  General    rents  SDoken  of 
Course  of  Oceanic  Currents. 

Application. — We    will  now  apply 

these  principles  in  the  explanation  of  the  currents  of  the  Atlantic 
Ocean,  for  these  are  best  known. 

Currents  coming  from  the  north  and  south  on  the  African  coast,  and 

corresponding  to  b  b'  in  the  above  diagram,  unite  to  form  an  equatorial 

current,  c  c',  which  stretches  across  the  Atlantic  until,  striking  (Fig.  32) 

against  the  coast  of  South  America,  it  turns  north  and  south,  a  a'.     The 

1  Dana's  "  Manual,"  p.  88. 


OCEANIC  CURRENTS. 


39 


southern  branch  has  not  been  accurately  traced.  It  probably  turns 
gradually  eastward,  c?',  and  forming  a  grand  circle  in  the  southern 
Atlantic  joins  again  the  South  African  current  b'.  The  northern  branch, 
#,  runs  along  the  coast  of  South  America,  through  the  Caribbean  Sea 
and  into  the  Gulf  of  Mexico,  from  which  emerging  it  runs  with  great 
velocity  through  the  narrow  straits  of  Florida  and  thence  under  the 
name  of  the  Gulf  Stream  along  the  coast  of  North  America,  turning 


Strc 
glot 


FIG.  32.— General  Course  of  Currents  of  the  Atlantic. 

more  and  more  eastward  in  obedience  to  the  law  already  mentioned, 
until  it  becomes  an  eastward  current,  <?,  about  50°  to  60°  latitude ;  and 
then  stretches  across  to  the  coast  of  Europe,  and  turns  again  southward 
to  join  the  equatorial  current.  A  portion  of  it,  however,  in  its  east- 
ward course  turns  northward,  e,  and  returns  as  a  cold  polar  current  hug- 
ging the  shore  of  North  America  as  a  cold  wall  to  the  Gulf  Stream, 
and  thus  passes  south. 

Geological  Agency  of  Oceanic  Currents.— The  velocity  of  oceanic 
currents  is  generally  small,  although,  in  the  case  of  the  Gulf  Stream,  at 
the  Florida  Straits,  it  reaches  almost  the  velocity  of  a  torrent,  viz., 
three  and  a  half  to  five  miles  per  hour.  The  volume  of  water  carried 
by  them  is  almost  inconceivably  great ;  it  is  estimated  that  the  Gulf 
Stream  alone  carries  many  times  more  water  than  all  the  rivers  of  the 
globe.  According  to  Croll,  it  is  equal  to  a  current  fifty  miles  wide  and 


40  AQUEOUS  AGENCIES. 

one  thousand  feet  deep,  running  at  a  rate  of  four  miles  per  hour.  The 
geological  agency  of  these  powerful  currents  in  modifying  the  bottom 
of  the  sea  by  erosion  may  be,  and  by  sedimentary  deposit  must  be, 
very  important,  though  as  yet  comparatively  little  known. 

One  of  the  chief  functions  of  oceanic  currents  is  the  transportation 
and  distribution  over  the  open-sea  bottom  of  sediments  brought  down 
by  the  rivers.  By  far  the  larger  part  of  the  debris  of  the  land  is  cer- 
tainly dropped  near  the  shore,  and  marginal  sea-bottoms  are  everywhere 
the  great  theatres  of  sedimentation  ;  but,  without  the  agency  of  marine 
currents,  none  would  reach  open  sea,  all  would  be  dropped  near  shore. 
By  the  agency  of  these,  however,  the  finer  portions  are  carried  and 
widely  distributed  over  certain  portions  of  deep-sea  bottoms.  We 
have  undoubted  evidence  of  this  in  some  cases.  Thus  the  sediments 
brought  down  by  the  Amazon  are  swept  seaward  by  a  strong  tide,  and 
then  taken  by  the  oceanic  current  which  sweeps  along  that  coast,  and 
carried  300  miles  and  deposited  much  of  it  on  the  coast  of  Guiana. 
According  to  Humboldt,  the  same  stream  carries  sediment  from  the 
Caribbean  into  the  Gulf  of  Mexico.1  There  is  little  doubt,  too,  that 
much  of  the  sediments  brought  into  the  Gulf  of  Mexico  by  the  Gulf 
rivers  is  swept  along  by  the  Gulf  Stream,  and  a  part  of  it  deposited  on 
Florida  Point  and  the  Bahama  Banks.  The  surface  transparency  of 
the  Gulf  Stream  is  no  objection  to  this  view,  as  a  little  reflection  will 
show.  Ocean-currents  differ  from  rivers,  in  the  fact  that  the  former 
run  in  perfectly  smooth  beds  of  still  water.  There  are,  therefore,  no 
subordinate  currents  from  side  to  side,  or  up  and  down,  whereby  in  river- 
currents  the  water  is  thoroughly  mixed  up,  and  the  finer  sediments 
prevented  from  settling.  In  ocean-currents  the  conditions  are  as  favor- 
able for  subsidence  as  in  still  water.  It  is  evident,  therefore,  that  sedi- 
ments carried  by  ocean-currents  must  in  a  little  time  sink  out  of  sight, 
although  from  the  great  depth  of  these  currents  they  mav  still  be  car- 
ried to  considerable  distances.  Deep-sea  deposits  have  until  recently 
received  little  attention,  although  they  are  acknowledged  to  be  of  the 
greatest  geological  importance. 

Submarine  Banks.— These  are  always  accumulations  of  material 
dropped  by  currents.  They  are  formed  under  conditions  similar  to 
those  which  determine  the  formation  of  bars;  i.  e.,  either  by  the  meet- 
ing of  opposing  sediment-laden  currents  or  else  bv  such  a  current  coming 
in  contact  with  still  water.  In  fact,  the  outer  bar  is.  a  true  submarine 
bank.  The  currents  may  be  either  tidal  or  oceanic  or  river.  Admira- 
ble examples  of  both  these  modes  of  formation  are  found  in  the  Ger- 
man Ocean.  The  tidal  wave  from  the  Atlantic  strikes  the  British  Isles, 
passes  round  in  both  directions,  and  enters  this  ocean  from  the  north 
around  the  north  point  of  Scotland,  and  from  the  south  through  the 
1  Lycll's  "  Principles  of  Geology." 


TIDES  AND   CURRENTS. 


41 


British  Channel  and  Straits  of  Dover  (Fig.  33).  These  two  currents 
coming  from  opposite  directions  meet  and  make  still  water,  and  there- 
fore deposit  their  sediment  and  form  banks.  Again,  the  tidal  current 
is  concentrated  in  the  British  Channel,  and  runs  with  great  velocity, 
scouring  out  this  channel,  and  in  addition  gathering  abundant  sediment 
from  the  rivers  emptying  into  the  channel.  Thus  loaded  with  sedi- 


FIG.  33.— Tides  of  the  German  Ocean. 


ment  it  rushes  through  the  narrow  Straits  of  Dover,  and,  coming  in 
contact  with  the  still  water  of  the  German  Sea,  forms  eddies  on  either 
side,  and  deposits  its  sediments.  Besides  the  banks  thus  formed,  there 
are,  of  course,  bars  formed  at  the  mouths  of  the  rivers  emptying  into 
this  shallow  sea.  By  a  combination  of  all  these  causes,  we  explain  the 
numerous  banks  which  render  the  navigation  of  this  sea  so  dangerous. 

But  great  banks  far  away  from  shore  are  usually  formed  by  oceanic 
currents.  Thus  the  Banks  of  Newfoundland  are  evidently  formed  by 
the  meeting  of  the  polar  current  (e,  Fig.  32),  bearing  icebergs  loaded 
with  earth,  and  the  warm  current  of  the  Gulf  Stream,  perhaps  also  bear- 
ing its  share  of  fine  sediment.  Again,  the  Gulf  Stream,  rushing  at 


42 


AQUEOUS  AGENCIES. 


high  velocity  (four  miles  per  hour)  through  the  narrow  Straits  of  Florida, 
coming  in  contact  with  the  still  water  of  the  Atlantic  beyond  and  form- 
ing eddies  on  each  side,  and  depositing  sediment,  has  certainly  con- 
tributed to  form,  if  it  has  not  wholly  formed,  the  Bahama  Banks  on  one 
side,  and  the  bank  on  which  the  Florida  reefs  are  built  on  the  other. 
It  is  probable  that  many  other  peculiarities  of  the  Atlantic  bottom  in 
the  course  of  the  Gulf  Stream  may  be  similarly  accounted  for.1 

Land  formed  by  Ocean  Agencies. — Upon  submarine  banks,  however 
these  may  be  produced,  are  gradually  formed  islands.  These  islands 
are  always  formed  by  the  immediate  agency  of  waves.  As  soon  as  the 
submarine  bank  rises  so  near  the  surface  that  the  waves  touch  bottom 
and  form  breakers,  these  commence  to  throw  up  the  sand  or  mud  until  an 
island  is  formed,  which  continues  to  grow  by  the  same  agency,  until  it 
becomes  inhabited  by  plants  and  animals,  and  finally  by  man.  The 

height  of  such  islands  above  the  sea 
will  depend  upon  the  height  of  the 
tides  and  the  force  of  the  waves. 
They  are  seldom  more  than  fifteen 
feet  above  high  water.  Thus,  we 
find  that  extensive  banks  are  always 
dotted  over  with  islands.  In  this 
manner  are  formed  the  low  islands  so 
common  about  the  mouths  of  har- 
bors and  estuaries,  also  the  narrow 
sand-spits  all  along  our  Southern 
coast,  separating  the  harbors  and 
sounds  from  the  ocean.  Fig.  34, 
which  is  a  map  of  the  North  Caro- 
lina coast,  will  give  a  good  idea  of 
these  sand-spits.  In  the  course  of 
time  such  sounds,  being  protected 
in  some  measure  by  the  sand-spits 
from  the  scouring  action  of  the 
tides,  are  gradually  filled  up  with 
sediments  brought  down  by  the 
rivers,  leaving  only  narrow  passages 
for  the  flow  of  the  tide.  In  this 
manner  were  formed  the  sea-islands 
all  along  our  Southern  coast,  separated  from  the  mainland  only  by 
narrow  tidal  inlets.  These  tidal  inlets  may  become  filled  up,  and  the 
whole  coast-line  transferred  farther  seaward. 

A  large  portion  of  the  coasts  of  the  world  is  thus  bordered  by  wave- 


FIG.  84. — Coast  of  North  Carolina. 


1  See  the  author's  views  on  this  subject,  American  Journc 
46,  1857. 


of  Science,  vol.  xxiii.,  p. 


GLACIERS.  43 

formed  islands.  We  have  already  seen,  however,  that  on  some  coasts, 
e.  g\,  Norway,  Scotland,  etc.,  islands  are  formed  by  the  destructive  action 
of  waves.  Bordering  islands,  so  common  along  all  coasts,  are  there- 
fore of  two  classes,  and  formed  by  two  opposite  effects  of  waves — the 
one  land-destroying,  the  other  land-forming.  The  islands  of  one  class 
are  high  and  rocky,  of  the  other  low  and  sandy  or  muddy ;  the  former 
are  the  scattered  remains  of  an  old  coast -line,  the  latter  the  commencing 
points  of  a  new  coast-line. 

SECTION  3. — ICE. 

The  agency  of  ice  will  be  considered  under  the  heads  of  Glaciers  and 
Icebergs;  the  effects  of  frost  in  disintegrating  rocks  having  been  already 
treated  of  under  Atmospheric  Agencies.  It  is  only  comparatively  re- 
cently that  the  great  importance  of  ice  as  a  geological  agent  has  been 
recognized.  To  Agassiz  is  due  the  credit  of  having  first  fully  recognized 
this  importance. 

Glaciers. 

Definition. — In  many  parts  of  the  earth,  where  the  mountains  reach 
into  the  region  of  perpetual  snow,  and  other  favoring  conditions  are 
present,  we  find  that  the  mountain-valleys  are  occupied  by  masses  of 
compact  ice,  connected  with  the  snow-cap  above,  but  extending  far 
below  the  snow-line  into  the  region  of  cultivated  fields,  and  moving 
slowly  but  constantly  down  the  slope  of  the  valley.  Such  valtey-pro- 
longations  of  the  perpetual  snow-caps  are  called  glaciers.  The  exist- 
ence of  glaciers,  and  their  motion,  is  necessitated  by  the  great  law  of 
circulation,  so  universal  in  Nature.  For  in  those  countries  where  gla- 
ciers exist,  the  waste  of  perpetual  snow  by  evaporation  is  small  in  com- 
parison with  the  supply  by  the  fall  of  snow.  There  would  be  no  limit, 
therefore,  to  the  accumulation  of  snow  on  mountain-tops,  if  it  did  not 
run  off,  down  the  slope,  by  these  ice-streams,  and  thus  return  into  the 
general  circulation  of  meteoric  waters.  Glaciers  extend  not  only  far 
below  the  snow-line,  but  even  far  below  the  mean  line  of  32°.  In  the 
Alps  the  snow-line  is  about  9,000  *  feet  above  the  sea-level,  while  some 
of  the  glaciers  extend  down  to  within  3,400  feet  of  the  same  level,  i.  e., 
more  than  5,000  feet  below  the  snow-line. 

Necessary  Conditions. — The  conditions  necessary  to  the  formation 
of  glaciers  are :  1.  The  mountain  must  rise  into  the  region  of  perpetual 
snow,  for  the  snow-cap  is  the  fountain  of  glaciers.  2.  There  must  be 
considerable  changes  of  temperature,  and  therefore  alternate  tha wings 
and  freezings.  This  condition  seems  necessary  to  the  gradual  compact- 
ing of  snow  into  glacier-ice.  The  want  of  this  condition  is  apparently 
the  cause  of  the  non-existence,  or  small .  development,  of  glaciers  in 
1  Dana's  "  Manual  of  Geology." 


44  AQUEOUS  AGENCIES. 

tropical  regions.  3.  A  moist  atmosphere  is  favorable  to  their  produc- 
tion, for  the  moister  the  climate  the  greater  is  the  snow-fall,  and  the 
smaller  is  the  waste  by  evaporation,  and  therefore  the  greater  the 
excess  which  must  run  off  by  glaciers.  This  is  an  additional  reason 
why  glaciers  are  not  formed  under  the  equator  ;  for  the  great  capacity 
for  moisture  of  the  air  in  this  zone  increases  the  waste  while  it  decreases 
the  fall  of  snow.  This  is  also  the  reason  of  the  scanty  formation  of  gla- 
ciers in  the  Sierra  Mountains,  and  their  abundance  and  magnitude  in 
the  Alps. 

Ramifications  of  Glaciers. — We  have  said  glaciers  are  valley-prolon- 
gations of  the  ice-cap.  Now,  mountain-valleys  are  of  two  kinds,  viz.: 
1.  The  deeper  and  larger  longitudinal  valleys,  between  parallel  ranges; 
and,  2.  The  transverse  or  radiating  valleys,  transverse  in  case  of  ridges, 
and  radiating  in  case  of  peaks.  The  longitudinal  valleys  may  be  formed 
either  by  erosion  or  by  igneous  agencies  folding  the  crust  of  the  earth  ; 
but  the  transverse  or  radiating  valleys  are  always  formed  by  erosion. 
It  is  these  valleys  of  erosion  which  are  occupied  by  glaciers.  In  coun- 
tries where  there  are  no  glaciers  they  are  occupied,  of  course,  by 
streams.  We  have  already  shown  (p.  9)  how  these  valleys  commence 
near  the  top  of  the  mountain  as  furrows,  which,  uniting,  form  gullies, 
and  these,  in  their  turn,  forming  ravines  and  gorges,  thus  becoming  less 
and  less  numerous,  but  larger  as  we  approach  the  base  of  the  mountain. 
In  the  same  manner,  therefore,  as  streams  ramify,  so  also  do  glaciers. 
The  only  difference  is  the  degree  of  ramification.  Streams  ramify 
almost  infinitely,  while  glaciers  seldom  have  more  than  three  or  four 
tributaries.  Fig.  35  is  a  map  of  the  Mont  Blanc  glacier-region.  By 
inspection  of  this  map  it  will  be  seen  that  the  Merde  Glace,  m,  receives 
four  tributaries,  marked  ta,  I,  g,  etc.  On  page  51  is  an  enlarged  view 
of  the  same  glacier,  with  its  tributaries. 

Motion  of  Glaciers. — Again,  we  have  said  in  our  definition  that  gla- 
ciers are  in  constant  motion.  By  the  law  of  circulation,  constant  down- 
ward motion  is  as  necessary  to  the  idea  of  a  glacier  as  it  is  to  that  of  a 
river,  since  both  the  glacier  and  the  river  carry  away  the  excess  of  sup- 
ply over  evaporation.  But  a  glacier,  though  in  constant  motion,  never 
passes  beyond  a  certain  point,  where  the  slow  downward  motion  is^ 
exactly  balanced  by  the  melting  of  the  ice  by  sun  and  air.  This  point 
is  called  the  lower  limit  of  the  glacier.  As  long  as  conditions  remain 
unchanged,  the  lower  end  of  the  glacier  remains  exactly  at  the  same 
point,  although  the  substance  of  the  glacier  moves  always  downward. 
But  if  external  conditions  change,  the  point  of  the  glacier  may  move 
upward  or  downward.  Thus,  during  a  succession  of  cool,  damp  years, 
the  melting  being  less  rapid,  the  point  of  the  glacier  moves  slowly  down, 
sometimes  invading  cultivated  fields  and  overturning  huts,  until  it  finds 
a  new  point  of  equilibrium.  During  a  succession  of  warm  and  dry  years, 


GLACIERS. 


46 


AQUEOUS  AGENCIES. 


on  the  contrary,  the  melting  being  more  rapid,  the  point  retreats,  to 
find  a  new  point  of  equilibrium  higher  up  the  mountain.  But,  whether 
the  point  be  stationary,  or  advance  or  recede,  the  substance  of  the  gla- 
cier is  ever  moving  steadily  onward.  It  may  be  compared  to  those 
rivers,  in  dry,  sandy  countries,  which  run  ever  toward  the  sea,  but  never 
reach  beyond  a  certain  point,  being  absorbed  by  the  sand. 

Graphic  Illustration. — These  facts  may  be  conveniently  represented 
as  follows :  Let  a  d  (Fig.  36)  equal  the  length  of  the  mountain-slope, 
and  the  line  a  b  (=  c  d)  the  velocity  of  the 
a.  glacier-motion  taken  as  uniform.  This  velocity 
varies  with  the  slope,  as  will  be  seen  hereafter, 
but  is  little  affected  by  the  elevation.  It  may 
be  taken,  therefore,  as  the  same  in  every  part 
of  the  slope,  and  therefore  correctly  repre- 
sented by  equal  lines,  i.  e.,  by  the  ordinates  of 
the  parallelogram  abed.  The  melting  power 
of  the  sun  and  air,  on  the  contrary,  regularly 
increases  from  the  top,  where  it  is  almost  noth- 
ing, to  the  bottom  of  the  mountain.  We  will, 
therefore,  represent  it  by  the  increasing  ordi- 
nates  of  the  triangle  a  e  d.  At  a?,  therefore, 
where  the  ordinates  of  the  triangle  and  of  the 
parallelogram  are  equal  to  each  other,  will  be 
the  lower  limit  of  the  glacier.  During  a  suc- 
cession of  cool  years  the  rate  of  melting  will 
be  represented  by  the  ordinates  of  the  smaller 
triangle  a  g  d,  and  the  point  of  the  glacier  will  advance  to  z.  During 
a  succession  of  warm,  dry  years,  the  rate  of  melting  will  be  represented 
by  the  larger  triangle  afd,  and  the  point  of  the  glacier  will  recede  to  y. 
Line  of  the  Lower  Limit  of  Glaciers. — We  have  said,  again,  that 'the 
glacier  reaches  below  the  snow-line.  There  are  three  lines,  or  rather 
spheroidal  surfaces,  running  above  the  surface  of  the  earth,  which  are 
apt  to  be  confounded  with  one  another,  and  must,  therefore,  be  now 
defined.  These  are  the  line  of  perpetual  snow,  the  mean  line  of  32°, 
and  the  line  of  the  lower  limit  of  glaciers.  The  line  of  perpetual  snow, 
at  the  equator,  is  about  16,000  to  17,000  feet  above  the  sea-level.  As 
we  approach  the  poles  it  gradually  approaches  the  sea-level,  until  it 
touches  at  or  near  the  poles,  forming  thus  a  spheroid  more  oblate  than 
the  earth  itself  (Fig.  37).  Next  follows  the  mean  line  of  32°.  This 
commences  at  the  equator,  E,  coincident  with  the  snow-line  (it  may 
be  even  above  it — Dana),  but  diverges  as  we  pass  toward  the  pole, 
and  finally  touches  the  sea-level  at  about  66°  north  and  south  lati- 
tude, at  b  b.  Below  this,  again,  is  the  line  of  lower  limit  of  glaciers, 
which,  commencing  again  nearly  coincident  with  the  two  preceding,  at 


u      c 
FIG.  86. 


GLACIERS. 


47 


the  equator,  approaches  and  touches  the  sea-level  at  about  50°  latitude, 
or,  under  favorable  circumstances,  at  even  lower  latitudes.  The  differ- 
ence between  these  lines  is  often  several  thousand  feet.  In  the  Alps, 
the  line  of  32°  is  2,000  feet,  and  the  line  of  lower  limit  of  glaciers  5,000 
feet,  below  the  snow- 
line.  In  some  parts 
of  the  arctic  region, 
the  line  of  32°  is  3,500 
feet  below  the  snow- 
line,  and  in  Norway 
the  lower  limit  of  gla- 
ciers is  4,000  feet  be- 
low the  line  of  32° 
(Dana).  For  the  sake 
of  simplicity  we  have 
represented  the  sur- 
faces, of  which  these 
lines  are  the  sections, 
as  regular  spheroids ; 
but,  in  fact,  they  are 
very  irregular,  being 
much  influenced  by 
climate.  Their  inter- 
section with  the  sea- 
level  will,  therefore,  not  be  along  lines  of  latitude,  but  will  be  irregular, 
like  isotherms.  As  the  line  a  c  marks  the  lower  limit  of  glaciers  in 
different  latitudes,  it  is  evident  that  at  c  glaciers  will  touch  the  sea,  and 
beyond  this  point  will  run  far  into  the  sea.  It  is  in  this  manner,  as  we 
will  see  hereafter,  that  icebergs  are  formed.  In  Chili,  glaciers  touch 
the  sea-level  at  46°  40'  south  latitude.1 

General  Description. — In  glacial  regions  a  mountain-valley  is  occu- 
pied in  its  highest  part  by  perpetual  snow;  below  this,  farther  down 
the  valley,  by  neve — a  granular  snow,  intermediate  between  snow  and 
ice ;  still  farther  down,  by  true  glacier-ice ;  and,  finally,  by  a  river 
(Fig.  41).  This  river  is  formed  partly  by  the  melting  of  the  whole 
surface  of  the  glacier,  both  above  and  below,  and  partly  by  the  natural 
drainage  of  the  valley.  The  glacier,  however,  is  the  principal  source. 
From  the  point  of  every  glacier,  therefore,  runs  a  river. 

The  size  of  glaciers  varies  very  much.  Alpine  glaciers  are  some  of 
them  fifteen  miles  long,  and  vary  from  half  a  mile  to  three  miles  in 
breadth,  and  from  one  hundred  to  six  hundred  feet  in  thickness.  In 
the  region  about  Mont  Blanc  and  Finsteraarhorn  alone  there  are  about 
four  hundred  glaciers.  In  the  temperate  regions  of  North  America,  gla- 
1  D'Archiac,  "  Histoire  de  Geologic." 


FIG.  3T.-General  Relation  of  Limit  of  Glaciers  to  Snow-Line. 


4$  AQUEOUS  AGENCIES. 

ciers  are  found  only  on  the  Pacific  coast,  in  the  Sierra  and  Cascade 
Ranges.  On  Mount  Shasta,  and  'especially  on  Mount  Rainier,  glaciers 
equal  to  those  of  the  Alps  have  been  recently  found.  In  the  Himalaya 
Mountains  they  are  developed  upon  a  much  more  gigantic  scale  ;  but  it 
is  only  in  arctic  regions  that  we  can  form  any  just  conception  of  their 
immense  importance  as  geological  agents.  In  Spitzbergen,  a  glacier 
was  seen  eleven  miles  wide  and  four  hundred  feet  thick  at  the  point.1 
Of  course,  this  thickness  only  represents  the  part  above  water.  By  far 
the  larger  part,  or  six-sevenths,  is  below  water-level.  In  Greenland  the 
great  Humboldt  Glacier  enters  the  sea  with  a  point  forty-five  miles  wide 
and  three  hundred  feet  thick  (Kane).  But  even  these  examples  give 
an  incomplete  idea  of  the  whole  truth.  Greenland  is  apparently  en- 
tirely covered  with  an  immense  sheet  of  'ice •,  several  thousand  feet  thick, 
which  moves  slowly  seaward,  and  enters  the  ocean  through  immense 
fiords.8  Judging  from  the  immense  barrier  of  icebergs  found  by  Cap- 
tain Wilkes  (United  States  Exploring  Expedition)  on  its  coast,  the  an- 
tarctic continent  is  probably  even  more  thickly  covered  with  ice  than 
Greenland. 

We  are  apt  to  suppose  that  the  surface  of  a  glacier  must  be  smooth. 
This  is,  however,  very  far  from  being  true.     On  the  contrary,  the  ex- 
treme roughness  of  the  ice-surface  renders  the  ascent  along  the  glacier 
extremely  difficult.     This  inequality  of  surface  is  due  partly  to  unequal 
melting,  and  partly  to  crevasses,  or  fissures.     The  unequal  melting  is 
produced  as  follows :  A  stone,  lying  on  the  surface  of  a  glacier,  pro- 
tects the  surface  beneath  from  the  rays  of  the  sun.     Meanwhile  the 
surrounding  ice  is  melted,  until  finally  the  slab 
of  stone  stands  on  a  column  of  ice  often  several 
feet  in  height  (Fig.  38).     A  slab  seen  by  Forbes 
measured  23  feet  long,  17  feet  wide,  and  3£  feet 
thick,  and  rested  on  a  column  13  feet  high.  In  such 
/        \  cases  the  stone  finally  falls  off,  leaving  a  sharp 

___--<^  pinnacle,  and  another  column  commences  to  form 

FIG.  88.— Mod^of^Formaaon  under  the  stone.     In  this  manner  are  formed  what 
are  called  needles.     When  we  consider  that  there 

are  immense  numbers  of  stones  on  the  glacier-surface,  we  can  easily  see 
that  these  needles  will  multiply  indefinitely.  If,  on  the  other  hand,  a 
thin  stratum  of  earth  stains  the  surface  of  the  glacier  in  spots,  these 
spots  will  melt  faster  than  the  surrounding  ice,  because  more  absorb- 
ent of  heat,  and  thus  form  deep  holes. 

Again,  fissures  or  crevasses,  often  of  great  size,  ten  to  twenty  feet 
wide,  one  hundred  feet  deep,  and  sometimes  running  entirely  across  the 
glacier,  are  very  abundant.     As  the  surface  of  the  glacier  is  often  cov- 
ered with  snow,  and  the  fissures  thus  concealed,  they  form  the  most 
1  Dana's  "  Manual."  2  Dr.  Rink,  "  Archives  des  Sciences,"  vol.  xxvii.,  p.  155. 


GLACIERS.  49 

dangerous  feature  connected  with  Alpine  travel.  The  law  which  gov- 
erns their  formation  will  be  discussed  hereafter ;  suffice  it  to  say  that 
the  great  transverse  fissures  are  formed  by  the  glacier  passing  over  an 
angle  formed  by  a  sudden  change  in  the  slope  of  the  bed.  Streams, 
produced  by  the  melting  of  ice,  running  on  the  surface  of  the  glacier, 
plunge  into  these  fissures  with  a  thundering  noise,  and  hollow  out  im- 
mense wells,  called  moulins,  and  magnificent  ice-caves.  Although  the 


FIG.  39.— Inequalities  of  the  Surface  of  a  Glacier  (after  Agassiz). 


glacier  moves,  the  great  crevasses  and  the  wells  with  their  falls  remain 
stationary,  precisely  as  the  position  of  a  rapid  or  breaker  remains  sta- 
tionary, although  the  river  runs  onward  ;  and  for  the  same  reason,  viz., 
that  it  is  reformed  alwaj^s  on  the  same  spot. 

From  all  these  causes  the  surface  of  a  glacier  is  often  studded  over 
with  conical  masses  and  projecting  points  of  every  conceivable  shape. 
This  is  well  shown  in  the  accompanying  figure  (Fig.  39). 

Earth  and  Stones,  etc. — The  surface  of  a  glacier  is,  moreover, 
largely  covered  with  earth  and  stones  gathered  in  its  course  from  the 
crumbling  cliffs  on  either  side.  These  are  often  so  abundant  as  almost 
to  cover  the  surface.  More  usually,  however,  they  are  distributed  in 
two  or  more  rows,  called  moraines.  Fig.  40  is  a  view  of  a  glacier,  with 
its  moraines  and  lateral  crevasses. 

Such  is  a  general  description  of  the  appearance  of  a  glacier.  There 
are,  however,  several  points  which,  by  their  importance  and  interest, 
require  special  notice.  These  are  :  1.  Moraines ;  2.  Glaciers  as  a  geo- 
logical agent ;  3.  Glacier-motion ;  and,  4.  Glacier-structure. 


50 


AQUEOUS  AGENCIES. 


FIG.  40.— Zermatt  Glacier  (Agassiz). 

Moraines. 

There  are  three  kinds  of  moraines  described  by  writers,  viz.,  lateral 
moraines,  medial  moraines,  and  terminal  moraines.  Lateral  moraines 
are  continuous  lines  of  earth  and  stones,  arranged  on  either  margin  of 
the  glacier,  and  evidently  formed  from  the  ruins  of  the  crumbling  cliffs 
of  the  inclosing  valley.  This  debris  does  not  fall  from  every  part  of 
the  valley-sides,  but  generally  only  from  certain  bold,  projecting  cliffs. 
It  is  converted  into  a  continuous  line  by  the  motion  of  the  glacier,  just 
as  light  materials  thrown  constantly  into  a  river  at  one  poirit  would 
appear  as  a  continuous  line  on  the  stream. 

Medial  moraines  are  similar  lines  of  debris,  occupying  the  central 
portions  of  the  glacier.  Sometimes  there  is  but  one ;  sometimes  two,  or 
more ;  sometimes  the  whole  surface  of  the  glacier  is  almost  covered  with 
them.  The  true  explanation  was  first  pointed  out  by  Agassiz.  They 
are  formed  by  the  coalescence  of  the  interior  lateral  moraines  of  tribu- 
tary glaciers,  carried  down  the  main  trunk  by  the  motion  of  the  ice- 
current.  The  accompanying  map  (Fig.  41)  of  the  Mer  de  Glace  and 
its  tributaries  shows  clearly  the  manner  in  which  these  moraines  are 
formed.  Both  lateral  and  medial  moraines  are  generally  situated  on  a 
ridge  of  ice,  sometimes  fifty  to  eighty  feet  high,  evidently  formed  by 
the  protection  of  the  ice,  in  this  part,  from  the  melting-power  of  the 


GLACIERS  AS  A  GEOLOGICAL  AGENT. 


51 


sun.     The  fragments  of  rock  brought  down  by  glaciers  are  often  of 
enormous  size.     One  described  by  Forbes  contained  244,000  cubic  feet. 

Everything  which  falls  upon 
the  surface  of  the  glacier  is 
slowly  and  silently  carried  down- 
ward by  this  ice-stream,  and 
finally  dropped  at  its  point. 
Much  finely-triturated  matter  is 
also  pushed  along  beneath  the 
glacier,  and  finds  its  way  to 
the  same  point.  In  the  course 
of  time  an  immense  accumula- 
tion is  formed,  of  somewhat 
crescentic  shape,  as  seen  in 
Fig.  41. 

This  accumulation  is  called 
the  terminal  moraine.  It  is  the 
delta  of  this  ice-river.  The  ex- 
istence of  moraines  is  a  con- 
stant witness  of  the  motion  of 
the  glaciers. 

Glaciers  as  a  Geological  Agent. 
Glaciers,  like  rivers,  erode  the 
surface  over  which  they  move, 
carry  the  materials  gathered  in 
their  course  often  to  great  dis- 
tances, and  finally  deposit  them. 
In  all  these  respects,  however,  the  effects  of  their  action  are  perfectly 
characteristic. 

Erosion. — When  we  consider  the  weight  of  a  glaciers  and  their  un- 
yielding nature  as  compared  with  water,  it  is  easy  to  see  that  their 
erosive  power  must  be  very  great.  This  is  increased  immensely  by 
fragments  of  stone  of  every  conceivable  size  carried  along  between  the 
glacier  and  its  bed.  These  partly  fall  in  at  the  sides  and  become 
jammed  between  the  glacier  and  the  confining  rocks,  partly  fall  into 
crevasses  and  work  their  way  to  the  bed,  and  partly  are  torn  from  the 
rocky  bed  itself.  The  effects  of  glacier  erosion  differ  entirely  from 
those  of  water :  1.  Water,  by  virtue  of  its  perfect  fluidity,  wears  away 
the  softer  spots  of  rock  and  leaves  the  harder  standing  in  relief ;  while 
a  glacier,  like  an  unyielding  rubber,  grinds  both  hard  and  soft  to  one 
level.  This,  however,  is  not  so  absolutely  true  of  glaciers  as  might 
be  supposed.  Glaciers,  for  reasons  to  be  discussed  hereafter,  conform 
to  large  and  gentle  inequalities  of  their  beds,  though  not  to  small  ones, 


FIG.  41.— Mer  de  Glace. 


52  AQUEOUS  AGENCIES. 

acting  thus  like  a  very  stiffly  viscous  body.  Thus  their  beds  are  worn 
into  very  remarkable  and  characteristic  smooth  and  rounded  depressions 
and  elevations  called  roches  moutonnees  (Fig.  42).  Sometimes  large 
and  deep  hollows  are  swept  out  by  a  glacier  at  some  point  where  the 


FIG.  42.— Eoches  Moutonnees  of  an  Ancient  Glacier,  Colorado  (after  Hayden). 

rock  is  softer  or  where  the  slope  of  the  bed  changes  suddenly  from  a 
greater  to  a  less  angle.  If  the  glacier  should  subsequently  retire,  water 
accumulates  in  these  excavations  and  forms  lakelets.  Such  lakelets  are 
common  in  old  glacial  beds. 

2.  The  lines  produced  by  water-erosion,  if  detectible  at  all,  are 
always  more  or  less  irregular  and  meandering ;  while  those  produced  by 
glaciers  are  straight  and  parallel  (Fig.  43). 

Thus,  smooth,  gently-billowy  surfaces,  marked  with  straight  parallel 
scratches,  are  very  characteristic  of  glacial  action.  We  will  call  such 
surfaces  glaciated,  and  the  process  glaciation. 

Transportation. — The  transporting  power  of  glaciers  follows  no  law 
similar  to  that  pointed  out  under  rivers — in  fact,  it  has  no  relation  at  all 
to  velocity.  The  reason  is,  that  the  stone  rests  on  the  surface  as  afloat- 
ing  body.  There  is,  therefore,  no  limit  to  the  transporting  power. 
Bowlders  of  250,000  cubic  feet  are  carried  with  the  same  ease  and  the 
same  velocity  as  the  finest  dust. 

Deposit — Balanced  Stones. — A  water-current  carrying  stones  bruises 
and  rounds  their  corners,  and  deposits  them  always  in  the  most  secure 
positions ;  but  glaciers  often  deposit  huge  angular  fragments  of  rock 


GLACIERS   AS  A   GEOLOGICAL  AGENT.  53 

in  the  most  insecure  positions — so  nicely  balanced,  sometimes,  that  a 
touch  of  the  hand  will  dislodge  them.  The  reason  is,  they  are  set 
down  by  the  gradually  melting  ice  with  inconceivable  gentleness.  Thus 
balanced  stones,  rocking-stones,  etc.,  are  common  in  glacial  regions. 
In  using  these  as  a  sign  of  glacial  action,  however,  we  must  recollect 


FIG.  43.— Glacial  Scorings  (after  Agassiz). 


that  a  bowlder  dropped  by  any  agent,  or  even  a  bowlder  of  disintegra- 
tion (p.  6),  may  in  time  become  a  rocking-stone,  by  slow  but  irregular 
disintegration  changing  the  position  of  the  centre  of  gravity.  But  angu- 
lar erratics  in  insecure  positions  are  very  characteristic  of  glacial  action. 

Material  of  the  Terminal  Moraine. — The  material  of  the  terminal 
moraine  is  very  characteristic  :  1.  It  consists  of  fragments  of  every  con- 
ceivable size,  from  huge  bowlders  down  to  fine  earth,  mixed  together 
into  an  heterogeneous  mass  entirely  different  from  the  neatly-sorted  de- 
posits from  water.  It  is,  therefore,  entirely  unsorted  and  unstratified, 
and  without  organic  remains.  2.  The  mass  consists  of  two  parts,  viz.,  that 
which  was  carried  on  the  top  of  the  glacier,  and  that  which  was  forced 
out  beneath  (moraine  profonde).  The  first  consists  of  loose  material 
containing  angular,  unworn  fragments ;  the  other  of  fine  compact  mate- 
rial containing  fragments  worn  and  polished,  and  scratched  with 
straight  parallel  scratches,  but  in  both  cases  entirely  different  from 
water-worn  pebbles.  In  all  respects,  therefore,  the  action  of  glaciers  is 
characteristic  and  cannot  be  confounded  with  that  of  water. 

Evidences  of  Former  Extension  of  Glaciers. — It  is  by  evidence  of 

this  kind  that  the  former  great  extension  of  glaciers  in  regions  where 
they  now  exist,  and  the  former  existence  of  glaciers  in  regions  where 
they  no  longer  exist,  have  been  proved.  We  have  already  stated  that 
during  a  succession  of  cool,  damp  seasons,  a  glacier  may  extend  far 


54 


AQUEOUS  AGENCIES. 


FIG.  44.— Section  across  Glacial  Valley,  showing 
old  Lateral  Moraines. 


beyond  its  previous  limits.  Similar  changes  take  place  also  in  the 
depth  of  a  glacier.  In  a  word,  glaciers  are  subject  to  floods  like  rivers ; 
only  these  floods,  instead  of  being  annual,  are  secular.  Now,  as  rivers 
after  floods  leave  floating  material  stranded  on  the  banks,  showing  the 

height  of  the  flood-water,  so,  in 

the  decrease  of  a  glacier,  lines 
of  bowlders  are  left  stranded, 
often  delicately  balanced,  on 
ledges  high  up  the  sides  of  the 
valley.  These 
lines  of  bowlders 
mark  the  former 
height  of  the  gla- 
cier. Some  of  these  lines  have  been  found  in  the  Alps 
2,000  feet  above  the  present  level.  Fig.  44  is  a  cross-sec- 
tion of  a  glacial  valley.  The  dotted  lines  show  the  for- 
mer level.  In  the  same  valleys  we  find  old  terminal  mo- 
raines (Fig.  45,  «')  miles  beyond  the  present  limit  of  the 
glacier.  The  characteristic  planing,  polishing,  and  par- 
allel scoring,  have  been  found  equally  far  above  the 
present  level  and  beyond  the  present  limit  of  Alpine 
glaciers. 

Glacial  Lakes. — When  a  glacier  retreats,  the  water  of 
the  river  which  flows  from  its  point  may  accumulate  in 
great  rock-basins  scooped  out  by  the  glacier,  or  else  be- 
hind the  old  terminal  moraines.  In  these  two  ways  lakes 
are  often  formed. 


Motion  of  Glaciers  and  its  Laws. 

Evidences  of  Motion. — That  glaciers  move  slowly  down  their  valleys 
was  long  known  to  Alpine  hunters.  Rude  experiments  of  the  first  scien- 
tific explorers,  confirmed  this  popular  notion.  Hugi  in  1827  built  a  hut 
upon  the  Aar  glacier.  This  hut  was  visited  from  year  to  year  by  scien- 
tific explorers  and  its  change  of  position  measured.  In  1841  Agassiz 
found  that  it  had  moved  in  all  1,428  metres  in  fourteen  years,  or  about 
100  metres  (330  feet)  per  annum.  Numerous  other  observations  from 
year  to  year  by  Agassiz  and  others,  on  the  position  of  conspicuous  bowl- 
ders lying  on  the  surface  of  glaciers,  confirmed  these  results  and  placed 
the  fact  of  glacier-motion  beyond  doubt.  But  the  most  important  obser- 
vations determining  both  the  rate  and  the  laws  of  glacier-motion  were 
made  in  1842  by  Prof.  Agassiz  on  the  Aar  glacier,  and  Prof.  Forbes  on 
the  Mer  de  Glace.  By  these  experiments,  carefully  made  by  driving 
stakes  into  the  glacier,  in  a  straight  row  from  one  side  to  the  other,  and 
observing  the  change  in  the  relative  position  of  the  stakes,  it  was  deter- 


<L'  T,    c  rif 


MOTION   OF   GLACIERS   AND   ITS   LAWS.  55 

mined  that  the  centre  of  the  glacier  moved  faster  than  the  margins. 
This  differential  motion  is  the  capital  discovery  in  relation  to  the  motion 
of  glaciers.  It  is  claimed  by  both  Agassiz  and  Forbes.  It  had,  how- 
ever, been  previously  distinctly  stated,  though  not  proved,  by  Bishop 
Rendu. 

Laws  Of  Glacier-Motion. — The  term  differential  motion  is  a  con- 
densed expression  for  all  the  laws  of  glacier-motion.  It  asserts  that 
the  different  parts  of  a  glacier  do  not  move  together  as  a  solid,  but 
move  among  themselves  in  the  manner  of  a  fluid.  A  glacier  moves 
like  a  fluid,  though  a  very  stiff,  viscous  fluid ;  its  motion  may  there- 
fore be  rightly  called  viscoid.  We  will  mention  some  of  the  most  im- 
portant laws  of  fluid  motion,  and  show  that  glaciers  conform  to  them. 

1.  The  Velocity  of  the  Central  Parts  is  greater 
than  that  of  the  Margins. — This  well-known  law  of 
currents,  the  result  of  friction  of  the  fluid  against 
the  containing  banks,  was  completely  proved  in  the 

case  of  glaciers  by  the  experiments  of  Agassiz  and      ^*\^--o—v-<>-'^''/ 

Forbes,  and  recently  confirmed  in  the  most  perfect     a/ 

manner  by  Tyndall.    A  line  of  stakes,  a  b  c  d  efg, 

placed  in  a  straight  row  across  a  glacier,  becomes 

every  day  more  and  more  curved,  as  seen  in  Fig. 

46.      The   exact  rate  of  motion   for  each    stake 

is  easily  measured  by  the   theodolite.      The  rate 

of  the  centre  is  often  many  times  greater  than  that  of  the  margins. 

2.  The  Velocity  of  the  Surface  is  greater  than  that  of  the  Bottom. 
— This  law  of  currents,  which  is  the  necessary  result  of  friction  on  the 
bed,  is  more  difficult  to  prove  in  the  case  of  glaciers,  because  it  is  dif- 
ficult to  get  a  vertical  section.     The  necessary  observation  was,  how- 
ever, successfully  accomplished  by  Prof.  Tyndall  in  1857.     We  have 

already  said  (page  51)  that  glaciers 
po'  conform  to  large  but  not  to  small 

;  inequalities    of    their    channels :    a 

glacier,  therefore,  passing  by  a  nar- 
row side-ravine  will  expose  its  whole 

thickness  on  the  side.     Prof.  Tyn- 

dall,  having  found  such  a  side  ex- 
posure more  than  140  feet  vertical, 

placed  three  pegs  in  a  vertical  line,  one  near  the  top,  one  near  the  mid- 
dle, and  one  at  the  bottom  (Fig.  47,  a  b  c).  The  vertical  line  became  more 
and  more  inclined  daily.  The  daily  motion  at  top  was  six  inches,  in  the 
middle  4.5  inches,  and  at  the  bottom  2.5  inches.  Thus,  glaciers,  like 
rivers,  slide  on  their  beds  and  banks,  producing  erosion ;  but,  also,  the 
several  layers,  both  horizontal  and  vertical,  slide  on  each  other. 

3.   The    Velocity  increases  with  the  Slope. — Fig.  48  represents   the 


56 


AQUEOUS  AGENCIES. 


surface-slope  of  the  glacier  Du  Geant,  G  ;  the  Mer  de  Glace,  M ;  and 
the  glacier  De  J3ois,  B ;  and  their  daily  motion.  The  increase  of  ve- 
locity with  the  slope  is  evident. 

4.   The  Velocity  increases  with  the  Fluidity. — The  daily  motion  of 


glaciers  is  greater  in  summer,  when  the  ice  is  rapidly  melting,  than  in 
winter  ;  and  in  mid-day  than  at  night. 

5.  The  Velocity  increases  with  the  Depth. — In  the  Alps,  where  the 
thickness  is  200  to  300  feet,  the  mean  daily  motion  is  one  to  three 
feet ;  but  in  Greenland,  where  the  thickness  is  2,000  to  3,000  feet,  the 
daily  motion,  in  spite  of  the  much  lower  temperature,  is  in  some  cases 
60  feet.1 

6.  Fluid  Currents  conform  to  the  Irregularities  of  their  Channel 
— Glaciers,  like  water-currents,  conform  to  the  inequalities  of  the  bot- 
tom  and   sides   of    their  channels.     They 

have  their  shallows  and  their  deeps,  their 
narrows  and  their  lakes, 
their  cascades,  their  rap- 
ids, and  their  tranquil  por- 
tions.    Fig.  49   shows  a 
glacier  running  through  a 
narrow  gorge  into  a  wide 
lake   of    ice,   and    again 
through    another    gorge. 
There   is  this   difference, 
however,  between  a  gla- 
cier and  a  water-current,  viz.,  that,  while  the  latter 
conforms  to  even  the  minutest  and  sharpest  outlines, 
the  former  conforms  only  to  the  larger  or  more  gentle. 
In  this,  a  glacier  acts  like  a  stiff,  viscous  fluid. 

7.  The  Line  of  Swiftest  Motion  is  more  sinuous 
than  the  Channel. — We  have  already  seen  that  this  is 
true  of  rivers  (page  21).  The  line  of  swiftest  current 
is  reflected  from  side  to  side,  increasing  the  curves  by  erosion.  The 
same  has  been  recently  proved  by  Tyndall  to  be  the  case  with  glaciers. 
Fig.  50  represents  a  portion  of  a  sinuous  glacier,  like  the  Mer  de  Glace : 
the  dotted  line  represents  the  line  of  swiftest  motion. 

1  Helland,  Journal  of  Geological  Society,  vol.  xxxiii.,  p.  142,  et  scq. 


FIG.  50. 


THEORIES   OF   GLACIER-MOTION.  57 

Theories  of  Glacier- Motion. 

There  are  few  subjects  connected  with  the  physics  of  the  earth 
which  have  excited  more  interest  than  that  of  glacier-motion.  The 
subject  is  one  of  exceeding  beauty,  and  not  without  geological  im- 
portance. Passing  over  several  very  ingenious  theories  which  have 
now  been  abandoned,  the  first  theory  which  was  conceived  in  the  true 
inductive  spirit,  and  which  explains  the  differential  motion,  is  that  of 
Prof.  James  Forbes. 

Viscosity  Theory  of  Forbes. 

Statement  of  the  Theory. — According  to  Forbes,  ice,  though  ap- 
parently so  hard  and  solid,  is  really,  to  a  slight  extent,  a  viscous  body. 
In  small  masses  this  property  is  not  noticeable,  but  in  large  masses  and 
under  long-continued  pressure  it  slowly  yields,  and  will  flow  like  a  stiffly 
viscous  fluid.  In  large  masses  like  a  glacier,  this  steady,  powerful  press- 
ure is  furnished  by  the  immense  weight  of  the  superincumbent  ice. 

Argument. — It  is  evident  that  this  theory  completely  accounts  for 
all  the  phenomena  of  glacier-motion,  even  in  their  minutest  details.  A 
glacier,  beyond  all  doubt,  moves  like  a  viscous  body,  but  it  is  still  a 
question  whether  it  does  so  by  virtue  of  a  property  of  viscosity.  The 
proposition  that  ice  is  a  viscous  substance  seems  at  first  palpably  ab- 
surd. It  is  necessary,  therefore,  to  show  that  this  proposition  is  not  so 
absurd  as  it  seems. 

The  properties  of  solidity  and  liquidity,  though  perfectly  distinct 
and  even  incompatible  in  our  minds,  nevertheless,  in  Nature,  shade  into 
one  another  in  the  most  imperceptible  manner.  Malleability,  plasticity, 
and  viscosity,  are  intermediate  terms  of  a  connecting  series.  The  idea 
which  underlies  all  these  expressions  is  that  of  capacity  of  motion  of 
the  molecules  among  themselves  without  rupture :  the  difference  among 
them  being  the  greater  or  less  resistance  to  that  motion.  In  the  case 
of  malleable  bodies,  like  the  metals,  great  force  is  required  to  produce 
motion ;  in  plastic  bodies,  like  wax  or  clay,  less  force  is  required ;  in 
viscous  bodies,  like  stiff  tar,  motion  takes  place  spontaneously  but 
slowly ;  while  in  liquids  it  takes  place  freely  and  with  little  or  no  resist- 
ance. In  all  of  these  cases,  if  the  pressure  be  sufficient,  the  body  will 
change  its  form  without  rupture — in  other  words,  will  flow.  Now,  by 
increasing  the  mass  we  may  increase  the  pressure  to  any  extent. 
Therefore,  all  malleable,  ductile,  plastic,  or  viscous  bodies,  if  in  suffi- 
ciently large  masses,  will  flow  like  water.  Thus,  a  mass  of  lead,  suffi- 
ciently thick,  would  certainly  flow  under  the  pressure  of  its  own  weight. 

But  solid  bodies  may  be  divided  into  two  great  classes,  viz.,  bodies 
which  are  malleable,  plastic,  or  viscous,  and  bodies  which  are  brittle  y 
the  very  idea  of  brittleness  being  that  of  total  incapacity  of  motion 


58  AQUEOUS   AGENCIES. 

among  the  particles  without  rupture.  Now,  ice  belongs  to  the  class  of 
brittle  bodies.  Forbes  attempts  to  remove  this  difficulty  by  showing 
that  many  apparently  brittle  bodies  will  also  flow  under  their  own 
weight ;  for  instance,  pitch,  so  hard  and  brittle  that  it  flies  to  pieces 
under  a  blow  of  the  hammer,  will,  if  the  containing  barrel  be  removed, 
flow  and  spread  itself  in  every  direction.  So,  also,  molasses-candy, 
made  quite  hard  and  brittle,  will  still  flow  by  standing.  A  remarkable 
pitch-lake,  about  three  miles  in  circumference,  occurs  in  Trinidad.  The 
pitch  is  described  as  in  constant,  slow-boiling  motion,  coming  up  in 
the  centre,  flowing  over  to  the  circumference,  and  again  sinking  down. 
Yet  this  pitch,  in  small  masses,  would  be  called  solid  and  brittle. 
Struck  with  a  hammer,  it  flies  to  pieces  like  glass.  In  fact,  the  essen- 
tial peculiarity  of  a  stiff,  viscous  body,  in  which  it  differs  from  mal- 
leable or  plastic  bodies,  is,  that  it  yields  only  to  slowly-applied  force. 

Forbes,  therefore,  thinks  that  glacier-ice  is  an  exceedingly  stiff,  vis- 
cous substance,  which,  though  apparently  brittle  in  small  quantities  and 
to  sudden  force,  yet,  under  the  slow-acting  but  powerful  pressure  of 
its  own  weight,  flows  down  the  slope  of  its  bed,  squeezing  through 
narrows  and  spreading  out  into  lakes,  conforming  to  all  the  larger  and 
gentler  inequalities  of  bed  and  banks,  but  not  to  the  sharper  ones. 
The  velocity  of  motion  is  small  in  the  same  proportion  as  the  viscous 
mass  is  stiff.  The  descent  of  the  Mer  de  Glace  from  the  cascade  of  the 
Glacier  du  Geant  to  the  point  of  Glacier  de  Bois,  a  distance  of  ten  miles, 
is  4,000  feet.  Water,  under  these  circumstances,  would  rush  with  fear- 
ful velocity.  The  glacier  moves  but  two  feet  in  twenty-four  hours. 

Regelation  Theory  of  Tyndall. 

If  ice  be  indeed  a  viscous  body,  then  there  seems  no  reason  why  it 
should  not  yield  to  pressure  even  in  small  masses,  if  the  pressure  be 
sufficiently  slowly  graduated.  In  the  hands  of  a  skillful  experiment- 
alist it  ought  to  exhibit  this  property.  Prof.  Tyndall  tried  the  ex- 
periment. Masses  of  ice  of  various  forms  were  subjected  to  slowly- 
graduated,  hydrostatic  pressure.  In  every  case,  however  slowly  grad- 
uated the  pressure,  the  ice  broke ;  but  if  the  broken  fragments  were 
pressed  together,  they  reunited  into  new  forms.  In  this  manner,  ice  in 
the  hands  of  Prof.  Tyndall  proved  as  plastic  as  clay  :  spheres  of  ice 
(a,  Fig.  51)  were  flattened  into  lenses  (£>),  hemispheres  (c)  were  changed 
into  bowls  (f£),  and  bars  (e)  into  semi-rings  (./")•  He  even  asserts  that 
ice  may  be  moulded  into  any  desirable  form  ;  e.  g.,  into  vases,  statuettes, 
rings,  coils,  knots,  etc.  Here,  then,  we  have  a  power  of  being  moulded 
such  as  was  not  dreamed  of  before  ;  but  this  power  was  not  depend- 
ent on  a  property  of  viscosity,  but  upon  another  property  long  known, 
but  only  recently  investigated  by  Faraday,  viz.,  the  property  of  rege- 
lation. 


THEORIES   OF  GLACIER-MOTION. 


59 


Regelation. — If  two  slabs  of  ice  be  laid  one  atop  of  the  other,  they 
soon  freeze  into  a  solid  mass.  This  will  take  place  not  only  in  cold 
weather,  but  in  midsummer,  or  even  if  boiling1  water  be  thrown  over 
the  slabs.  If  a  mass  of  ice  be  broken  to  pieces,  and  the  fragments  be 
pressed,  or  even  brought  in  contact  with  one  another,  they  will  quickly 
unite  into  a  solid  mass.  Snow  pressed  in  the  warm  hand,  though  con- 


FIG.  51.—  A  B  C,  moulds ;  a  c  e,  original  forms  of  the  ice ;  6  df,  the  forms  into  which  they 
were  moulded. 

stantly  melting,  gradually  becomes  compacted  into  solid  ice.  This  very 
remarkable  but  imperfectly  understood  property  of  ice  completely  ex- 
plains the  phenomena  of  moulding  ice  by  experiment.  By  this  property 
the  broken  fragments  reunite  in  a  new  form  as  solid  as  before.  We 
may  possibly  call  this  property  of  moulding  under  pressure  plasticity 
(although  it  is  not  true  plasticity,  since  it  does  not  mould  without  rupt- 
ure, but  by  rupture  and  regelation} ;  but  it  cannot  in  any  sense  be  called 
viscosity,  for  the  true  definition  of  viscosity  is  the  property  of  yielding 
under  tension  —  the  property  of  stretching  like  molasses-candy,  or 
melted  glass ;  but  ice  in  the  experiments,  according  to  Tyndall,  did  not 
yield  in  the  slightest  degree  to  tension.  In  the  experiment,  if,  instead 
of  placing  the  straight  bar  at  once  into  the  curved  mould,  it  had  been 
placed  successively  in  a  thousand  moulds,  with  gradually-increased 
curvature,  or,  still  better,  if  placed  in  a  straight  mould,  and  this  mould, 
while  under  pressure,  curved  slowly,  then  there  would  have  been  no 
sudden  visible  ruptures,  but  an  infinite  number  of  small  ruptures  and 
regelations  going  on  all  the  time.  The  ice  would  have  behaved  pre- 
cisely like  a  viscous  body.  Now,  this  is  precisely  what  takes  place  in 
a  glacier. 

Application  to  Glaciers. — A  glacier,  on  account  of  its  immense  mass, 
is,  in  its  lower  parts,  under  the  immense  pressure  of  its  own  weight 
tending  to  mould  it  to  the  inequalities  of  its  own  bed,  and  in  every  part 
under  a  still  more  powerful  pressure — a  pressure  proportioned  to  the 
height  of  the  head  of  the  glacier — urging  it  down  the  slope  of  its  bed. 
Under  the  influence  of  this  pressure  the  mass  is  continually  yielding  by 


60  AQUEOUS  AGENCIES. 

fracture,  but  again  uniting  by  regelation.  By  this  constant  process  of 
crushing,  change  of  form,  and  reunion,  the  glacier  behaves  like  a 
plastic  or  viscous  body;  though  of  true  plasticity  or  true  viscosity  there 
is,  according  to  Tyndall,  none.  In  fact,  we  have  in  the  phenomena  of 
glaciers  the  most  delicate  test  of  viscosity  conceivable ;  but  we  find  the 
glaciers  will  not  stand  the  test.  For  instance,  the  slope  of  the  Mer  de 


Glace  at  one  point  changes  from  4°  to  9°  25' '  (Fig.  52),  and  yet  the 
glacier,  although  moving  but  two  feet  a  day,  cannot  make  this  slight 
bend  without  rupture ;  for  at  this  point  there  are  always  large  trans- 
verse fissures  which  heal  up  below  by  pressure  and  regelation.  In  an- 
other place  the  glacier  is  similarly  broken  by  passing  an  angle  produced 
by  a  change  of  slope  of  only  2°.  It  seems  almost  impossible  that  a 
body  having  the  slightest  viscositv  should  be  fractured  under  these  cir- 
cumstances. Tyndall  concludes,  therefore,  that  the  motion  of  glaciers 
is  viscoid,  but  the  body  is  not  viscous — the  viscoid  motion  being  the 
result,  not  of  the  property  of  viscosity,  but  of  fracture  and  regelation. 

Comparison  Of  the  Two  Theories. — Forbes's  theory  supposes  motion 
among  the  ultimate  particles  without  rupture.  Tyndall's  supposes 
motion  among  discrete  particles  by  rupture,  change  of  position,  and 
regelation.  The  undoubted  viscoid  motion  is  equally  explained  by 
both :  by  the  one,  by  a  property  of  viscosity  ;  by  the  other,  by  a  prop- 
erty of  regelation. 

Conclusion. — It  seems  almost  certain  that  both  views  are  true,  and 
that  both  properties  are  concerned  in  glacial  motion.  Eecent  observa- 
tions and  experiments  have  shown  an  undoubted  viscosity  in  ice — es- 
pecially in  ice  containing  much  inclosed  and  dissolved  air,  as  is  the  case 
with  glacier-ice.  Ice  boards  supported  at  the  two  ends  gradually  bend 
into  an  arc  under  their  own  weight.  Cylinders  of  snow  compacted  into 
'ice  may  be  bent  in  the  hand  to  a  semicircle  without  rupture.8 

Structure  of  Glaciers. 

There  are  two  points  connected  with  the  structure  of  glaciers  which 
require  notice,  viz.,  the  veined  structure  and  the  fissures. 

Veined  Structure. — The  ice  of  glaciers  is  not  homogeneous,  but  con- 
sists of  white  vesicular  ice  (white  because  vesicular),  banded,  often  very 
beautifully,  with  solid  transparent  blue  ice  (transparent  blue  because 
solid),  the  banding  sometimes  so  delicate  that  a  hand-specimen  looks 

1  Tyndall,  "  Glaciers  of  the  Alps." 

8  Altkin,  American  Journal  of  Science,  vol.  v.,  p.  305,  third  series. 


THEORIES   OF   GLACIER-MOTION.  61 

like  striped  agate.  These  blue  veins  are  not  continuous  planes,  but 
apparently  very  flat  lenticular  in  shape,  varying  in  thickness  from  a 
line  to  several  inches,  and  in  length  from  a  few  inches  to  several  feet. 
Their  direction  being  parallel  to  one  another,  they  give  a  stratified  or 
cleavage  structure  to  the  glacier,  and,  in  melting,  the  glacier  often 
splits  or  cleaves  along  these  planes.  According  to  Prof.  Forbes,  look- 
ing upon  the  glacier  as  a  whole,  we  may  regard  the  strata  as  taking 
the  form  represented  by  the  subjoined  figures.  In  a  section  parallel  to 


FIG.  53.— Sections  of  a  Gkcier. 


FIG.  5t.— Ideal  Diagram,  showing  Struct- 
ure of  Glaciers  (after  Forbes). 


the  surface  (Fig.  53,  a),  the  strata  outcrop  in  the  form  of  loops.  A 
cross-section  (Fig.  53,  b)  shows  them  lying  in  troughs,  and  a  longi- 
tudinal vertical  section  (Fig.  53,  c)  shows  the  manner  in  which  they 
dip.  Fig.  54  is  an  ideal  glacier  cut  in  several  directions,  and  combining 
in  one  view  the  three  sections  given  above.  It  is  generally  impossible 
to  trace  the  veins  around  from  side  to  side.  Sometimes  they  are  most 
distinct  on  the  margins,  and  then  are  called  marginal  veins ;  some- 
times at  the  point  of  the  loop — transverse  veins  /  sometimes  tributaries 
running  together,  as  in  the  figure  (Fig.  54) — the  interior  branches  of  the 
two  loops  coalesce,  and  are  flattened  against  one  another,  and  form 
longitudinal  veins. 

Fissures. — These  are  also  marginal,  transverse,  and  longitudinal. 
The  marginal  fissures  are  shown  in  Fig.  40  ;  they  are  always  at  right 
angles  to  the  marginal  veins. 


62  AQUEOUS  AGENCIES. 

Theories  of  Structure. 

Fissures. — There  can  be  no  doubt  that  the  great  fissures  or  crevasses 
are  produced  by  tension  or  stretching,  and  that  their  direction  is  always 
at  right  angles  to  the  line  of  greatest  tension.  Thus  the  transverse 
fissures  are  produced  by  the  stretching  of  the  glacier  in  passing  over  a 
salient  angle.  The  marginal  fissures  are  produced  by  the  dragging  or 
pulling  of  the  swifter  central  portions  upon  the  slower  marginal  por- 
tions. It  has  been  proved  by  Hopkins,  the  English  physicist  and  geolo- 
gist, that  the  line  of  greatest  tension  from  this  cause  would  be  inclined 
45°,  with  the  course  of  the  glacier  as  shown  by  the  arrows  (Fig.  55). 


The  fissures  should  be  at  right  angles  to  these  lines,  and,  therefore,  also 
inclined  45°  with  the  margin,  and  running  upward  and  inward.  The 
longitudinal  fissures  are  best  seen  where  a  glacier  runs  through  a  nar- 
row gorge  out  on  an  open  plain.  The  lateral  spreading  of  the  glacier 
causes  it  to  crack  longitudinally  (Fig.  56).  Fig.  57  is  a  longitudinal 
vertical  section  of  the  same. 

Veined  Structure. — Tyndall  has  shown  conclusively  that  veins  are 
always  at  right  angles  to  the  line  of  greatest  pressure,  and  that,  there- 
fore, they  are  produced  by  pressure.  Thus  fissures  and  veins,  being 
produced  by  opposite  causes — one  by  tension  and  the  other  by  pressure 

— are  found  under  opposite  con- 
ditions. Thus  as  transverse  fis- 
sures are  produced  by  the  longi- 
tudinal stretching  of  a  glacier 
passing  over  a  salient  angle,  so 
transverse  veins  are  formed  by 
the  longitudinal  compression  of 
a  glacier  passing  over  a  re'enter- 
ing  angle.  Fig.  57  is  a  section 
of  the  Rh6ne  glacier  (Fig.  56), 
showing  the  crevasses  (c  c  c) 


THEORIES   OF   STRUCTURE. 


63 


produced  by  the  steep  declivity,  and  the  veined  structure  (s  s  s)  pro- 
duced by  the  compression  consequent  upon  the  change  of  angle  on 
coming  out  on  the  plain.  The  relation  of  crevasses  and  vein-structure 
is  still  better  shown  in  the  ideal  section  (Fig.  58). 

Again,  as  marginal  fissures  are  produced  by  the  pulling  of  the  cen- 
tral portions  upon  the  lag- 
ging  margins   behind,   so        /n  c 
the  marginal  veins  are  pro- 
duced  by  the  crowding  or 
pushing  of  the  swifter  cen- 
tral  parts  upon  the  mar- 
ginal parts  in  front  (Fig. 
59).     The  marginal  veins 
are.  therefore,  inclined  to 

the  margin  about  45°,  but  pointing  inward  and  downward,  and,  there- 
fore, at  right  angles  to  the  crevasses.  The  relation  of  these  to  one 
another  is  shown  in  Fig.  60. 

Finally,  as  longitudinal  fissures  are  produced  by  lateral  spreading 
(Fig.  56),  so  longitudinal  veins  are  produced  by  lateral  compression. 


This  is  best  seen  where  two  tributaries  meet  at  high  angle  (Fig.  61) — 
for  instance,  where  the  Glacier  du  G6ant  and  the  Glacier  de  Lechaud 
form  the  Mer  de  Glace  (Fig.  41).  All  these  facts  have  been  experi- 
mentally illustrated  by  Tyndall. 

Physical  Theory  of  Veins. — There  is  little  doubt  that  veins  are 
formed  by  pressure  at  right  angles  to  the  direction  of  the  veins ;  but 
how  pressure  produces  this  structure  is  very  imperfectly  understood. 
Probably  at  least  a  partial  explanation  is  contained  in  the  following 
proposition  :  1.  White  vesicular  ice  by  powerful  pressure  is  crushed,  the 
air  escapes,  and  the  ice  is  refrozen  into  solid  blue  transparent  ice.  2. 
Ice  being  a  substance  which  expands  in  freezing,  and,  therefore,  con- 
tracts in  melting,  its  freezing  and  melting  point  is  lowered  by  pressure. 
Therefore,  ice  at  or  near  32°  Fahr.  is  melted  by  pressure.  Now,  the 
glacier  is  under  powerful  pressure  of  its  own  weight,  and  the  stress  of 


64 


AQUEOUS  AGENCIES. 


this  pressure  is  ever  changing  from  point  to  point  by  the  changing 
position  of  the  particles  produced  by  the  motion.  Thus  the  glacier  in' 
places  is  ever  melting  under  pressure,  and  again  refreezing  by  relief  of 
pressure.  The  melting  discharges  the  air-bubbles,  and,  in  refreezing, 
the  ice  is  blue.  3.  No  substance  is  perfectly  homogeneous,  and  of 
equal  strength  in  all  parts ;  therefore,  this  crushing  and  melting,  and 
consequent  conversion  of  white  into  blue  ice,  take  place  irregularly  in 
spots.  4.  As  ice  of  a  glacier  acts  like  a  viscous  substance,  the  final 
effect  of  pressure  would  be  to  flatten  these  spots,  both  white  and  blue, 
in  the  direction  of  greatest  pressure,  and  extend  them  in  a  direction  at 
right  angles  to  the  pressure,  and  thus  create  bands  in  this  direction.  5. 
Differential  motion  would  also  tend  to  bring  the  veins  into  the  direction 
indicated  by  Forbes. 

Floating  Ice — Icebergs. 

We  have  already  seen  (page  47)  that  at  a  certain  latitude,  varying 
from  46°  in  South  America  to  about  65°  in  Norway,  glaciers  touch  the 
surface  of  the  ocean.  Beyond  this  latitude,  they  run  out  to  sea  often  to 
great  distances.  By  the  buoyant  power  of  water,  assisted  by  tides  and 
waves,  these  projecting  floating  masses  are  broken  off,  and  accumulate 
as  immense  ice-barriers  in  polar  seas,  or  are  drifted  away  by  currents 
toward  the  equator.  Such  floating  fragments  of  glaciers  are  called  ice- 


FIG.  62.— Formation  of  Icebergs. 

bergs.  Fig.  62  is  an  ideal  section,  through  a  glacial  valley,  in  which 
a  g  is  the  glacier,  b  the  cliffs  beyond,  Ij  the  sea-level,  and  *  an  iceberg. 
The  principal  source  of  the  icelfergs  of  the  north  Atlantic  is  the 
coast  of  Greenland.  This  country  is  an  elevated  table-land,  sloping 
in  every  direction  to  a  coast  deeply  indented  like  Norway,  with  alter- 
nate deep  fiords  and  jutting  headlands.  The  whole  table-land  is  com- 
pletely covered  with  an  ice-sheet,  probably  several  thousand  feet  thick, 
moving  slowly  seaward,  and  discharging  through  the  fiords  as  immense 


FLOATING   ICE— ICEBERGS.  65 

glaciers,1  which,  as  already  explained,  form  icebergs.  In  this  remarkable 
country  no  water  falls  from  the  atmosphere  except  in  the  form  of 
snow,  and  all  the  rivers  are  glaciers.  The  geological  effects  of  such  a 
moving  ice-sheet  may  be  easily  imagined.  The  whole  surface  of  the 
country  rock  must  be  polished  and  scored,  the  general  direction  of  the 
striae  being  parallel  over  large  areas. 

The  antarctic  continent  is  probably  similarly,  and  even  more  thick- 
ly, ice-sheeted,  for  the  humid  atmosphere  of  that  region  is  very  favorable 
to  the  accumulation  of  snow  and  ice.  Captain  Wilkes  found  an  impen- 
etrable ice-barrier,  in  many  places  150  to  200  .feet  high,  for  1,200 
miles  along  that  coast.  From  this  ice-barrier,  icebergs  separate  and 
are  drifted  toward  the  equator. 

The  formation  of  icebergs  in  polar  regions,  and  their  drifting  into 
warmer  latitudes,  to  be  melted  there,  is  evidently  a  necessary  conse- 
quence of  the  great  law  of  circulation,  for  otherwise  ice  would  accumu- 
late without  limit  in  these  regions. 

General  Description. — The  number  of  icebergs  accumulated  about 
polar  coasts  is  almost  inconceivable.  Scoresby  counted  500  at  one 
view.  Kane  counted  280  of  the  first  magnitude  at  one  view.  They 
are  often  200  and  sometimes  even  300  feet  high,  and  the  mass  above 


water  66,000,000  cubic  yards  (Dr.  Rink).  As  the  specific  gravity  of 
ice  is  0.918,  if  these  were  solid  ice,  there  would  be  but  one-twelfth 
above  water;  but  as  glacier-ice  is  somewhat  vesicular,  there  is  about 
one-seventh  above  water.  The  thickness  of  some  of  these  icebergs 
must  therefore  be  2,000  to  3,000  feet,  and  their  volume  near  500,000,000 
cubic  yards,  which  is  about  equivalent  to  a  mass  one  mile  square,  and 
500  feet  thick.  Under  the  influence  of  the  melting  power  of  the  sun  un- 

1  Dr.  Rink,  "Archives  des  Sciences,"  vol.  xxvii.,  p.  155. 

5 


66  AQUEOUS  AGENCIES. 

equally  affecting  different  parts,  they  assume  various  and  often  strange 
forms.  The  accompanying  figure  (Fig.  63)  gives  the  usual  appearance 
in  the  northern  Atlantic.  Those  separated  from  the  antarctic  barrier 
present,  before  they  have  been  much  acted  upon  by  the  sun,  a  much 
more  regularly  prismatic  appearance.  Fig.  64  gives  the  appearance  of 


one  of  these  prismatic  blocks  or  tables,  180  feet  high,  seen  by  Sir  James 
Ross  in  the  antarctic  seas. 

Icebergs  as  a  Geological  Agent— Erosion. — The  polishing  and  scor- 
ing effects  of  the  ice-sheets  and  of  their  discharging  glaciers  must,  of 
course,  extend  over  the  sea-bottoms  about  polar  coasts  as  far  as  the 
glaciers  touch  bottom,  which,  considering  their  immense  thickness, 
must  be  for  considerable  distance  (Fig.  62,  j  to  g).  This,  however,  is 
glacier  agency  rather  than  iceberg  agency.  On  being  separated  they 
float  away,  and  are  carried  by  currents  with  their  immense  loads  of 
earth  and  bowlders,  amounting  often  to  100,000  tons  or  more,  as  far  as 
40°  or  even  36°  latitude,  where,  being  gradually  melted,  they  drop 
their  burden.  If  the  water  be  not  sufficiently  deep,  they  ground,  and 
being  swayed  by  waves  and  tides  they  chafe  and  score  the  bottom  in  a 
somewhat  irregular  manner ;  or,  packing  together  in  large  fields,  and 
urged  onward  by  powerful  currents,  they  may  possibly  score  the  bot- 
tom over  considerable  areas  somewhat  in  the  manner  of  glaciers.  A 
large  iceberg  will  ground  in  water  2,000  and  2,500  feet  deep ;  they 
have  been  found  by  James  Ross  actually  aground  in  1,560  feet  of  water 
off  Victoria  Land.  A  true  glaciated  surface,  however,  cannot  be  pro- 
duced by  icebergs. 

Deposits. — The  bottom  of  the  sea  about  polar  shores  is  found  deep- 
ly covered  with  materials  brought  down  by  glaciers  and  dropped  by 
icebergs  (Fig.  52).  Again,  similar  materials  are  carried  by  icebergs  as 
far  as  these  are  drifted  by  currents,  and  spread  on  the  bottom  of  the 
sea  everywhere  in  the  course  of  these  currents.  Where  stranded  ice- 
bergs accumulate,  as  on  the  banks  of  Newfoundland,  large  quantities  of 
such  materials  are  deposited.  These  banks  are  in  fact  supposed  to 


MECHANICAL  AGENCIES   OF  WATER.  6f 

have  been  formed  in  this  way.  Such  deposits  have  not  been  sufficient- 
ly examined  ;  they  are  probably  somewhat  similar  to  those  of  glaciers, 
exhibiting,  however,  some  signs  of  the  sorting  power  of  water.  Bal- 
anced stones  or  bowlders  in  insecure  positions  can  hardly  be  left  by 
icebergs. 

Shore-Ice. 

In  cold  climates  the  freezing  of  the  surface  of  the  water  forms 
sheets  of  ice  many  inches  or  even  feet  thick,  and  of  great  extent, 
about  the  shores  of  rivers,  bays,  and  seas.  They  often  inclose  stones 
and  bowlders  of  considerable  size,  and  when  loosened  in  spring  from 
the  shore  they  bear  these  away,  and  again  drop  them  at  considerable 
distances  from  their  parent  rock.  Also  such  sheets  packed  together  in 
large  masses,  and  driven  ashore  by  river  and  tidal  currents,  and  chafed 
back  and  forth  by  waves,  produce  effects  on  the  shore-rocks  somewhat 
similar  to  the  scoring,  polishing,  and  even  the  roches  moutonnees  of 
glacier-action.  These  effects  are  well  seen  on  the  shores  of  the  St. 
Lawrence  River  and  Gulf. 

The  importance  of  the  study  of  ice-agencies  will  be  seen  when  we 
come  to  explain  the  phenomena  of  the  Drift  or  Glacial  period,  a  remark- 
able period  in  the  history  of  the  earth. 

Comparison  of  the  Different  Forms  of  the  Mechanical  Agencies  of 
Water. 

Rivers  and  glaciers  are  constantly  cutting  down  all  lands,  bearing 
away  the  materials  thus  gathered,  and  depositing  them  on  the  sea- 
margins.  Acting  alone,  therefore,  their  effect  must  be  to  diminish  the 
height  and  to  extend  the  limits  of  the  land.  Ocean  agencies,  on  the 
other  hand,  by  tides  and  currents  bear  away  to  the  open  sea  the  mate- 
rials brought  down  from  the  land,  and  thus  tend  to  prevent  marginal 
accumulations  ;  and  by  waves  and  tides  constantly  eat  away  the  coast- 
line,  and  thus  strive  to  extend  the  domain  of  the  sea.  Thus,  while  river 
and  ocean  agencies  are  in  conflict  with  one  another  at  the  coast-line, 
the  one  striving  to  extend  the  limits  of  the  land,  and  the  other  of  the 
sea,  yet  they  cooperate  with  each  other  in  destroying  the  inequalities 
of  the  earth's  surface,  and  are  therefore  called  leveling  agencies.  More- 
over, it  is  evident  that  the  erosion  of  the  land  and  the  filling  up  of  the 
seas  are  correlative,  and  one  is  an  exact  measure  of  the  other.  Now, 
we  have  seen  (page  11)  that  the  probable  rate  at  which  all  continents 
are  being  cut  down  by  rivers  is  about  one  foot  in  4,500  to  5,000  years. 
But  since  the  ocean  is  about  three  times  the  extent  of  the  land,  this 
spread  evenly  over  the  bottom  of  the  sea  would  make  a  stratum  about 
four  inches  thick.  Therefore,  we  conclude  that,  neglecting  the  destruc- 
tive effects  of  waves  and  tides  on  the  coast-line,  which,  according  to 


68  AQUEOUS  AGENCIES. 

Phillips,1  are  small  in  amount  compared  with  general  erosion  of  the 
land-surface,  we  may  say  that  stratified  deposits  are  now  forming,  or 
the  ocean-bed  filling  up,  at  the  rate  of  about  four  inches  in  5,000  years. 

SECTION  4. — CHEMICAL  AGENCIES  OF  WATER. 
Subterranean  Waters,  Springs,  etc. 

As  we  have  already  seen  (page  9),  of  the  rain  which  falls  on  any  hy- 
drographical  basin,  a  part  runs  from  the  surface,  producing  universal  ero- 
sion. A  second  part  sinks  into  the  earth,  and,  after  a  longer  or  shorter 
subterranean  course,  comes  up  as  springs,  and  unites  with  the  surface- 
water  to  form  rivers ;  while  a  third  portion  never  comes  up  at  all,  but 
continues  by  subterranean  passages  to  the  sea.  This  last  portion  is 
removed  from  observation,  and  our  knowledge  concerning  it  is  very 
limited.  But  there  are  numerous  facts  which  lead  to  the  conviction 
that  it  is  often  very  considerable  in  amount.  In  many  portions  of  the 
sea  near  shore,  springs,  and  even  large  rivers,  of  fresh  water,  are  known 
to  well  up.  Thus,  in  the  Mediterranean  Sea,  "  a  body  of  fresh  water 
fifty  feet  in  diameter  rises  with  such  force  as  to  cause  a  visible  con- 
vexity of  the  sea-surface."  *  Similar  phenomena  have  been  observed 
in  many  other  places  in  the  same  sea,  and  also  in  the  Gulf  of  Mexico 
near  the  coast  of  Florida,  among  the  West  India  isles,  and  near  the 
Sandwich  Islands.  Besides  the  last  mentioned,  there  is  still  another 
portion  of  subterranean  water  existing  permanently  in  every  part  of 
the  earth  far  beneath  the  sea-level,  filling  fissures  and  saturating  sedi- 
ments to  great  depths,  and  only  brought  to  the  surface  by  volcanic 
forces.  This,  in  contradistinction  from  the  constantly-circulating  me- 
teoric water,  may  be  called  volcanic  water. 

Springs. — The  appearance  of  subterranean  waters  upon  the  surface 
constitutes  springs.  They  occur  in  two  principal  positions,  viz. :  1. 


FIG.  65.— Hill-side  Spring. 

Upon  hill-sides,  just  where  porous,  water-bearing  strata  such  as  sand 
outcrop,  underlaid  by  impervious  strata  like  clay  ;  2.  On  fissures,  pene- 
trating the  country  rock  to  great  depth. 

Most  of  the  small  springs  occurring  everywhere  belong  to  the  first 
class.  The  figure  (Fig.  65)  represents  a  section  of  a  hill  composed  mostly 

1  Phillips,  "Life  on  the  Earth,"  p.  131.  *  Herschel's  "Physical  Geography." 


SUBTERRANEAN   WATERS,   SPRINGS,   ETC. 


69 


of  porous  strata,  #,  but  underlaid  by  impervious  clay  stratum,  c.  Wa- 
ter falling  upon  the  surface  sinks  through  b  until  it  comes  in  contact 
with  c,  and  then  by  hydrostatic  pressure  moves  laterally  until  it  emerges 
at  a.  Sometimes  this  is  a  geological  agent  of  considerable  importance, 
modifying  even  the  forms  of  mountains,  and  producing  land-slips,  etc. 
Thus  the  Lookout  and  Raccoon  Mountains,  in  Tennessee,  are  table- 
mountains  of  nearly  horizontal  strata,  separated  by  erosion-valleys. 
These  mountains  are  all  of  them  capped  by  a  sandstone  stratum  about 
100  feet  thick,  underlaid  by  shale.  The  water  which  falls  upon  the 
mountain  emerges  in  numerous  springs  all  around  where  the  sandstone 
cap  comes  in  contact  with  the  underlying  shale.  The  sandstone  is  gradu- 
ally undermined,  and  falls  from  time  to  time,  and  thus  the  cliff  remains 
always  perpendicular  (Fig.  66). 

Large  springs  generally  is- 
sue from  fissures.  Water  pass- 
ing along  the  porous  stratum 
#,  perhaps  from  great  distance, 
and  prevented  from  rising  by 
the  overlying  impervious  strat- 
um c,  coming  in  contact  with  a  fissure,  immediately  rises  through  it  to 
the  surface  at  a  (Fig.  67). 

Artesian  Wells. — If  subterranean  streams  have  their  origin  in  an  ele- 
vated region,  a  d,  composed  of  regular  strata  dipping  under  a  lower  flat 

country,  c,  then  the  subter- 
n.  d 


FIG.  67.— Fissure-Spring. 


§ 
Rc 


;.— Artesian  Well. 


ranean  waters  passing  along 
any  porous  stratum,  as  a 
(Fig.  68),  and  confined  by 
two  impermeable  strata,  b 
and  £?,  will  be  under  power- 
ful hydrostatic  pressure,  and 
will,  therefore,  rise  to  the 
surface,  perhaps  with  considerable  force,  if  the  stream  be  tapped  by  bor- 
ing at  c.  Borings  by  which  water  is  obtained  in  this  manner  are  called 
artesian  wells,  from  the  French  province  Artois,  where  they  were  first 
successfully  attempted.  The  source  of  the  water  may  be  100  miles  or 
more  distant  from  the  well.  Some  of  these  wells  are  very  deep.  The 
Greville  artesian  in  Paris  is  2,000  feet  deep.  At  the  moment  of  tapping 
the  stream,  a  powerful  jet  was  thrown  112  feet  high.  One  in  West- 
phalia, Germany,  is  2,385  feet  deep;  one  at  St.  Louis,  3,843  feet;  one 
at  Louisville,  Kentucky,  2,852.  In  parts  of  Alabama  and  California, 
the  principal  supply  of  water  for  agricultural  purposes  is  drawn  from 
these  wells. 

Thus  there  is  on  all  coasts  a  constant  flowing  of  water,  both  super- 
ficial and  subterranean,  into  the  sea.    Their  relative  amount  it  is  impos- 


70  AQUEOUS  AGENCIES. 

sible  to  determine.  Much,  no  doubt,  depends  upon  the  configuration 
of  the  country  and  the  nature  of  the  strata.  The  heavy  hydrostatic 
pressure  to  which  subterranean  water  is  subjected,  especially  in  ele- 
vated countries,  brings  by  far  the  larger  portion  of  it  to  the  surface  in 
the  form  of  springs.  But,  in  limestone  regions  (this  rock  being  affected 
with  frequent  and  large  fissures,  and  open  subterranean  passages,  as 
will  be  hereafter  explained),  large  subterranean  rivers  often  exist,  and 
these,  even  after  coming  to  the  surface,  are  often  reengulfed,  and  finally 
reach  the  sea  by  subterranean  passages.  The  largest  springs,  there- 
fore, generally  occur  in  limestone  countries.  From  the  Silver  Spring, 
in  Florida,  issues  a  stream  navigable  for  small  steamers  up  to  the  very 
spring  itself.  The  country  for  sixty  miles  around  is  entirely  destitute 
of  superficial  streams,  the  whole  drainage  of  the  country  being  subter- 
ranean, and  coming  up  in  this  spring. 

Chemical  Effects  of  Subterranean  Waters.  —  We  have  already  seen 
(page  6)  how  atmospheric  water  disintegrates  rocks,  dissolving  out 
their  soluble  parts,  and  reducing  their  unsoluble  parts  to  soils.  Springs, 
therefore,  always  contain  these  soluble  matters.  In  granite  regions 
they  contain  potash  ;  in  limestone  regions  they  contain  lime,  and  are 
called  hard  ;  in  other  cases  they  contain  salt,  and  are  brackish  ;  when 
the  saline  ingredients  are  unusual  in  quantity,  or  in  kind,  they  are 
called  mineral  waters. 

Limestone  Caves.  —  In  most  rocks,  the  insoluble  part  left  as  soil  is 
far  the  largest,  only  a  small  percentage  being  dissolved  ;  but  in  the 
case  of  limestone  the  whole  rock  is  soluble.  Therefore,  in  limestone 
regions,  percolating  waters  passing  through  fissures  and  between  strata 
dissolve  the  limestone  and  hollow  out  open  passages,  and  form  immense 
caves.  Water  charged  with  limestone,  dripping  from  the  roofs  and 
falling  on  the  floors  of  these  caves,  deposit  their  limestone  by  evapora- 

tion, and  form  stalactites 


b  b,  which,  meet- 
ing  each  other,  form  lime- 
stone  pMars,  c  c.  The 
great  Mammoth  Cave,  in 
Kentucky  ;  Wier's  Cave, 
iQ  Virginia,  and  Nico- 
Jack  Cave,  m  Tennessee, 
are  familiar  examples. 

FIG.  eO.-Limestone  Cave.  "     As    might    be    expected, 

subterranean    rivers    are 

often  found  running  through  these  caves.    This  is  the  case  in  the  Mam- 
.moth  Cave,  and  in  Nicojack  Cave. 


CHEMICAL  DEPOSITS  IN  SPRINGS.  71 

Chemical  Deposits  in  Springs. 

Deposits  of  Carbonate  Of  Lime. — We  have  just  seen  that  ordinary 
subterranean  waters  in  limestone  districts,  and,  therefore,  containing 
small  quantities  of  carbonate  of  lime,  deposit  this  substance  only  very 
slowly  by  drying,  as  stalactites  and  stalagmites ;  but  in  carbonated 
springs  in  limestone  districts  a  very  rapid  deposit  of  lime  carbonate 
often  occurs. 

Explanation. — In  order  to  understand  this,  it  is  necessary  to  re- 
member :  1.  That  lime  carbonate  is  insoluble  in  pure  water,  but  soluble 
in  water  containing  carbonic  acid ;  2.  That  the  amount  of  carbonate 
dissolved  is  proportionate  to  the  amount  of  carbonic  acid  contained ;  3. 
That  the  amount  of  carbonic  acid  which  may  be  taken  in  solution  by 
water  is  proportionate  to  the  pressure. 

Now,  there  are  two  sources  of  carbonic  acid,  viz.,  atmospheric  and 
subterranean.  All  water  contains  carbonic  acid  from  the  atmosphere, 
and  will,  therefore,  dissolve  limestone,  but  this  deposits  only  slowly  by 
drying,  as  already  explained.  But  in  many  districts,  especially  in  vol- 
canic districts,  there  are  abundant  subterranean  sources  of  carbonic 
acid.  If  subterranean  waters  come  in  contact  with  such  carbonic  acid, 
being  under  heavy  pressure,  they  will  take  up  a  large  quantity  of  this 
gas ;  and  if  such  water  comes  to  the  surface,  the  pressure  being  re- 
moved, the  gas  will  escape  in  bubbles.  This  is  a  carbonated  spring. 
If,  further,  the  subterranean  waters,  thus  highly  charged  with  carbonic 
acid,  come  in  contact  with  limestone  rocks,  or  rocks  of  any  kind  con- 
taining lime  carbonate,  they  will  dissolve  a  proportionably  large  amount 
of  this  carbonate  ;  and  when  they  come  to  the  surface,  the  escape  of 
the  carbonic  acid  causes  the  lime  carbonate  to  deposit  abundantly. 
Thus  around  carbonated  springs  in  limestone  districts,  and  along  the 
course  of  the  streams  which  issue  from  them,  are  generally  found  ex- 
tensive deposits  of  this  substance.  Being  found  mostly  in  volcanic 
regions,  these  springs  are  commonly  hot. 

Kinds  of  Materials. — The  material  thus  deposited  is  usually  called 
travertine,  but  is  very  diverse  in  appearance.  If  the  deposit  is  quiet, 
the  material  is  dense ;  if  tumultuous,  the  material  is  spongy ;  if  no 
iron  is  present,  it  is  white  like  marble ;  but  if  iron  be  present,  its  oxida- 
tion colors  it  yellow,  brown,  or  reddish.  If  the  amount  of  iron  be  vari- 
able, the  stone  is  beautifully  striped.  If  objects  of  any  kind,  branches, 
twigs,  leaves,  are  immersed  in  such  waters,  they  are  speedily  incrusted, 
often  in  the  most  beautiful  manner. 

Examples  of  such  deposits  are  found  in  all  countries.  At  the  baths 
of  San  Vignone,  Italy,  a  carbonated  spring  issuing  from  the  top  of  a 
hill  has  covered  the  hill  with  a  stratum  of  white,  compact  travertine  250 
feet  thick.  In  the  conduit-pipe  which  leads  the  water  to  the  baths,  the 


AQUEOUS  AGENCIES. 


deposit  accumulates  six  inches  thick  every  year.  A  similar  deposit  of 
travertine  occurs  at  the  baths  of  San  Filippo.  At  this  latter  place, 
beautiful  fac-similes  of  medallions,  coins,  etc.,  are  formed  by  placing 
these  objects  of  art  in  the  spray  of  an  artificial  cascade.  In  Virginia, 
around  the  "  Old  Sweet "  and  the  "  Red  Sweet "  Springs,  and  in  the 
course  of  the  stream  which  flows  from  them 
for  several  miles,  a  brownish-yellow  deposit 
of  travertine  has  accumulated  to  the  depth 
of  at  least  thirty  feet.  The  spray  of  Beaver 
Dam  Falls,  about  three  miles  below  the 
springs,  incrusts  every  object  in  its  reach 
with  this  deposit. 

In  California,  all  about  the  shores  of  Lake 
Mono,  abundance  of  beautiful  and  strangely- 
branched  coralline   forms  are  found, 
which  have  evidently  been  formed  in 
a  somewhat  similar  way.     In  the  re- 
gion  of   the   Yellowstone    Park,  de- 
posits  of   traver- 
tine  from    waters 
of  hot  springs  run- 
ning down  a  steep 
incline,  in  a  suc- 
cession    of    cas- 
cades, assume  the 
most  beautiful 
forms,   as    shown 
in  the  accompany- 
ing figure,  taken 
from  Hayden. 

Deposits  of 
Iron.  —  Iron  car- 
bonate, like  lime 

carbonate,  is  to  some  extent  soluble  in  water  containing  carbonic  acid. 
Subterranean  waters,  therefore,  which  always  contain  atmospheric  car- 
bonic acid,  when  they  meet  this  carbonate,  will  take  up  a  small  quan- 
tity in  solution.  Such  waters  are  called  chalybeate.  On  coming  to 
the  surface  the  iron  gives  up  its  carbonic  acid,  is  peroxidized,  becomes 
insoluble,  and  is  deposited.  As  the  presence  of  organic  matter  is  usu- 
ally necessary  to  bring  the  iron  into  a  soluble  condition,  the  full  dis- 
cussion of  this  very  interesting  subject  is  reserved  until  we  take  up 
organic  agencies. 

Deposits  of  Silica. — Silica  is  soluble  in  alkaline  waters,  especially  if 
the  waters  be  hot.     Such  waters  reaching  the  surface  and  cooling,  de- 


FIG.  70.— Deposits  from  Carbonated  Springs. 


CHEMICAL  DEPOSITS   IN  LAKES.  73 

posit  the  silica  in  great  abundance,  often  at  first  in  a  gelatinous  con- 
dition, but  drying  to  a  white  porous  material  called  siliceous  sinter. 
Examples  of  such  deposits  are  found  in  all  geysers,  as  in  those  of  Ice- 
land, and  in  the  Steamboat  Springs  in  Nevada,  and  especially  in  the 
wonderful  geysers  of  Yellowstone  Park.  Such  deposits  are  confined 
to  volcanic  regions,  the  volcanic  rocks  furnishing  both  the  alkali  and 
the  heat.  We  will  discuss  these  again  under  Igneous  Agencies. 

Deposits  of  Sulphur  and  Gypsum. — Springs  containing  sulphide  of 
hydrogen  (H2S),  usually  called  sulphur  -  springs,  sometimes  deposit 
sulphur  by  oxidation  of  the  hydrogen  (H2S  +  O=H2O  +  S),  and  some- 
times gypsum.  This  latter  deposit  is  caused  by  the  more  complete  oxi- 
dation of  the  sulphide  of  hydrogen,  forming  sulphuric  acid  (H2S  +  4O 
=H2SO4),  and  the  reaction  of  this  acid  on  lime  carbonate  held  in  solu- 
tion in  the  same  water. 

Chemical  Deposits  in  Lakes. 

Salt  Lakes  and  Alkaline  Lakes. — Salt  lakes  may  be  formed  either 
— 1.  By  the  isolation  of  a  portion  of  sea-water  in  the  elevation  of  sea- 
bottom  into  land  ;  or,  2.  By  indefinite  concentration  of  river-water  in 
a  lake  without  an  outlet.  Thus,  the  Dead  Sea,  Lake  Elton,  and  the 
brine-pools  of  the  Russian  steppes,  are  probably  concentrated  remains 
of  isolated  portions  of  the  sea,1  for  their  waters  are  highly-concen- 
trated mother-liquors  of  sea-water,  having  a  composition  very  similar 
to  the  mother-liquors  of  the  salt-maker.  The  Caspian  Sea,  on  the  other 
hand,  although  elevated  lake-margins  show  that  much  of  its  waters 
has  dried  away,  is  still  much  fresher  than  sea-water.  This  fact,  to- 
gether with  the  composition  of  its  waters,  is  usually  supposed  to  indi- 
cate that  it  has  been  formed  by  the  simple  concentration  of  the  waters 
of  a  once  fresh  lake.2  Yet  there  are  some  evidences,  as  we  shall  see 
hereafter,  of  this  sea  having  been  once  connected  with  the  Black  and 
with  the  Arctic  Ocean.  The  composition  of  the  waters  of  the  Great 
Salt  Lake  of  Utah  would  seem  to  indicate  its  origin  in  the  isolation  of 
sea-water ;  but  there  are  also  some  evidences  of  its  once  having  had  an 
outlet,  in  which  case  it  must  have  been  fresh,  or  at  least  brackish.3 

Alkaline  lakes  can  only  be  formed  by  the  second  way-.  Both  salt 
and  alkaline  lakes,  therefore,  may  be  formed  by  indefinite  concentra- 
tion of  river-water  in  a  lake  without  outlet.  Whether  the  one  or  the 
other  is  formed  depends  on  the  composition  of  the  river-water.  If  al- 
kaline chlorides  predominate,  a  salt  lake  will  be  formed ;  but  if  alka- 
line carbonates,  an  alkaline  lake.  Such  alkaline  lakes  are  found  in 
Hungary,  in  Lower  Egypt,  and  in  Persia.  In  our  own  country,  Lake 
Mono,  fifteen  miles  long  and  twelve  miles  wide,  and  Lake  Owen,  of  at 

1  Bischof,   "  Chemical  and  Physical  Geology,"  vol.  i.,  p.  396. 

2  Ibid.,  p.  91.  3  Gilbert,  "  Wheeler  Report  for  1872,"  p.  49. 


74:  AQUEOUS  AGENCIES. 

least  equal  dimensions,  are  examples  of  alkaline  lakes.  The  waters  of 
Lake  Mono  consist  principally  of  a  strong  solution  of  carbonate  of 
soda,  with  a  little  carbonate  of  lime  and  borate  of  soda.1 

Conditions  of  Salt-Lake  Formation. — Spring  and  river  waters  always 
contain  a  small  quantity  of  saline  matter  derived  from  the  rocks  and 
soils.  Suppose,  then,  we  have  a  lake  supplied  by  rivers:  1.  If  the 
supply  of  water  by  rivers  is  greater  than  the  loss  by  evaporation  from 
the  lake-surface,  then  the  water  will  rise  until,  finding  an  outlet  in  the 
rim  of  the  lake-basin,  it  flows  into  the  sea.  In  this  case  the  lake  will 
remain  fresh,  or  the  quantity  of  saline  matter  will  be  inappreciable. 
But  if,  on  the  other  hand,  the  loss  by  evaporation  is  greater  than  the 
supply  by  rivers,  the  lake  will  decrease  in  extent,  and  therefore  in  evap- 
orating surface,  until  an  equilibrium  is  established.  Now  all  the  saline 
matters  constantly  leached  from  the  earth  accumulate  in  the  lake  with- 
out limit ;  the  lake,  therefore,  must  eventually  become  saturated  with 
saline  matter,  and  afterward  begin  to  deposit  salt.  It  is  evident,  then, 
that  whether  a  lake  is  fresh  or  salt  depends  upon  whether  or  not  it  has 
an  outlet,  and  this  latter  depends  upon  the  relation  of  supply  by  rivers 
to  loss  by  evaporation.  Lakes  are  mostly  fresh,  because  much  more 
water  falls  on  continents  than  evaporates  from  the  same  surface,  the 
excess  running  back  to  the  sea  by  rivers.  It  is  only  in  certain  parts 
of  the  continents,  where  the  climate  is  very  dry,  that  there  is  no  such 
excess.  In  these  regions  alone,  therefore,  can  salt  lakes  exist.  Such 
regions  occur  in  the  interior  of  Asia,  on  the  plateau  of  Mexico,  in  the 
basin  of  Utah,  and  in  several  other  places. 

Even  in  case  a  salt  lake  is  originally  formed  by  the  isolation  of  a 
portion  of  sea-water,  whether  it  remains  a  salt  lake  or  gradually  becomes 
fresh  will  depend  upon  the  conditions  we  have  already  mentioned. 
For  example :  if  the  Mediterranean  should  be  separated  from  the  At- 
lantic at  the  straits  of  Gibraltar,  it  would  not  only  remain  a  salt  lake, 
but  would  diminish  in  area,  and  finally  deposit  salt.  This  we  conclude, 
because  the  water  of  the  Mediterranean  seems  to  be  a  little  more  salt 
than  that  of  the  Atlantic.  If,  on  the  contrary,  the  Black  Sea  were  sepa- 
rated from  the  'Mediterranean,  or  the  Baltic  from  the  Atlantic,  or  the 
bay  of  San  Francisco  from  the  Pacific,  the  supply  by  rivers,  in  the  case 
of  these  inland  seas  being  greater  than  their  loss  by  evaporation,  they 
would  rise  until  they  found  an  outlet,  and  then  would  be  gradually 
rinsed  out,  and  become  fresh.  Lake  Champlain  was,  in  very  recent 
geological  time,  an  arm  of  the  sea.  When  first  isolated  it  was  salt.  It 
has  become  fresh  by  this  process. 

1  The  probability  of  Great  Salt  Lake  having  been  produced  by  simple  evaporation  of 
river-water  is  increased  by  the  difference  in  the  composition  of  the  waters  of  lakes  in 
this  general  region.  Where  sedimentary  rocks  prevail,  as  in  Utah,  they  are  salt ;  where 
volcanic  rocks  prevail,  as  about  Mono  and  Owen,  they  are  alkaline. 


CHEMICAL   DEPOSITS  IX  LAKES.  75 

Deposits  in  Salt  Lakes. — The  nature  of  the  chemical  deposits  in  salt 
lakes  will  depend  upon  the  manner  in  which  these  lakes  have  been 
formed.  We  will  take  the  simplest  case,  viz.,  that  of  a  lake  formed 
by  the  isolation  of  sea-water,  and  its  concentration  by  evaporation.  In 
this  case  the  substance  first  deposited  would  be  gypsum  ;  for  this  siib- 
stance  is  insoluble  in  a  saturated  brine,  and  therefore  always  deposits 
first  in  the  artificial  evaporation  of  sea-water  in  salt-making.  Upon  the 
gypsum  would  be  deposited  salt.  Meanwhile,  however,  the  rivers  dur- 
ing their  flood-season  would  bring  down  sediments.  During  the  flood- 
season,  the  supply  of  water  being  greater  than  loss  by  evaporation,  the 
deposit  of  salt  or  gypsum  would  cease ;  while  during  the  dry  season  the 
deposit  of  sediment  would  cease,  and  the  evaporation  being  now  in  ex- 
cess, the  deposit  of  salt  would  recommence.  Thus  the  deposits  in  the 
bottom  of  salt  lakes  probably  consist  of  alternations  of  salt  and  sedi- 
ment, the  whole  underlaid  by  layers  of  gypsum.  These  views  have 
been  confirmed  by  observation.  During  the  dry  season  Lake  Elton 
deposits  annually  a  considerable  layer  of  salt.  Wells  dug  near  the 
margin  of  this  lake  revealed  100  alternations  of  salt  and  mud,  the  salt- 
beds  being  many  of  them  eight  or  nine  inches  thick.1  Most  of  the  salt 
has  already  deposited ;  for  the  water  of  this  lake  is  an  almost  pure  bit- 
tern. The  great  predominance  of  chloride  of  magnesium  in  Dead  Sea 
water  shows  that  it  is  a  mother-liquor,  from  which  immense  quantities 
of  common  salt  have  already  been  deposited.  Similar  alternations, 
therefore,  no  doubt  exist  in  the  bottom  of  this  sea.2  The  Great  Salt 
Lake,  in  Utah,  is  also  a  saturated  brine  depositing  salt,  as  is  proved  by 
the  incrustations  of  salt  about  its  margin  in  dry  seasons  ;  but  the  de- 
posit has  not  progressed  so  far  in  this  case  as  in  the  preceding.  The 
great  extent  to  which  the  waters  of  this  lake  have  dried  away  and  be- 
come concentrated  is  further  shown  by  old  lake-margins  far  beyond 
the  limits,  and  several  hundred  feet  above  the  level,  of  the  present  shore- 
line. Similar  phenomena  are  observed  about  other  salt  lakes,  especially 
about  the  Caspian  Sea  (Murchison). 

In  the  case  of  salt  lakes,  either  formed  entirely,  or  modified,  by  river- 
water,  the  deposits  are  probably  much  more  complex  and  various — some- 
times salt,  sometimes  carbonate  of  lime,  and  sometimes  sulphate  of  lime. 
This  subject,  however,  has  been  but  little  investigated. 

Deposits  are  also  sometimes  formed  in  lakes  which  are  not  salt.  For 
example :  the  Solfatara  Lake,  Italy,  is  formed  by  the  accumulation  of 
the  water  from  warm  carbonated  springs,  similar  to  those  of  San  Filippo 
and  San  Vignone.  In  this  lake,  therefore,  deposits  of  travertine  are 
forming.  Although  these  deposits  take  place  in  a  lake,  they  properly 
belong  to  deposits  from  springs,  since  they  do  not  take  place  by  concen- 
tration. 

a  Bischof,  "  Chemical  and  Physical  Geology,"  vol.  i.,  p.  405.  2  Ibid.,  p.  400. 


76  IGNEOUS  AGENCIES. 

Chemical  Deposits  in  Seas. 

Concerning  these  little  is  known.  It  is  certain,  however,  that  all 
rivers  carry  to  the  sea  carbonate  of  lime  in  solution,  and  some  of  them 
in  considerable  quantities.  There  is  scarcely  any  river-water  which 
contains  less  carbonate  of  lime  than  sea-water  ;  many  rivers  contain 
four  times  as  much.1  This  carbonate  of  lime  thus  constantly  carried 
into  the  sea  must  eventually  deposit  in  some  form.  Usually,  however, 
sea-water  is  kept  below  the  saturating  point  for  this  substance,  by  its 
constant  withdrawal  by  shells  and  corals,  as  will  be  explained  under  Or- 
ganic Agency.  But  in  shallow  bays  nearly  cut  off  from  the  sea,  or  in 
salt  lagoons  on  the  sea-margin  near  the  mouths  of  rivers  in  dry  climates, 
and  subject  to  occasional  overflows  by  the  sea  and  floodings  by  rivers, 
carbonate  of  lime  and  sulphate  of  lime  may  deposit  by  evaporation.  At 
the  mouths  of  many  rivers,  whose  waters  contain  much  carbonate  of 
lime,  as,  for  instance,  the  Rhine,  the  delta  deposit  is  cemented  into 
hard  rock  by  means  of  this  substance.  On  shores  of  coral  seas,  as 
upon  the  Keys  of  Florida,  the  coast  of  the  West  India  Islands,  and  the 
islands  of  the  Pacific,  shore-material  is  consolidated  into  hard  rock  by 
the  same  means.  On  many  shores  in  tropical  regions,  the  waves,  being 
driven  up  on  flat  beaches  far  inland,  leave  sea-water  inclosed  in  shallow 
pools,  which  by  evaporation  give  rise  to  calcareous  deposits  which  are 
increased  by  the  frequent  alternate  influx  and  evaporation  of  sea-water. 
Conglomerate  rocks  are  thus  forming  at  the  present  time  in  the  Canaries 
and  many  other  places. 


CHAPTER  III. 
IGNEOUS   AGENCIES. 

THE  agencies  thus  far  considered  tend  to  reduce  the  inequalities  of 
the  earth  by  cutting  down  the  continents  and  filling  up  the  seas.  Their 
final  effect,  if  unopposed,  would  be  to  bring  the  whole  surface  to  one 
level,  and  thus  to  make  the  empire  of  the  sea  universal.  This  is  pre- 
vented by  igneous  agencies,  which  tend,  by  elevation  of  land  and  de- 
pression of  sea-bottoms,  to  increase  the  inequalities  of  the  earth-surface, 
and  thus  to  increase  the  area  and  the  height  of  the  land.  All  the  dif- 
ferent forms  of  igneous  agency  are  connected  with  the  interior  heat  of 
the  earth.  This  must,  therefore,  be  first  considered. 

SECTION  1. — INTERIOR  HEAT  OF  THE  EARTH. 

Stratum  of  Invariable  Temperature. — The  mean  surface  temperature 
of  the  earth  varies  from  80°  at  the  equator  to  nearly  0°  at  the  poles. 
1  Bischof,   "  Chemical  and  Physical  Geology,"  vol.  i.,  p.  179. 


INTERIOR  HEAT  OF  THE  EARTH.  77 

The  rate  of  decrease  in  passing  from  the  equator  to  the  poles  is  not  the 
same  in  all  longitudes ;  the  isotherms,  or  lines  joining  places  of  equal 
mean  temperatures,  are  therefore  not  parallel  to  the  lines  of  latitude, 
but  quite  irregular.  The  mean  temperature  of  the  whole  earth-surface 
is  about  58°.  There  is  also  in  every  locality  a  daily  and  an  annual 
variation  of  temperature.  As  we  pass  below  the  surface  both  the  daily 
and  annual  variations  become  less,  until  they  cease  altogether.  The 
stratum  of  no  daily  variation  is  but  a  foot  or  two  beneath  the  surface; 
but  the  stratum  of  no  annual  variation,  or  stratum  of  invariable  temper- 
ature in  temperate  climates,  is  about  sixty  to  seventy  feet  deep.  The 
temperature  of  the  invariable  stratum  is  nearly  the  mean  temperature 
of  the  place.  The  depth  of  the  invariable  stratum  depends  upon  the 
amount  of  annual  variation ;  it  is,  therefore,  least  at  the  equator,  and 
increases  toward  the  poles.  At  the  equator  it  is  only  one  or  two  feet 
beneath  the  surface  ; 1  in  middle  latitudes  about  sixty  feet,  and  in  high 
latitudes  probably  more  than  100  feet.  It  is,  therefore,  a  spheroid  more 
oblate  than  the  earth  itself.  The  temperature  of  the  earth  everywhere 
within  this  spheroid  is  unaffected  by  external  changes. 

Increasing  Temperature  of  the  Interior  of  the  Earth. — Beneath  the 
invariable  stratum  the  temperature  of  the  earth  everywhere  increases, 


a.     o 


for  all  depths  to  which  it  has  been  penetrated,  at  an  average  rate  of  1° 
for  every  53  feet.  This  very  important  fact  has  been  determined  by 
numerous  observations  on  the  temperature  of  mines  and  of  artesian 
wells  in  almost  every  part  of  the  earth.  All  the  facts  thus  far  stated 
are  graphically  illustrated  in  the  accompanying  figure  (Fig.  71),  in 
which  the  line  a  b  represents  depth  below  the  surface,  and  the  diverging 
1  Humboldt,  "Cosmos,"  Sabine's  edition,  vol.  i.,  p.  165. 


78  IGNEOUS  AGENCIES. 

line  c  d  the  increasing  heat ;  m  the  invariable  stratum ;  n  the  line  of  no 
daily  variation ;  the  curves  p  e,  c  e,  o  e,  the  temperatures  in  summer, 
autumn,  and  winter,  respectively ;  the  space  p  e  o  the  annual  swing  of 
temperature;  and  the  smaller  curves  meeting  on  the  line  n,  the  daily 
variation  or  swing  of  temperature. 

We  have  given  the  rate  of  increase  as  about  1°  in  53  feet.  It 
varies,  however,  in  different  places,  from  1°  in  30  feet  to  1°  in  90  feet. 
Except  in  the  vicinity  of  volcanic  action,  this  difference  is  probably 
due  to  varying  conductivity  of  the  rocks.  The  lines,  or  rather  surfaces, 
which  join  places  in  the  interior  of  the  earth,  having  equal  tempera- 
tures, may  be  called  isogeotherms.  If  the  rate  of  increase  were  every- 
where the  same,  the  isogeotherms  would  be  regularly  concentric  ;  but, 
as  this  is  not  the  case,  they  are  irregular  surfaces  (Fig.  72),  rising 
nearer  the  earth-surface,  and  closing  upon  one  another  where  the  con- 
ductivity is  poor,  and  sinking  deeper  and  separating  where  the  con- 
ductivity is  greater. 

Constitution  Of  the  Earth's  Interior. — From  the  facts  given  above 
it  is  probable  that  the  temperature  of  the  interior  of  the  earth  is  very 
great.  A  rate  of  increase  of  1°  for  every  53  feet  would  give  us,  at  the 
depth  of  twenty-five  or  thirty  miles,  a  temperature  sufficient  to  fuse  most 
rocks.  Hence  it  has  been  confidently  concluded  by  many,  that  the 
earth,  beneath  a  comparatively  thin  crust  of  thirty  miles,  must  be  liquid. 
A  crust  of  thirty  miles  on  our  globe  is  equivalent  to  a  crust  of  less  than 
one-tenth  of  an  inch  in  a  globe  two  feet  in  diameter.  There  are,  how- 
ever, many  objections  to  this  conclusion.  The  question  of  the  interior 
constitution  of  the  earth  is  one  of  extreme  difficulty  and  complexity, 
and  science  is  not  yet  in  a  position  to  solve  it  completely.  Neverthe- 
less, it  can  be  proved  that  the  solid  crust  must  be  much  thicker  than  is 
usually  supposed,  if,  indeed,  there  be  any  general  interior  fluid  at  all. 

The  argument  for  the  interior  fluidity  of  the  earth,  beneath  a  crust 
of  only  thirty  miles,  proceeds  upon  two  suppositions,  viz.  :  1.  That  the 
interior  temperature  increases  at  the  same  rate  for  all  depths •  and, 
2.  That  the  fusing -point  of  rocks  is  the  same  for  all  depths.  Now, 
neither  of  these  can  be  true. 

1.  Rate  of  Increase  not  uniform. — Although  we  have  spoken  of  1° 
for  every  30  feet  or  50  feet  or  90  feet,  yet  it  must  not  be  supposed 
that  observation  gives  a  uniform  rate  of  increase  at  any  place.  On  the 
contrary,  the  rate  is  sometimes  faster  and  sometimes  slower,  depending 
on  the  conductivity  of  the  rock  penetrated,  and  on  other  causes  little 
understood.  The  rate  given  is  always  an  average.  In  other  words, 
observation  gives  the  fact  of  increase,  but  not  the  law.  We  are  thus 
thrown  back  on  general  reasoning. 

If  two  bars,  one  a  good  conductor,  like  metal,  and  the  other  a  bad 
conductor,  like  charcoal,  be  heated  red  hot  at  one  end,  and  the  rate  of 


INTERIOR   HEAT   OF  THE  EARTH.  79 

decreasing  temperature — fall  of  heat — toward  the  other  be  observed,  it 
will  be  found  that  the  rate  is  very  rapid  in  the  case  of  the  charcoal ;  so 
that  a  temperature  of  60°  is  reached  at  the  distance  of  two  or  three 
inches,  while  in  the  case  of  the  metal  the  rate  of  decrease  is  much 
slower,  and  60°  is  only  reached  at  a  distance  of  several  feet.  Con- 
versely, the  rate  of  increase,  or  rise,  in  passing  toward  a  source  of 
heat,  is  rapid  in  the  case  of  the  bad  conductor,  and  slow  in  the  case  of 
the  good  conductor.  Now,  the  average  density  of  materials  at  the  sur- 
face of  the  earth  is  about  2.5,  but  the  average  density  of  the  whole 
earth  is  more  than  5.5  ;  therefore  the  density  of  the  central  portions 
must  be  much  more  than  5.5.  It  has  been  estimated  at  16.27.1  There 
can  be  no  doubt,  therefore,  that  the  density  of  the  earth  increases 
toward  the  centre ;  and  as  this  increase  is  probably  largely  the  result 
of  pressure,  it  is  probably  somewhat  regular.  Whatever  be  the  cause, 
the  effect  would  be  to  increase  the  conductivity  for  heat,  and  there- 
fore to  diminish  the  rate  of  increasing  temperature.  Thus  it  follows 
that,  though  in  an  homogeneous  globe  the  melting-point  of  rocks 
(3,000°)  would  be  reached  at  the  depth  of  30  miles,  yet,  in  a  globe 
increasing  in  density  toward  the  centre,  we  must  seek  this  temperature 
at  a  greater  depth. 

If  Alt  (Fig.  73),  representing  depth  from  the  surface  S  S,  be  taken  as 
absciss,  and  heat  be  represented  by  ordi- 
nates,  then,  in  an  homogeneous  earth, 
CD  would  represent  uniform  increase 
of  heat,  and  the  heat  ordinate  of  3,000°, 
m  m,  would  be  reached  at  the  depth  of 
A  m  =  thirty  miles.  But  in  an  earth  in- 
creasing in  density,  and,  therefore,  in 
conductivity,  the  rate  would  not  be  uni- 
form, but  gradually  decreasing.  This 
would  be  represented,  not  by  a  straight 
line,  C  D,  but  by  a  curved  line,  C  E ;  D 
and  the  ordinate  of  3,000°  would  not 
be  reached  at  thirty  miles,  but  at  a 
much  greater  depth— say  at  m',  of  fifty 
miles. 

2.  Fusing-Point  not  the  same  for  all  Depths.— Nearly  all  substances 
expand  in  the  act  of  melting,  and  contract  in  the  act  of  solidifying. 
Only  in  a  few  substances,  like  ice,  is  the  reverse  true.  Now,  the  fusing- 
point  of  all  substances  which  expand  in  the  act  of  fusing  must  be 
raised  by  pressure,  since  the  expanding  force  of  heat,  in  this  case,  must 
overcome  not  only  the  cohesion,  but  also  the  pressure.  That  this  is 

1  "  Cosmos,"  vol.  iv.,  p.  33. 


80  IGNEOUS  AGENCIES. 

true,  has  been  proved  experimentally  for  many  substances  by  Hopkins.1 
But  granite  and  other  rocks  have  been  proved  to  expand  in  fusing ; 
therefore  the  fusing-point  of  rocks  is  raised  by  pressure,  and  must  be 
greatly  raised  by  the  inconceivable  pressure  to  which  they  are  sub- 
jected in  the  interior  of  the  earth.  For  this  reason,  therefore,  we 
must  again  go  deeper  to  find  the  interior  fluid.  In  the  figure,  m'  is  the 
point  where  we  last  found  the  fusing-point  of  3,000°.  But  this  is  the 
fusing-point  on  the  surface,  or  under  atmospheric  pressure.  The  press- 
ure of 'fifty  miles  of  rock  would  certainlv  greatly  raise  the  fusing- 
point.  Suppose  it  is  thus  raised  to  3,500°  :  to  find  this  we  must  go 
still  deeper,  to  m",  perhaps  seventy-five  miles  in  depth.  But  the  in- 
creased pressure  would  again  raise  the  fusing-point ;  and  thus,  in  this 
chase  of  the  increasing  heat  after  the  flying  fusing-point,  where  the 
former  would  overtake  the  latter,  or  whether  it  would  overtake  it  at  all, 
science  is  yet  unable  to  answer. 

From  this  line  of  reasoning,  therefore,  we  conclude  that  the  solid 
crust  of  the  earth  must  be  much  thicker  than  is  usually  supposed,  and 
there  may  be  even  no  interior  liquid  at  all. 

Astronomical  Reasons. — There  is  another  and  an  entirely  different 
line  of  reasoning  which  has  led  some  of  the  best  mathematicians  and 
physicists  to  the  same  result.  According  to  the  thin-crust  theory,  the 
earth  is  still  substantially  a  liquid  globe,  and  therefore  under  the  at- 
tractive influence  of  the  sun  and  moon  it  ought  to  behave  like  a  yielding 
liquid.  Now,  according  to  Hopkins,  Thomson,  and  others,  the  earth  in 
all  its  astronomical  relations  behaves  like  a  rigid  solid — a  solid  more 
rigid  than  a  solid  globe  of  glass — and  the  difference  between  the  be- 
havior of  a  liquid  globe  and  a  solid  globe  could  easily  be  detected  by 
astronomical  phenomena.9  A  complete  exposition  of  the  proof  would 
be  unsuitable  to  an  elementary  work.  Suffice  it  here  to  say  that  the 
force  of  these  arguments  has  led  many  of  the  most  advanced  geologists 
to  the  conclusion  that  the  earth,  if  not  solid  to  the  centre,  must  have  a 
crust  so  thick  that  for  all  purposes  of  the  geologist  it  may  be  regarded 
as  substantially  solid ;  that  volcanoes  are  openings  into  local  masses 
of  liquid,  not  into  a  general  interior  liquid — into  subterranean  fire-lakes, 
not  into  a  universal  Jire-sea — in  a  word,  that  all  the  theories  of  igneous 
phenomena  must  be  reconstructed  on  the  basis  of  a  solid  earth.  A  few 
geologists,  however,  find  a  compromise  in  the  view  that  there  exists  a 
semi-liquid  stratum  between  the  solid  crust  and  a  solid  nucleus. 

The  interior  heat  of  the  earth  manifests  itself  at  the  surface  in  three 
principal  forms,  viz.,  volcanoes,  earthquakes,  and  gradual  oscillations 
of  the  earth's  crust. 

1  American  Journal  of  Science  and  Art,  II.,  vol.  xxxii.,  p.  367. 

8  Thomson  has  recently  reaffirmed  these  conclusions  with  still  greater  positiveness. — 
Nature,  vol.  xiv.,  p.  426 ;  American  Journal  of  Science  and  Art,  vol.  xii.,  p.  336,  1876. 


VOLCANOES.  81 

SECTION  2. — VOLCANOES. 

Definition. — A  volcano  is  usually  a  conical  mountain,  with  a  funnel- 
shaped,  or  pit-shaped,  or  cup-shaped  opening  at  the  top,  through  which 
are  ejected  materials  of  various  kinds,  always  hot  and  often  in  a  fused 
condition.  The  activity  of  volcanoes  is  sometimes  constant,  as  in  the 
case  of  Stromboli,  in  Italy,  from  which  continuous  lava-streams  flow 
("  Cosmos,"  vol.  iv.),  but  more  commonly  intermittent,  i.  e.,  having  pe- 
riods of  eruption  alternating  with  periods  of  more  or  less  complete  re- 
pose. Volcanoes  which  have  not  been  known  to  erupt  during  historic 
times  are  said  to  be  extinct.  It  is  impossible,  however,  to  draw  the 
line  of  distinction  between  active  and  extinct  volcanoes.  Vesuvius, 
until  the  great  eruption  which  overthrew  the  ancient  cities  of  Hercu- 
laneum  and  Pompeii,  was  regarded  as  an  extinct  volcano.  Since  that 
time  it  has  been  very  active. 

Size,  Number,  and  Distribution. — Some  volcanoes  are  among  the 
loftiest  mountains  on  our  globe.  Aconcagua,  in  Chili,  is  23,000  feet, 
Cotopaxi,  in  Peru,  19,660  feet  in  height.  These  volcanic  cones,  how- 
ever, are  situated  on  a  high  plateau ;  their  height,  therefore,  is  not  due  to 
volcanic  eruptions  entirely.  But  Mauna  Loa,  in  Hawaii,  nearly  14,000 
feet,  and  Mount  Etna,  11,000  feet  high,  seem  to  be  due  entirely,  and 
Mount  Shasta,  California,  14,440  feet,  Rainier,  Washington  Territory, 
14,444,  almost  entirely  to  this  cause.  The  crater  of  Mauna  Loa  is  two 
and  a  half  miles  across ;  that  of  Kilauea  three  miles  across  and  1,000 
feet  deep. 

The  number  of  known  volcanoes,  according  to  Humboldt,  is  407, 
and  of  these  225  are  known  to  have  been  active  in  the  last  160  years. 
The  actual  number  is,  however,  probably  much  greater.  It  has  been 
estimated  that,  in  the  archipelago  about  Borneo  alone,  there  are  900  vol- 
canoes.1 The  distribution  of  volcanoes  is  remarkable,  (a.)  They  are 
almost  entirely  confined  to  the  vicinity  of  the  sea.  Two-thirds  of  them 
are  found  on  islands  in  the"  midst  of  the  sea,  and  the  remainder,  with 
the  exception  of  a  few  in  the  interior  of  Asia,  are  near  the  sea-coast. 
Those  on  islands  in  the  sea  probably  commenced,  most  of  them,  at  the 
bottom  of  the  sea,  the  islands  having  been  formed  by  their  agency. 
New  islands  have  been  suddenly  formed  under  the  eye  of  observers  in 
the  Mediterranean.  The  basin  of  the  Pacific  is  the  great  theatre  of 
volcanic  activity,  nearly  seven-eighths  of  all  known  volcanoes  being  situ- 
ated on  its  coasts,  or  on  islands  in  its  midst,  (b.)  Volcanoes  are,  more- 
over, distributed  in  groups  (as  the  Hawaiian  volcanoes,  the  Mediter- 
ranean volcanoes,  the  West  Indian  volcanoes,  the  volcanoes  of  Auvergne, 
etc.),  or  along  extensive  lines  as  if  connected  with  a  great  fissure 
of  the  earth's  crust.  The  most  remarkable  linear  series  of  volcanoes 
1  Herschel,  "  Physical  Geology,"  p.  113. 


82  IGNEOUS  AGENCIES. 

is  that  which  belts  the  Pacific  coast.  Commencing  with  the  Fuegian 
volcanoes  it  runs  along  the  whole  extent  of  the  Andes,  then  along  the 
Cordilleras  of  Mexico,  the  Rocky  Mountains,  then  along  the  Aleutian 
chain  of  .islands,  Kamtchatka,  the  Kurile  Islands,  Japan  Islands,  Philip- 
pines, New  Guinea,  New  Zealand,  to  the  antarctic  volcanoes  Mounts 
Erebus  and  Terror,  thence  back  by  Deception  Island  to  Fuegia  again, 
thus  completely  encircling  the  globe.  (c.)  Volcanoes  are  generally 
formed  in  comparatively  recent  strata.  This  seems  to  be  connected 
with  their  relation  to  the  sea  ;  for  recent  strata  are  abundant  about  the 
sea-coast,  and  the  most  recent  are  now  forming  in  the  bed  of  the  sea. 
The  extinct  volcanoes  of  France  and  Germany  are  in  tertiary  regions. 
Possibly  the  retiring  of  the  sea  has  extinguished  them.  In  the  oldest 
strata  volcanic  activity  has  apparently  died  out  long  ago. 

Phenomena  of  an  Eruption. — The  phenomena  of  an  eruption  are  very 
diverse.  Sometimes  there  is  a  gradual  melting  of  the  floor  of  the  crater, 
and  then  a  rising  and  boiling  of  the  liquid  contents  until  they  quietly  over- 
flow and  form  immense  streams  of  lava,  extending  fifty  to  sixty  miles. 
After  the  eruption,  the  melted  lava  again  sinks  and  cools,  and  solidifies, 
to  form  the  floor  of  the  crater  until  another  eruption.  This  is  the  case 
with  the  Hawaiian  and  many  other  volcanoes  in  the  South  Seas.  In 
other  cases,  as  in  the  Mediterranean  volcanoes,  and  especially  in  many 
in  the  Indian  Ocean,  the  eruption  is  fearfully  explosive.  In  such  cases  the 
eruption  is  usually  preceded  by  premonitory  earthquakes  and  sounds 
of  subterranean  explosions ;  then  the  bottom  of  the  crater  is  blown  out 
with  a  violent  explosion,  throwing  huge  rocky  fragments  to  great  dis- 
tances, often  many  miles ;  then  the  melted  lava  rises  and  overflows  in 
streams  running  down  the  side  of  the  mountain.  The  rise  and  overflow 
of  lava  are  accompanied  with  violent  explosions  of  gas  which  throw  up 
immense  quantities  of  ashes  and  cinders  6,000  and  even  10,000  feet 
above  the  crater.1  In  the  great  eruption  of  Tomboro,  in  the  island  of 
Sumbawa  near  Java,  in  1815,  these  explosions  were  heard  in  Sumatra, 
970  miles  distant.2  The  emission  of  gas  usually  continues  after  all  other 
ejections  cease.  Violent  storms  and  heavy  rain  accompany  the  eruption, 
and  when  the  mountain  reaches  into  the  region  of  perpetual  snow,  as 
in  many  of  the  South  American  volcanoes,  the  fearful  deluges  produced 
by  the  sudden  melting  of  the  snows  are  often  the  most  destructive 
phenomenon  connected  with  the  eruption. 

Volcanic  eruptions,  therefore,  may  be  divided  into  two  great  types, 
viz.,  the  quiet  and  the  explosive.  In  the  one,  lava-flows  predominate ; 
in  the  other,  cinders  and  ashes,  and  steam  and  gas.  The  Hawaiian  vol- 
canoes are  perhaps  the  best  examples  of  the  former,  and  the  Javanese 
volcanoes  of  the  latter.  The  Mediterranean  and  most  other  volcanoes 
.  are  .mixtures  of  these  two  types  in  varying  proportions. 

-1  Dana's  "  Manual,"  p.  692.  *  Lyell,  "  Principles  of  Geology." 


VOLCANOES.  33 

The  quantity  of  materials  ejected  during  an  eruption  is  sometimes 
almost  inconceivable.  During  the  great  eruption  of  Tomboro,  already 
mentioned,  ashes  and  cinders  were  ejected  sufficient  to  make  three 
Mont  Blancs,  or  to  cover  the  whole  of  Germany  two  feet  deep.1  The 
lava  which  streamed  from  Skaptar  Jokull,  Iceland,  in  1783,  has  been 
computed  to  be  equivalent  to  about  twenty-one  cubic  miles,  or  to  the 
whole  quantity  of  water  poured  by  the  Nile  into  the  sea  in  one  year! 
These  were,  however,  very  extraordinary  eruptions.  In  the  greatest 
eruptions  of  Vesuvius  the  quantity  of  lava  poured  out  was  not  more 
than  600,000,000  cubic  feet  =  one  square  mile  covered  twenty-two  feet 
deep.  The  volume  of  lava  poured  out  by  Kilauea,  in  1840,  is  estimated 
by  Dana  as  sufficient  to  cover  one  square  mile  of  surface  800  feet  deep. 

Great  destruction  of  life  is  often  produced  by  volcanic  eruptions. 
The  overthrow  of  Herculaneum  and  Pompeii  by  ejections  from  Vesu- 
vius is  well  known.  The  great  eruption  of  Skaptar  Jokull  destroyed 
1,300  human  lives  and  150,000  domestic  animals.  The  eruption  of 
Etna,  in  1669,  overwhelmed  fourteen  towns  and  villages.  In  the  prov- 
ince of  Tomboro,  out  of  a  population  of  12,000,  only  twenty-six  persons 
escaped  the  great  eruption  of  1815.' 

Monticules. — Eruptions  occur  not  only  from  the  summit-crater,  but 
also  frequently  from  fissures  in  the  side  of  the  mountain.  By  the  im- 
mense upheaving  force  necessary  to  raise  lava  to  the  mouth  of  the 
crater  of  a  lofty  volcano,  the  mountain  is  fissured  by  cracks  radiating 
from  the  crater  in  all  directions.  These  cracks  are  filled  with  lava, 
which  on  hardening  form  radiating  dikes  which  intersect  the  successive 
layers  of  ejections,  and  bind  them  into  a  stronger  mass.  Through  these 
fissures  the  principal  streams  of  lava  often  pass.  During  an  eruption 
of  Mauna  Loa,  in  1852,  the  immense  pressure  of  the  lava  in  the  princi- 
pal crater  fissured  the  side  of  the  mountain,  and  a  fiery  fountain  of 
liquid  lava,  1,000  feet  wide,  was  projected  upward  through  the  fissure 
to  the  height  of  700  feet,  and  continued  to  play  for  several  days.  Upon 
these  fissures  subordinate  craters,  and  finally  cones,  are  formed.  These 
subordinate  cones  about  the  base,  and  upon  the  slopes  of  the  principal 
cone,  are  called  monticules  or  hornitos.  There  are  about  600  monticules 
on  Etna— one  of  them  over  700  feet  high  (Jukes). 

Materials  erupted. — As  we  have  already  stated,  the  materials  erupted 
are  stones,  lava-streams,  cinders,  ashes,  and  gases. 

Stones.— In  explosive  eruptions  the  solid  floor  of  the  crater  is  often 
blown  out  with  violence,  and  rock-fragments,  sometimes  of  great  size, 
are  thrown  to  great  distances. 

Lava. — The  term  lava  is  applied  to  the  liquid  matter  poured  from  a 
volcano  during  eruption,  and  also  to  the  same  when  it  has  hardened 
into  rock. 

1  Herschel,  "Physical  Geology,"  p.  111. 


84  IGNEOUS  AGENCIES. 

Liquid  Lava, — At  the  time  of  eruption  the  liquidity  of  lava  varies 
very  much,  depending  partly  upon  the  heat,  partly  on  the  fusibility  of 
the  material,  and  partly  upon  the  kind  of  fusion.  In  the  Hawaiian 
volcanoes  the  lava  is  a  melted'  glass  almost  as  thin  as  honey.  In  Ki- 
lauea  this  lava  is  often  thrown  into  the  air  by  the  bursting  of  gas-bubbles, 
and  drawn  out  into  long  threads  like  spun  glass,  which  is  carried  by  the 
winds,  and  collects  in  places  as  a  soft,  brownish,  towy  mass,  called 
"  Pele's  hair."  Completely  fused  lava,  when  cooled  rapidly,  forms  vol- 
canic slag  or  volcanic  glass  (obsidian)  ;  but  if  cooled  slowly,  so  that 
the  several  minerals  of  which  it  is  composed  have  time  to  separate  and 
crystallize,  forms  stony  lava.  If  it  is  full  of  gas-bubbles  (rock-froth), 
and  hardens  in  this  condition,  it  forms  vesicular  or  scoriaceous  lava. 
If  the  quantity  of  gas  and  steam  be  very  great,  the  whole  liquid  mass 
may  swell  into  a  rock-froth,  which  rises  to  the  lip  of  the  crater,  and 
outpours  much  as  porter  or  ale  from  a  bottle  when  the  cork  is  drawn. 
Or  the  rock-froth  may  be  thrown  violently  into  the  air,  and,  hardening 
there,  may  fall  again  in  cindery  or  scoriaceous  masses  ;  or,  thrown  with 
still  greater  violence,  the  rock-froth  may  be  broken  into  fine  rock-spray, 
and  fall  as  volcanic  sand  and  ashes'.  Ashes,  when  consolidated  by  time 
and  percolating  water,  form  tufa.  Thus,  there  are  four  physical  condi- 
tions in  which  we  find  lava — viz., stony,  glassy,  scoria ceous,  and  tufaceous. 

Again,  the  liquidity  of  lava  and  its  character  depend  much  on  the 
kind  of  fusion.  Daubree  has  shown  that  all  siliceous  rocks  and  glass 
mixtures,  in  the  presence  of  superheated  water  even  in  small  quanti- 
ties, and  under  pressure,  will  become  more  or  less  liquid,  at  tempera- 
tures far  below  that  necessary  to  produce  true  fusion.  At  400°  Fahr., 
such  rocks  become  pasty ;  at  800°,  completely  liquid.  The  same 
change  takes  place  at  even  lower  temperatures  if  a  little  alkali  be  pres- 
ent. To  distinguish  this  liquidity  from  that  of  true  igneous  fusion, 
which  requires  a  temperature  of  2,500°  to  3,000°,  it  has  been  called  aqueo- 
igneous  fusion.  Now,  very  much  lava  at  the  time  of  eruption  is  in  this 
condition.  Such  lava,  when  the  pressure  is  suddenly  removed  by  break- 
ing up  of  the  floor  of  the  crater,  and  the  contained  water  suddenly 
changed  into  steam,  is  blown  into  the  finest  dust,  which  is  then  carried 
to  great  height  by  the  out-rushing  steam,  and  falls  again  as  volcanic 
ashes,  which  may  consolidate  into  tufa.  If  the  heat  be  not  sufficient 
to  produce  complete  aqueo-igneous  fusion,  the  lava  is  outpoured  as  a 
kind  of  rock-broth,  consisting  of  unfused  particles  in  a  semifused  mass, 
which  concretes  into  an  earthy  kind  of  rock.  Or  the  material  may  pour 
out  only  as  hot  mud,  which  concretes  into  a  kind  of  tufa.  In  fact, 
every  variety  of  fusion  and  semifusion,  depending  on  the  degree  of  heat 
and  the  quantity  of  water,  may  be  traced,  from  perfect  igneous  fusion 
through  various  grades  of  aqueo-igneous  fusion,  to  the  condition  of  hot 
mud. 


VOLCANOES.  85 

It  is  evident  that,  of  the  two  kinds  of  eruption  mentioned  above,  the 
quiet  type  is  characterized  by  igneous  fusion,  the  explosive  type  by 
aqueo-igneous  fusion.  In  the  former  the  heat  is  great,  but  the  amount 
of  water  is  small ;  while  in  the  latter  the  heat  is  less,  but  the  amount 
of  water  far  greater. 

The  rapidity  of  the  flow  of  a  lava-stream  depends  on  its  fluidity.  In 
the  Hawaiian  volcanoes  the  lava,  where  it  issues  from  the  crater,  has  been 
seen  to  flow  with  a  velocity  of  fifteen  miles  an  hour  ;  while  Vesuvian  lava 
seldom  flows  at  a  rate  of  more  than  two  or  three  miles  an  hour.  Lava, 
like  glass,  passes  through  various  grades  of  viscous  fluidity  in  cooling. 
It  gradually  becomes  so  stiff  that  it  may  flow  only  a  few  feet  per  day. 
The  froth  or  scum  which  covers  the  surface  of  a  lava-stream  quickly 
cools  and  hardens  into  a  crust  of  vesicular  lava,  which  may  even  be 
walked  upon  while  the  interior  is  still  flowing  beneath.  In  this  way 
are  often  formed  long  galleries.  Also  the  running  together  of  the  con- 
tained gas-bubbles  and  steam-bubbles  forms  huge  blisters  in  the  viscous 
mass,  which,  on  hardening,  form  cavities  often  of  great  size. 

Hardened  Lava. — Miner  alogically,  lava  consists  essentially  of  feld- 
spar and  augite,  either  intimately  mixed,  as  in  glassy  lava,  or  aggre- 
gated in  more  or  less  distinct  particles  or  crystals,  as  in  the  stony  varie- 
ties. Now,  feldspar  is  a  light-colored  mineral,  having  a  specific  gravity 
of  about  2.5,  while  augite  is  usually  a  very  dark-colored  mineral,  having 
a  specific  gravity  of  about  3.5.  It  is  evident,  therefore,  that  in  propor- 
tion as  feldspar  predominates  the  lava  is  lighter  colored  and  of  less  spe- 
cific gravity,  and  in  proportion  as  augite  predominates  the  rock  is  darker 
and  heavier.  Chemically,  feldspar  is  a  silicate  of  alumina  and  alkali 
(the  latter  being  either  potash  or  soda),  with  excess  of  silica  (acid  sili- 
cate) ;  while  augite  is  a  silicate  of  lime,  magnesia,  and  iron,  with  excess 
of  base  (basic  silicate).  Therefore,  lavas  may  be  divided  into  two 
classes — the  feldspathic  or  acidic  lavas  and  the  augitic  or  basic  lavas. 
Further,  it  is  seen  that  all  lavas  are  multiple  silicates,  like  glass :  they 
are,  therefore,  true  glass-mixtures.  Now,  the  feldspathic  or  acid  lavas 
are  a  more  difficultly  fusible,  the  augitic  or  basic  lavas  a  more  easily 
fusible,  glass-mixture.  Either  of  these  two  kinds  of  lava  may  exist  in 
any  of  the  conditions  mentioned  above — viz.,  as  stony,  glassy,  vesicular, 
or  tufaceous  lava.  Trachyte  is  an  example  of  feldspathic  lava,  and 
basalt  or  dolerite  of  augitic  lava  in  a  stony  condition.  Pumice  is  a 
peculiar  vesicular  variety  of  feldspathic  lava. 

It  is  highly  probable  that  the  fusion  and  subsequent  cooling  of 
granite,  or  gneiss,  or  even  of  the  purer  varieties  of  mixed  sandstones 
and  clays,  would  make  a  feldspathic  or  trachytic  lava ;  while  the  fusion 
and  cooling  of  impure  slates  and  shales  and  limestones  would  produce 
an  augitic  or  basaltic  lava. 

Gas,  Smoke,  and  Flame. — The  gases  emitted  by  volcanoes  are  princi- 


86  IGNEOUS  AGENCIES. 

pally  steam,  sulphurous  vapor  (S  and  SOJ,  hydrochloric  acid,  and  car- 
bonic acid.  By  far  the  most  abundant  of  these  is  steam.  In  violent, 
explosive  eruptions,  which  eject  principally  cinders  and  ashes,  it  is  prob- 
able that  water,  mostly  in  the  form  of  steam,  is  one  of  the  most  abun- 
dant of  all  the  ejected  materials.  In  quiet  lava-eruptions,  like  those  of 
the  Hawaiian  volcanoes,  the  quantity  of  steam  arid  gases  is  small.  It 
is  worthy  of  notice,  in  connection  with  the  position  of  volcanoes  near 
the  sea,  that  the  gases  ejected  are  such  as  might  be  formed  from  sea- 
water  and  from  limestone.  The  so-called  smoke  and  flame  of  volcanoes 
have  no  connection  with  combustion.  The  condensed  vapors  and  the 
ashes  suspended  in  the  air,  often  in  such  quantities  as  to  make  midnight- 
darkness  at  high  noon,  form  the  smoke ;  and  the  red  glare  of  the  same, 
reflecting  the  light  from  the  incandescent  lava  beneath,  forms  the 
apparent  flame. 

All  volcanic  ejections,  except  the  gases,  accumulate  about  the  crater, 
and  continue  to  increase  with  every  successive  eruption,  forming  a  sort 
of  stratified  deposit.  Sometimes  the  cone  is  made  up  of  successive  lay- 
ers of  lava,  as  in  Hawaiian  volcanoes ;  sometimes  it  is  made  up  of  suc- 
cessive layers  of  cinders  or  tufa ;  sometimes  of  alternate  layers  of  lava 
and  tufa.  Stratified  materials  of  this  kind,  however,  cannot  be  con- 
founded with  those  produced  by  the  action  of  water.  In  the  former  case 
the  stratification  is  not  the  result  of  the  sorting  of  the  materials. 

Kinds  of  Volcanic  Cones. — Volcanic  cones  and  craters  have  been 
divided  into  two  kinds — viz.,  cones  of  elevation  and  cones  of  eruption. 
A  cone  of  elevation  is  formed  by  interior  forces  lifting  the  crust  of  the 
earth  at  a  particular  point  until  the  latter  breaks  and  forms  a  crater, 
through  which  eruptions  take  place.  It  is  an  earth-bubble  which  swells 
and  breaks  at  the  top.  A  cone  of  eruption,  on  the  other  hand,  is 
formed  by  the  accumulation  around  a  crater  of  its  own  ejections.  There 
has  been  much  discussion  among  physical  geologists  as  to  whether 
existing  volcanic  cones  are  formed  mostly  by  the  one  method  or  the 
other.  We  will  not  enter  into  this  discussion.  It  seems  probable, 
however,  that  most  cones  are  principally  cones  of  eruption,  although 
their  height  and  size  have  been  increased  somewhat  also  by  elevating 
forces. 

Mode  of  Formation  of  a  Volcanic  Cone.— A  volcano  commences— 1. 
As  a  simple  opening  in  the  earth's  crust,  in  most  cases  with  little  or  no 


FIG.  74.— Section  across  Hawaii. 


VOLCANOES. 


elevation.  Through  this  opening  or  crater  are  ejected,  from  time  to 
time,  lava,  cinders,  ashes,  etc.,  which  accumulate  immediately  about  the 
crater,  and  continue  to  increase,  by  successive  layers,  with  every  erup- 
tion. Ejections  of  pure  lava,  particularly  if  the  lava  is  very  fluid, 
form  a  cone  of  broad  base  and  low  inclination.  This  is  the  case  with 
the  Pacific  volcanoes.  Fig.  74  is  a  section  through  Hawaii,  show- 
ing the  slope  of  the  pure  lava-cones  of  Mauna  Loa  (Z),  nearly  14,- 
000  feet  high,  and  of  Mauna  Kea  (K).  Tufa -cones  and  cinder-cones 
(Fig.  75)  take  a  much  higher  angle 
of  slope.  2.  With  every  eruption  the 
powerful  internal  forces  fissure  the 
mountain,  in  lines  radiating  from  the 
crater.  These  fissures  are  filled  with 
liquid  lava,  which,  on  hardening,  forms 

e'  FIG.  75.— Section  of  Cinder-Cone. 

radiating  dikes,  intersecting  the  lay- 
ers of  ejections,  and  binding  them  into  a  more  solid  mass.     Fig.  76 
shows  how  these  dikes,  rendered  more  visible  by  erosion,  intersect  the 


at  the  Base  of  the  Serra  del  Solflzio,  Etna. 


strata.  3.  After  a  time,  when  the  mountain  has  grown  to  considerable 
height,  the  force  necessary  to  raise  liquid  lava  to  the  lip  of  the  crater 
becomes  so  great  that  it  breaks  in  preference  through  the  fissured  sides 
of  the  mountain.  The  secondary  craters  thus  formed  immediately  com- 
mence to  make  accumulations  around  themselves,  and  thus  form  second- 


88 


IGNEOUS  AGENCIES. 


ary  cones  (Fig.  77,  c'),  or  monticules,  about  the  base  and  on  the  sides 
of  the  primary  cone.  If  a  secondary  cone  becomes  extinct,  it  is  finally 
buried  (Fig.  77,  c")  in  the  layers  of  the  primary  cone.  4.  Finally,  in 


FIG.  77.— Section  of  Volcano,  showing  Monticules. 


volcanoes  of  the  explosive  type,  during  great  eruptions  the  whole  top 
of  the  mountain  is  often  blown  off,  and  in  volcanoes  of  the  quieter  type 
is  melted  and  falls  in — in  either  case  forming  an  immense  crater,  within 
which,  by  subsequent  eruptions,  another  smaller  cone  of  eruption  is 


FIG.  78.— Section  of  Vesuvius  atod  Mount  Somma. 


built  up,  and  in  this  latter  often  a  still  smaller  cone  is  again  built. 
This  cone-within-cone  structure  is  well  illustrated  by  the  present  condi- 
tion, and  still  better  by  the  history,  of  Vesuvius.  Vesuvius  is  a  double- 
peaked  mountain,  with  a  deep,  semicircular  valley  between  the  peaks. 


FIG.  79.— Mount  Vesuvius  in  1756  (after  Scrope). 

The  present  active  cone  of  Vesuvius  is  encircled  by  a  rampart,  very  high 
on  one  side,  and  called  Mount  Somma,  but  traceable  to  some  degree  all 
around,  and  having  the  same  structure  as  Vesuvius  itself.  This  rampart 


VOLCANOES.  89 

is  the  remains  of  a  great  crater,  many  miles  in  diameter.  Fig.  78 
is  an  ideal  section  through  Mount  Somma  ($),  and  Vesuvius  (V}. 
S'  is  the  almost  obliterated  remains  of  the  old  crater  on  the  other 
side.  This  is  further  and  beautifully  illustrated  by  the  history  of  this 
mountain,  which  records  the  repeated  destruction  and  rebuilding  of 
these  cones  within  cones.  Fig.  79  is  an  outline  of  Vesuvius  as  it  existed 
in  1756 ; 1  S  is  Mount  Somma. 

Many  other  volcanoes  are  known  which  have  similar  circular  ram- 
parts made  up  of  layers  of  volcanic  ejections.  One  of  the  most  remark- 
able of  these  is  Barren  Island,  in  the  bay  of  Bengal  (Fig.  80).  The 


FIG.  80.— Section  of  Barren  Island. 

difference  between  this  and  Vesuvius  is,  that  the  circle  is  more  com- 
plete, and  the  valley  between  it  and  the  present  cone  is  occupied  by 
the  sea. 

Comparison  between  a  Volcanic  Cone  and  an  Exogenous  Tree.— It 
is  evident,  then,  that  a  cone  of  eruption  grows  by  layers  successively 
applied  on  the  outside.  Both  in  structure  and  growth  it  may,  there- 
fore, be  compared  to  an  exogenous  tree :  1.  As  the  sap  ascends 
through  the  centre  of  the  shoot  and  descends  on  the  outside,  forming 
layers  of  wood,  one  outside  of  the  other,  increasing  every  year  the 
height  and  the  diameter  of  the  tree,  so  in  a  volcano  lava  ascends  through 
the  centre  and  pours  over  the  outside,  forming  also  successive  layers, 
increasing  both  the  diameter  and  the  height.  2.  As  a  cross-section  of 
a  tree  shows  concentric  rings  around  (Fig.  81)  a  central  pith,  and  trav- 
ersed by  pith-rays,  so  a  cross-section  of  a  volcano 
would  show  a  central  crater,  with  concentric  layers, 
traversed  by  radiating  dikes.  3.  As  on  the  pith-rays, 
where  they  emerge  upon  the  surface,  arise  buds,  which 
grow  in  a  manner  similar  to  the  trunk,  so  on  the  radi- 
ating dikes  are  formed  monticules,  which  grow  like  the 
principal  cone.  If  buds  die,  they  are  covered  up  in  the  Fl«-  si- 

annual  layers  of  the  trunk ;  so,  in  like  manner,  extinct  monticules  are 
buried  in  the  layers  of  the  principal  cone. 

Estimate  of  the  Age  Of  Volcanoes. — The  age  of  exogenous  trees,  as 
is  well  known,  may  be  estimated  by  counting  the  annual  rings.  The 
age  of  volcanoes  cannot  be  estimated  accurately  in  a  similar  manner  : 
1.  Because  the  overflows  are  not  regularly  periodical ;  2.  Because 

1  Scrope,  Philosophical  Magazine,  vol.  xiv.,  p.  139. 


90  IGNEOUS   AGENCIES. 

in  the  case  of  lava-overflows  it  requires  many  overflows  to  make  one 
complete  layer  ;  and,  3.  Because  it  is  impossible  to  make  a  complete 
section  of  the  mountain.  Nevertheless,  Nature  gives  us  partial  sec- 
tions, which  reveal  an  almost  incalculable  antiquity.  Thus,  the  Val  de 
Bove,  of  Etna  (a  huge  valley  reaching  from  near  the  summit  to  the 
foot,  and  probably  formed  bv  an  engulfment  of  a  portion  of  the  moun- 
tain), gives  a  perpendicular  section  into  the  heart  of  the  mountain 
3,000  feet  deep.  Throughout  the  whole  of  this  section  the  mountain 
is  composed  entirely  of  layers  of  lava  and  cinders.  It  is  almost  cer- 
tain, therefore,  that  the  whole  mountain  to  its  very  base,  11,000  feet, 
is  similarly  composed.  That  the  time  necessary  to  accumulate  this  im- 
mense pile,  11.000  feet  high  and  ninety  miles  in  circumference  at  the 
base,  was  almost  inconceivably  great,  is  shown  by  the  fact  that  Etna 
had  already  attained  very  nearly  its  present  size  and  shape  2,500  years 
ago,  when  it  was  observed  by  the  early  Greek  writers.  The  lava-stream 
which  stopped  the  Carthaginians  in  their  march  against  Syracuse,  396 
years  before  Christ,  may  still  be  seen  at  the  surface,  not  yet  covered  by 
subsequent  eruptions.  And  yet  Etna  belongs  to  the  most  recent  geo- 
logical epoch,  for  it  has  broken  through,  and  is  built  upon,  the  newer 
tertiary  strata. 

Theory  of  Volcanoes. 

In  the  theory  of  volcanoes  there  are  two  things  to  be  accounted  for, 
viz. :  1.  The  force  necessary  to  raise  melted  lava  to  the  crater,  and  even 
to  project  it  with  violence  high  into  the  air ;  2.  The  heat  necessary  to 
fuse  rocks  and  form  lava. 

Force. — The  specific  gravity  of  lava  being  about  2.5  to  3,  it  would 
require  the  pressure  of  one  atmosphere,  or  fifteen  pounds  to  the  square 
inch,  for  every  eleven  or  twelve  feet  of  vertical  elevation  of  the  liquid 
mass.  The  following  table  gives  the  pressure  in  atmospheres  for  four 
well-known  volcanoes,  assuming  the  point  of  hydrostatic  equilibrium 
to  be  at  the  sea  level : 


NAME. 

Height. 

Pressure  in  Atmospheres. 

3  900  feet 

325 

Etna 

11  000    " 

920 

13  800     " 

1  150 

Cotopaxi  

19,660     " 

1.638 

The  lava  is  often,  however,  in  a  frothy  orVesicular  condition.  In  such 
cases  the  pressure  necessary  to  produce  overflow  would  be  much  less. 
But,  on  the  other  hand,  the  force  in  most  cases  is  not  only  sufficient  to 
lift  lava  to  the  top  of  the  crater,  but  to  project  it  thousands  of  feet  in 
the  air.  A  rock -mass  of  over  2,700  cubic  feet  was  projected  from  the 
crater  of  Cotopaxi  to  a  distance  of  nine  miles  (Lyell).  The  agent  of 


VOLCANOES.  91 

this  prodigious  force  is  evidently  gas  and  vapors,  especially  steam.  The 
great  quantity  of  steam  issuing  from  all  volcanoes,  but  especially  from 
those  of  the  explosive  type,  is  sufficient  proof.  Thus  far  theorists 
generally  agree,  but  from  this  point  opinions  diverge  into  the  most  op- 
posite directions. 

The  Heat. — There  are  many  and  diverse  opinions  as  to  the  source  of 
the  heat  associated  with  volcanic  eruptions.  Two  prominent  views, 
however,  may  be  said  to  divide  geologists.  According  to  the  one,  the 
heat  is  the  remains  of  the  primal  heat  of  the  once  universally  incandes- 
cent earth  ;  according  to  the  other,  the  heat  is  produced  by  chemical 
or  mechanical  action.  According  to  the  former,  the  heat  is  general,  and 
only  the  access  of  water  is  local ;  according  to  the  latter,  both  the  heat 
and  the  access  of  water  are  local.  According  to  the  former,  volcanoes 
are  openings  through  the  comparatively  thin  crust,  revealing  the  uni- 
versal interior  fluid  ;  according  to  the  latter,  they  are  openings  into 
isolated  interior  lakes  of  molten  matter.  The  former  may  be  called  the 
"  interior  fluidity  "  theory ;  the  latter  divides  into  two  branches,  which 
may  be  called  respectively  the  "  chemical  "  and  the  "  mechanical  "  the- 
ory. In  all,  access  of  water  to  the  hot  interior  furnishes  the  force. 

Internal  Fluidity  Theory. — This  theory  supposes  that  the  earth,  from 
its  original  incandescent  condition,  slowly  cooled  and  formed  a  surface- 
crust  ;  that  this  surface-crust,  though  ever  thickening  by  additions  to 
its  interior  surface,  is  still  comparatively  very  thin,  and  beneath  it  is 
still  the  universal  incandescent  liquid  ;  that  by  movements  of  the  sur- 
face the  solid  crust  is  fissured,  and  water  from  the  sea  or  from  other 
sources  finds  its  way  to  the  incandescent  liquid  mass,  and  develops 
elastic  force  sufficient  to  produce  eruption. 

By  this  view  the  focus  of  volcanoes  is  situated  at  the  lower  limit  of 
the  solid  crust.  The  theory  seems  clear  and  simple  enough,  but  when 
closely  examined  there  are  many  difficulties  in  the  way  of  its  accept- 
ance. 

Objections. — The  objections  to  this  view  are  :  1.  That  the  crust,  as 
already  shown,  must  be  far  thicker  than  this  theory  requires,  probably 
hundreds  of  miles  thick,  if,  indeed,  there  be  any  general  liquid  interior 
at  all ;  but  volcanoes  are  evidently  very  superficial  phenomena.  Under 
the  pressure  of  this  difficulty  these  theorists  have  been  driven  to  the 
acknowledgment  of  local  thinnings  of  the  solid  crust  in  the  region  of 
volcanoes. 

2.  Pressure  on  a  general  interior  liquid  from  any  cause  at  any  place 
would,  by  the  law  of  hydrostatics,  be  transmitted  equally  to  every  part 
of  the  crust,  which  would,  therefore,  yield  at  the  weakest  point,  wher- 
ever that  may  be,  even  though  it  be  on  the  opposite  side  of  the  globe  ; 
but  the  force  of  volcanic  eruption  is  evidently  just  beneath  the  volcano. 

3.  Volcanoes  belonging  to  the  same  group,  and  therefore  near  to- 


92  IGNEOUS  AGENCIES. 

gether,  often  erupt  independently,  as  if  each  had  its  own  reservoir  of 
liquid  matter.  The  pressure  of  these  two  objections  has  driven  many 
to  the  admission  of  a  sort  of  honey-combed  remains  of  the  interior 
liquid  inclosed  in  the  solid  crust,  and  now  isolated  both  from  the  in- 
terior liquid  and  from  each  other. 

4.  There  is  a  limit  to  the  descent  of  water  into  the  interior  of  the 
earth ;  gravity  urges  it  downward,  but  the  interior  heat  drives  it 
back.  The  limit,  therefore,  will  be  where  these  two  balance  each 
other,  i.  e.,  where  the  elastic  force  of  steam  is  equal  to  the  superincum- 
bent columja  of  water.  We  will  call  this  point  the  limit  of  volcanic 
waters.  It  is  evident  that  when  water  was  first  condensed  on  the  cool- 
ing earth,  this  limit  was  at  the  surface :  water  could  not  penetrate  at 
all.  As  the  earth  cooled,  this  limit  became  deeper  and  deeper ;  and,  if 
the  earth  became  perfectly  cool  to  the  centre,  there  is  little  doubt  that 
the  whole  of  the  water  on  the  earth  would  be  absorbed.  This  is  per- 
haps the  case  with  the  moon  now. 

Now,  it  seems  probable  that  at  the  limit  of  volcanic  water  the  in- 
terior heat  of  the  earth,  increasing  at  the  rate  of  1°  for  every  fifty  feet, 
would  be  far  short  of  the  temperature  necessary  for  igneous  fusion  of 
rocks.  Again,  the  elastic  force  necessary  to  sustain  the  superincum- 
bent water  would  by  no  means  be  sufficient  to  break  up  the  crust  of 
the  earth,  or  raise  melted  lava  to  the  surface. 

But  we  will  not  pursue  this  subject,  as  it  is  too  complex  to  be  yet 
solved  by  science.  We  rely,  therefore,  on  the  first  three  objections. 

Chemical  Theory. — Whether  or  not  the  earth  consist  of  solid  crust 
covering  an  interior  liquid,  it  almost  certainly  consists  of  an  oxidized 
crust  covering  an  unoxidized  interior.  Now,  the  oxidizing  agents  are 
water  and  air,  and  therefore  the  limit  of  the  oxidized  crust  is  the  limit 
of  volcanic  water.  Therefore,  the  oxidizing  agent  and  the  unoxidized 
material  are  in  close  proximity,  and  the  former  ever  encroaching  on  the 
latter,  and  therefore  liable  at  any  moment  to  set  up  chemical  action, 
the  intensity  of  which  would  vary  with  the  nature  of  the  material.  If 
the  action  be  intense,  heat  may  be  formed  sufficient  to  fuse  the  rocks 
and  to  develop  elastic  force  necessary  to  produce  eruption. 

In  this  general  form,  the  chemical  theory  seems  plausible,  but  many 
have  attempted  to  give  it  more  definiteness,  and  to  explain  the  special 
forms  of  oxidization  which  cause  volcanoes.  The  most  celebrated  of 
these  definite  forms  is  that  of  Sir  Humphry  Davy,  who  attributed  it  to 
the  contact  of  water  with  metallic  potassium,  sodium,  calcium  and 
magnesium,  in  the  interior  of  the  earth.  In  such  definite  forms  the 
theory  seems  far  too  hypothetical. 

Recent  Theories. — 1.  Aqueo-igneous  Theory. — Accumulation  of 
sediment  on  sea-bottoms  would  necessarily  produce  corresponding  rise 
of  isogeotherms,  and  thus  the  interior  heat  of  the  earth  would  invade 


VOLCANOES.  93 

the  sediments  with  their  contained  waters.  The  lower  portion  of 
sediments  10,000  feet  thick  would  be  raised  to  a  temperature  of  about 
260°,  and  of  40,000  feet  thick  (sediments  of  this  thickness  and  more  are 
known)  to  that  of  860°.  This  temperature,  or  even  a  less  temperature 
if  alkali  be  present,  would  be  sufficient  in  the  presence  of  the  contained 
water  of  the  sediments  to  produce  complete  aqueo-igneous  fusion,  and 
probably  to  develop  elastic  force  sufficient  to  produce  eruption.  This 
view  was  first  brought  forward  by  John  HerscheH  Observe  that  this 
temperature  and  the  corresponding  force  would  be  gradually  developed 
as  the  accumulation  progressed,  until  sufficient  to  produce  these  effects. 
Observe,  again,  that  in  this  case  the  water  does  not  seek  the  heat  by 
descending,  but  the  heat  seeks  the  already  imprisoned  water  by  as- 
cending. 

It  seems  very  probable  that  cases  of  eruption  of  hot  mud  and  of 
aqueo-igneously  fused  lavas  may  be  accounted  for  in  this  way,  but  the 
temperature  would  not  be  sufficient  to  account  for  true  igneous  fusion. 

Some  geologists  go  much  further,  and,  supposing  that  the  whole  sur- 
face of  the  earth  consists  of  sedimentary  rocks  of  great  thickness,  im- 
agine that  between  the  solid  surface  and  a  solid  nucleus  there  exists  a 
continuous  layer  of  aqueo-igneously  fused  matter  which  is  the  seat  of 
igneous  activity. 

2.  Mechanical  Theory. — As  we  shall  explain  hereafter  (p.  252),there 
is  much  reason  to  believe  that  the  interior  of  the  earth  is  contracting 
more  rapidly  than  the  exterior,  and  that  the  exterior  is  thus  necessarily 
thrust  upon  itself  by  irresistible  horizontal  pressure.     According  to  Mr. 
Mallet,  the  crushing  of  the  rocky  crust  in  places  under  this  pressure 
develops  heat  sufficient  to  fuse  the  rocks,  and  to  produce  eruption. 

3.  Issuing  of  Super-heated  Gases. — Very  recently  Rev.  O.  Fisher 
has  advanced  a  view  which  deserves  attention.     He  thinks  volcanoes 
are  vents  through  which  issue  from  the  earth's  interior  super-heated 
steam  and  gases,  melting  the  rocks  in  their  course  and  ejecting  them  by 
their  pressure.     According  to  this  view,  the  water  is  not  derived  from 
the  surface,  but  is  original  and  constituent.     This  view  is  independent 
of  the  condition  of  the  earth's  interior,  whether  solid  or  liquid ;  for  a 
temperature  which  would  permit  solidity  at  great  depths  would  produce 
fusion  under  less  pressure  near  the  surface.1     The  sun  may  be  regarded 
as  a  globe  in  an  earlier  and  more  active  stage  of  vulcanism.     From  its 
interior  gases  are  seen  to  issue  in  great  quantity,  and  almost  constantly. 

The  complete  development  of  these  later  theories  cannot  be  under- 
taken in  this  part  of  our  treatise.  We  will  take  the  subject  up  again 
under  the  head  of  Mountain  Formation  (p.  240). 

Cambridge  Philosophical  Society,  1875. 


94  IGNEOUS  AGENCIES. 

Subordinate   Volcanic  Phenomena. 

These  are  hot  springs,  carbonated  springs,  solfataras,  fumaroles, 
mud-volcanoes,  and  geysers.  They  are  all  secondary  phenomena,  i.  e., 
formed  by  the  percolation  of  meteoric  water  through  hot  volcanic  ejec- 
tions. Or  perhaps  in  some  cases  the  heat  may  be  produced  by  slow 
rock-crushing  by  horizontal  pressure,  as  explained  above. 

General  Explanation.— Thick  masses  of  lava  outpoured  from  vol- 
canoes remain  hot  in  their  interior  for  an  incalculable  time.  Water 
percolating  through  these  acquires  their  heat,  and  comes  up  again  as  hot 
springs ;  or,  if  it  contains  carbonic  acid,  as  carbonated  springs ;  or,  if  it 
contains  sulphurous  acid  and  sulphureted  hydrogen,  as  solfataras.  If 
condensable  vapors  issue  in  abundance  so  as  to  make  an  appearance  of 
smoke,  they  are  called  fumaroles.  If  the  hot  water  brings  up  with  it 
mud  which  accumulates  about  the  vent,  then  it  is  a  mud-spring  or  a 
mud-volcano.  If  the  heat  is  very  great,  so  that  violent  eruption  of  water 
takes  place  periodically,  then  it  becomes  a  geyser.  This  is  the  onty  one 
which  need  detain  us. 

Geysers. 

A  geyser  may  be  defined  as  a  periodically  eruptive  spring.  They 
are  found  only  in  Iceland,  in  the  Yellowstone  Park,  United  States,  and  in 
New  Zealand.  The  so-called  geysers  of  California  are  rather  fumaroles. 
Those  of  Iceland  have  been  long  studied  ;  we  will,  therefore,  describe 
these  first. 

Iceland  is  an  elevated  plateau  about  two  thousand  feet  high,  with  a 
narrow  marginal  habitable  region  sloping  gently  to  the  sea.  The  ele- 
vated plateau  is  the  seat  of  every  species  of  volcanic  action,  viz.,  lava- 
eruptions,  solfataras,  mud-volcanoes,  hot  springs,  and  geysers.  These 
last  exist  in  great  numbers ;  more  than  one  hundred  are  found  in  a 
circle  of  two  miles  diameter.  One  of  these,  the  Great  Geyser,  has  long 
attracted  attention. 

Description. — The  Great  Geyser  is  a  basin  or  pool  fifty-six  feet  in 
diameter,  on  the  top  of  a  mound  thirty  feet  high.  From  the  bottom  of 
the  basin  descends  a  funnel-shaped  pipe  eighteen  feet  in  diameter  at 
top,  and  seventy-eight  feet  deep.  Both  the  basin  and  the  tube  are  lined 
with  silica,  evidently  deposited  from  the  water.  The  natural  inference 
is,  that  the  mound  is  built  up  by  deposit  from  the  water,  in  somewhat 
the  same  manner  as  a  volcanic  cone  is  built  up  by  its  own  ejections. 
In  the  intervals  between  the  eruptions  the  basin  is  filled  to  the  brim 
with  perfectly  transparent  water,  having  a  temperature  of  about  170° 
to  180°. 

Phenomena  Of  an  Eruption. — 1.  Immediately  preceding  the  erup- 
tion sounds  like  cannonading  are  heard  beneath,  and  bubbles  rise  and 
break  on  the  surface  of  the  water.  2.  A  bulging  of  the  surface  is  then 


GEYSERS. 


95 


seen,  and  the  water  overflows  the  basin.  3.  Immediately  thereafter 
the  whole  of  the  water  in  the  tube  and  basin  is  shot  upward  one  hundred 
feet  high,  forming  a  fountain  of  dazzling  splendor.  4.  The  eruption  of 
water  is  immediately  followed  by  the  escape  of  steam  with  a  roaring 


noise.  These  last  two  phenomena  are  repeated  several  times,  so  that 
the  fountain  continues  to  play  for  several  minutes,  until  the  water  is 
sufficiently  cooled,  and  then  all  is  again  quiet  until  another  eruption. 


96 


IGXEOUS  AGENCIES. 


The  eruptions  occur  tolerably  regularly  every  ninety  minutes,  and  last 
six  or  seven  minutes.  Throwing  large  stones  into  the  tube  has  the 
effect  of  bringing  on  the  eruption  more  quickly. 

Yellowstone  Geysers.— In  magnificence  of  geyser  displays,  however, 
Iceland  is  far  surpassed  by  the  geyser  basin  of  Firehole  River.  This 
wonderful  geyser  region  is  situated  in  the  northwest  corner  of  Wyo- 
ming, on  an  elevated  volcanic  plateau  near  the  head-waters  of  the  Madi- 
son River,  a  tributary  of 
the  Missouri,  and  of  the 
Snake  River,  a  tributary 
of  the  Columbia.  The 
basin  is  only  about  three 
miles  wide.  About  it  are 
abundant  evidences  of  pro- 
digious volcanic  activity 
in  former  times,  and,  al- 
though primary  volcanic 
activity  has  ceased,  sec- 
ondary volcanic  phenom- 
ena are  developed  on  a 
stupendous  scale  and  of 
every  kind,  viz.:  hot 
springs,  carbonated  springs,  fumaroles,  mud-volcanoes,  and  geysers. 
In  this  vicinity  there  are  more  than  10,000  vents  of  all  kinds.  In 
some  places,  as  on  Gardiner's  River,  the  hot  springs  are  mostly  lime- 
depositing  (p.  71) ;  in  others,  as  on  Firehole  River,  they  are  geysers 
depositing  silica. 

In  the  upper  geyser  basin  the  valley  is  covered  with  a  snowy  de- 


FIG.  83.-(After  Hayden.) 


FIG.  84.— The  Turban  (after  Hayden). 


posit  from    the  hot  geyser-waters.      The  surface  of  the   mound-like, 
chimney-like,  and  hive-like   elevations,  immediately  surrounding  the 


GEYSERS. 


97 


vents,  is,  in  some  cases,  ornamented  in  the  most  exquisite  manner  by- 
deposits  of  the  same,  in  the  form  of  scalloped  embroidery  set  with 
pearly  tubercles ;  in  others,  the  siliceous  deposits  take  the  most  fan- 


Fio.  85.— Giant  Geyser  (after  Hay  den). 


tastic  forms  (Figs.  82,  83,  84).  In  some  places  the  silica  is  deposited  in 
large  quantities,  three  or  four  inches  deep,  in  a  gelatinous  condition 
like  starch-paste.  Trunks  and  branches  of  trees  immersed  in  these  wa- 
ters are  speedily  petrified. 

7 


98 


IGNEOUS  AGENCIES. 


We  can  only  mention  a  few  of  the  grandest  of  these  geysers  : 
1.  The  "  Grand  Geyser,"  according  to  Hayden,  throws  up  a  column 
of  water  six  feet  in  diameter  to  the  height  of  200  feet,  while  the  steam 
ascends  1,000  feet  or  more.  The  eruption  is  repeated  every  thirty- 
two  hours,  and  lasts  twenty  minutes.  In  a  state  of  quiescence  the 
temperature  of  the  water  at  the  surface  is  about  150°. 


FIG.  86. — Bee-Hive  Geyser  (from  a  Drawing  by  Holmes). 


2.  The  "  Giantess  "  throws  up  a  large  column  twenty  feet  in  diame- 
ter to  a  height  of  sixty  feet,  and  through  this  great  mass  it  shoots  up 
five  or  six  lesser  jets  to  a  height  of  250  feet.  It  erupts  about  once  in 
every  eleven  hours,  and  plays  twenty  minutes. 


GEYSERS. 


99 


3.  The  "  Giant "  (Fig.  85)  throws  a  column  five  feet  in  diameter 
140  feet  high,  and  plays  continuously  for  three  hours. 

4.  The  "  Bee-Hive  "  (Fig.  86),  so  called  from  the  shape  of  its  mound, 
shoots  up  a  splendid  column  two  or  three  feet  in  diameter  to  the  height 
by  measurement  of  219  feet,  and  plays  fifteen  minutes. 

5.  "  Old  Faithful,"  so  called  from  the  frequency  and  regularity  of  its 
eruptions,  throws  up  a  column  six  feet  in  diameter  to  the  height  of  100 
to  150  feet  regularly  every  hour,  and  plays  each  time  fifteen  minutes. 


FIG.  ST.— Forms  of  Geyser-Craters  (after  Hayden). 

Theories  of  Geyser-Eruption. — The  water  of  geysers  is  not  volcanic 
water,  but  simple  spring-water.  A  geyser  is  not,  therefore,  a  volcano 
ejecting  water,  but  a  true  spring.  There  has  been  much  speculation 
concerning  the  cause  of  their  truly  wonderful  eruptions. 

Mackenzie's  Theory. — According  to  Mackenzie,  the  eruptions  of  the 
Great  Geyser  may  be  accounted  for  by  supposing  its  pipe  connected 
by  a  narrow  conduit  with  the  lower  part  of  a  subterranean  cave, 
whose  walls  are  heated  by  the  near  vicinity  of  volcanic  fires.  Fig. 
88  represents  a  section  through  the  basin,  tube  and  supposed  cave. 
Now,  if  meteoric  water  should  run  into  the  cave  through  fissures  more 
rapidly  than  it  can  evaporate,  it  would  accumulate  until  it  rose  above, 
and  therefore  closed,  the  opening  at  a.  The  steam,  now  having  no  out- 
let, would  condense  in  the  chamber  b  until  its  pressure  raised  the  water 
into  the  pipe,  and  caused  it  to  overflow  the  basin.  The  pressure  still 
continuing,  all  the  water  would  be  driven  out  of  the  cave,  and  partly 
up  the  pipe.  Now,  the  pressure  which  sustained  the  whole  column  a  d 


100 


IGNEOUS  AGENCIES. 


would  not  only  sustain,  but  eject  with  violence,  the  column  c  d.  The 
steam  would  escape,  the  ejected  water  would  cool,  and  a  period  of  qui- 
escence would  follow.  If  there  were  but  one  geyser  in  Iceland,  this 
would  be  rightly  considered  a  very  ingenious  and  probable  hypoth- 
esis, for  without  doubt 
we  may  conceive  of  a 
cave  and  conduit  so  con- 
structed as  to  account 
for  the  phenomena.  But 
there  are  many  eruptive 
springs  in  Iceland,  and 
it  is  inconceivable  that 
all  of  them  should  have 
caves  and  conduits  so 
peculiarly  constructed. 
This  theory  is  therefore 
entirely  untenable. 


FIG.  88.— Mackenzie's  Theory  of  Eruption. 


Bunsen's  Investigations. — The  investigations  of  Bunsen  and  his 
theory  of  the  eruption  and  the  formation  of  geysers  are  among  the 
most  beautiful  illustrations  of  scientific  induction  which  we  have  in 
geology.  We  therefore  give  it,  perhaps,  more  fully  than  its  strict 
geological  importance  warrants. 

Bunsen  examined  all  the  phenomena  of  hot  springs  in  Iceland.  1. 
He  ascertained  that  geyser-water  is  meteoric  water,  containing  the 
soluble  matters  of  the  igneous  rocks  in  the  vicinity.  He  formed  iden- 
tical water  by  digesting  Iceland  rocks  in  hot  rain-water.  2.  He  ascer- 
tained that  there  are  two  kinds  of  hot  springs  in  Iceland,  viz.,  acid 
springs  and  alkaline-carbonate  springs,  and  that  only  alkaline-car- 
bonate springs  contain  any  silica  in  solution.  The  reason  is  obvious : 
alkaline  waters,  especially  if  hot,  are  the  natural  solvents  of  silica.  3. 
He  ascertained  that  only  the  silicated  springs  form  geysers.  Here  is 
one  important  step  taken — one  condition  of  geyser-formation  discov- 
ered. Deposit  of  silica  is  necessary  to  the  existence  of  geysers.  The 
tube  of  a  geyser  is  not  an  accidental  conduit,  but  is  built  up  try  its  own 
deposit.  4.  Of  silicated  springs,  only  those  with  long  tubes  erupt — 
another  condition.  5.  Contrary  to  previous  opinion,  the  silica  in  solu- 
tion does  not  deposit  on  cooling,  but  only  by  drying.  This  would  make 
the  building-up  of  a  geyser-tube  an  inconceivably  slow  process,  and  the 
time  proportionally  long.  This,  however,  is  not  true,  for  the  Yellow- 
stone geyser-waters,  which  deposit  abundantly  by  cooling,  evidently 
because  they  contain  much  more  silica  than  those  of  Iceland.  6.  The 
temperature  of  the  water  in  the  basin  was  found  to  be  usually  170°  to 
180°,  and  that  in  the  tube  to  increase  rapidly,  though  not  regularly, 
with  depth.  Moreover,  the  temperature,  both  at  the  surface  and  at 


GEYSERS. 


101 


Pressure  In 
Atmospheres. 

Bofflng-Point. 

1  Atmos. 

2       " 
3       " 
4       " 

212° 

250° 

275° 
293° 

all  depths,  increased  regularly  as  the  time  of  eruption  approached. 
Just  before  the  eruption  it  was,  at  the  depth  of  about  forty-five  feet, 
very  near  the  boiling-point  for  that  depth. 

Theory  of  Geyser-Eruption — Principles. — 1.  It  is  well  known  that 
the  boiling-point  of  water  rises  as  the  pressure  increases.  This  is 
shown  in  the  adjoining  table.  2.  It  follows 
from  the  above  that  if  water  be  under  strong 
pressure,  and  at  high  temperature,  though 
below  its  boiling-point  for  that  pressure,  and 
the  pressure  be  diminished  sufficiently,  it  will 
immediately  flash  into  steam.  3.  Water 
heated  beneath,  if  the  circulation  be  unim- 
peded, is  very  nearly  the  same  temperature  throughout.  That  it  is 
never  the  same  temperature  precisely  is  shown  by  the  circulation  itself, 
which  is  caused  by  difference  of  temperature,  producing  difference  in 
density.  The  phenomenon  of  simmering  is  also  a  well-known  evidence 
of  this  difference  of  temperature,  since  it  is  produced  by  the  collapse  of 
steam-bubbles  rising  into  the  cooler  water  above.  4.  But  if  the  circula- 
tion be  impeded,  as  when  the  water  is  contained  in  long,  narrow,  irreg- 
ular tubes,  and  heated  with  great  rapidity,  the  temperature  may  be 
greater  below  than  above  to  any  extent,  and  the  boiling-point  may  be 
reached  in  the  lower  part  of  the  tube,  while  it  is  far  from  this  point  in 
the  upper  part. 

Application  to  Geysers. — We  will  suppose  a  geyser  to  have  a  simple 
but  irregular  tube,  without  a  cave,  heated  below  by  volcanic  fires,  or 
by  still  hot  volcanic  ejections.  Now,  we  have  already  seen  that  the 
temperature  of  the  water  in  the  tube  increases  rapidly  with  the  depth, 
but  is,  at  every  depth  to  which  observation  extends,  short  of  the  boil- 
ing-point for  that  depth.  Let  absciss 

a  d  (Fig.  89)  represent  depth  in  the  ^  _  n|  I  I  |  1 1  | 
tube,  and  also  pressures ;  and  the  cor- 
responding temperature  be  measured 
on  the  ordinate  a  n.  If,  then,  a  #,  b  c, 
c  d,  represent  equal  depths  of  thirty- 
three  or  more  feet,  which  is  equal  to 
one  atmospheric  pressure,  the  curve 
e  f  passing  through  210°,  250°,  275°, 
and  293°,  at  the  horizontal  lines,  repre- 
senting one  atmosphere,  two  atmos- 
pheres, three  atmospheres,  etc.,  would 
correctly  represent  the  increasing  boil- 
ing-points as  we  pass  downward.  We 
shall  call  this  line,  e  /,  the  curve  of 
boiling-point.  The  line  a  g  commencing 


/ 

/ 

/ 

/ 

'• 

/> 

/ 

66.6  ft. 

1 

/ 

/ 

100  ft   jg 

at  the   surface  at   180°, 


102 


IGNEOUS  AGENCIES. 


and  gradually  approaching  the  boiling-point  line,  but  everywhere 
within  it,  would  represent  the  actual  temperature  in  a  state  of  qui- 
escence. Now,  Bunsen  found  that,  as  the  time  of  eruption  approached, 
the  temperature  at  every  depth  approached  the  boiling-point  for  that 
depth,  i.  e.,  the  line  a  g  moved  toward  the  line  ef.  There  is  no  doubt, 
therefore,  that,  at  the  moment  of  eruption,  at  some  point  below  the 
reach  of  observation,  the  line  a  g  actually  touches  the  line  ef — the  boiling- 
point  for  that  depth  is  actually  reached.  As  soon  as  this  occurs,  a  quan- 
tity of  water  in  the  lower  portion  of  the  tube,  or  perhaps  even  in  the  sub- 
terranean channels  which  lead  to  the  tube,  would  be  changed  into  steam, 
and  the  expanding  steam  would  lift  the  whole  column  of  water  in  the 
tube,  and  cause  the  water  in  the  basin  to  bulge  and  overflow.  As  soon 
as  the  water  overflowed,  the  pressure  would  be  diminished  in  every  part 
of  the  tube,  and  consequently  a  large  quantity  of  water  before  very 
near  the  boiling-point  would  flash  into  steam  and  instantly  eject  the 
whole  of  the  water  in  the  pipe  ;  and  the  steam  itself  would  rush  out 
immediately  afterward.  The  premonitory  cannonading  beneath  is  evi- 
dently produced  by  the  collapse  of  large  steam-bubbles  rising  through 
the  cooler  water  of  the  upper  part  of  the  tube  ;  in  other  words,  it  is 
simmering  on  a  huge  scale.  An  eruption  is  more  quickly  brought  on 
by  throwing  stones  into  the  throat  of  the  geyser,  because  the  circula- 
tion is  thus  more  effectually  impeded. 

The  theory  given  above  is  substantially  that  of  Bunsen  for  the  erup- 
tion of  the  Great  Geyser,  but  modified  to  make  it  applicable  to  all  gey- 
sers. In  the  Great  Geyser,  as  already  stated,  Bunsen  found  a  point,  forty- 
five  feet  deep,  where  the  temperature  was  nearer  the  boiling-point  than 
at  any  within  reach  of  observation,  though  doubtless  beyond  the  reach 
of  observation  the  temperature  again  approached  and  touched  the  boil- 
ing-point. This  point,  forty-five  feet  deep,  plays  an  important  part  in 
Bunsen's  theory.  To  illustrate  :  if  e  f  (Fig.  90)  represent  again  the 

curve  of  boiling-point,  then  the 
f        a  curve  of  actual  temperature  in 

the  Great  Geyser  tube  would  be 
the  irregular  line  a  g  h.  At  the 
moment  of  eruption,  this  line 
touched  boiling-point  at  some 
depth,  h,  beyond  the  reach  of 
observation.  Then  followed 
the  lifting  of  the  column,  the 
overflow  of  the  basin,  the  re- 
lief of  pressure  by  which  the 
point  g  was  brought  to  the  boil- 
ing-point, the  instantaneous 
formation  of  steam  at  <?,  and 


4 


e66ft 


GEYSERS. 


103 


: 


the  phenomena  of  an  eruption.     But  it  is  extremely  unlikely  that  this 
condition  should  exist  in  all  geysers ;  neither  is  it  at  all  nee-  _ 

essary  in  order  to  explain  the  phenomenon  of  an  eruption.  ,^'k 

To  prove  beyond  question  the  truth  of  his  theory,  Bun-        jjjl*j\ 
sen  constructed  an  artificial  geyser.    The  apparatus  (Fig.  91)       /:  : 
consisted  of  a  tube  of  tinned  sheet-iron  about  ten  feet  long, 
expanded  into  a  dish  above  for  catching  the  erupted  water.     ' 
It  may  or  may  not  be  expanded  below  for  the  convenience 
of  heating.    It  was  heated,  also,  a  little  below  the  middle,  by 
an  encircling  charcoal  chauffer,  to  represent  the  point  of 
nearest  approach  to  the  boiling-point  in  the  geyser-tube. 
When  this  apparatus  was  heated  at  the  two  points,  as  shown 
in  the  figure,  the  phenomena  of  geyser-eruption  were  com- 
pletely reproduced ;  first,  the  violent  explosive  simmering, 
then  the  overflow,  then  the  eruption,  and  then  the  state  of 
quiescence..     In   Bunsen's  experiment,   the    eruptions  oc- 
curred about  every  thirty  minutes. 

Bunsen's  Theory  of  Geyser  -  Formation. — According  to 
Bunsen,  a  geyser  does  not  find  a  cave,  or  even  a  perpen- 
dicular tube,  ready  made,  but,  like  volcanoes,  makes  its  own 
tube.  Fig.  92  is  an  ideal  section  of  a  geyser-mound,  show- 
ing the  manner  in  which,  according  to  this  view,  it  is  formed. 
The  irregular  line,  b  a  <?,  is  the  original  surface,  and  a  the 
position  of  a  hot  spring.  If  the  spring  be  not  alkaline,  it 
will  remain  an  ordinary  hot  spring;  but,  if  it  be  alkaline, 
it  will  hold  silica  in  solution,  and  the  silica  will  be  deposited  Fie.  91.— Arti- 
about  the  spring.  Thus  the  mound  and  tube  are  gradually 
built  up.  For  a  long  time  the  spring  will  not  be  eruptive,  for  the  cir- 
culation will  maintain  a  nearly  equal  temperature  in  every  part  of  the 

tube  —  it  may  be  a  boiling, 
but  not  an  eruptive  spring. 
But,  as  the  tube  becomes 
longer,  and  the  circulation 
more  and  more  impeded,  the 
difference  of  temperature  be- 
tween the  upper  and  lower 
parts  of  the  tube  becomes 
greater  and  greater,  until, 
finally,  the  boiling-point  is 
reached  below,  while  the  wa- 
ter above  is  comparatively 
cool.  Then  the  eruption  com- 
mences. Finally,  from  the  gradual  failure  of  the  subterranean  heat,  or 
from  the  increasing  length  of  the  tube  repressing  the  formation  of 


1 


FIG.  92.-Ideal  Section  of  a  Geyser-Tube,  according  to 


104  IGNEOUS  AGENCIES. 

steam,  the  eruptions  gradually  cease.  Bunsen  found  geysers  in  every 
stage  of  development — some  playful  springs  without  tubes ;  some  with 
short  tubes,  not  yet  eruptive ;  some  with  long  tubes,  violently  erup- 
tive ;  some  becoming  old  and  indisposed  to  erupt  unless  angered  by 
throwing  stones  down  the  throat. 

It  is  evident,  however,  that  Bunseii's  theory  of  geyser-eruption  is 
independent  of  his  theory  of  geyser-formation.  A  tube  or  fissure  of 
any  kind,  and  formed  in  any  way,  if  long  enough,  would  give  rise  to 
the  same  phenomena.  The  Yellowstone  geysers  have  mounds  or  chim- 
ney-like cones,  but  it  is  by  no  means  certain  that  the  whole  length  of 
their  eruptive  tubes  has  been  built  up  by  siliceous  deposit.  Bunsen's 
theory  of  eruption  none  the  less,  however,  applies  to  these  also.  The 
more  chimney-like  form  of  the  craters  in  the  case  of  the  Yellowstone 
geysers  is  probably  due  to  the  greater  abundance  of  silica  in  solution. 

SECTION  3. — EARTHQUAKES. 

Only  very  recently,  and  mainly  through  the  labors  of  Mr.  Mallet,  of 
England,  our  knowledge  on  the  subject  of  earthquakes  has  commenced 
to  take  on  scientific  form.  This  slowness  of  advance  has  arisen  not 
from  any  want  of  materials,  but  from  the  great  complexity  of  the  phe- 
nomena, their  origin  deep  within  the  bowels  of  the  earth  and  there- 
fore removed  from  observation,  and,  more  than  all,  from  the  surprise 
and  alarm  usually  produced  unfitting  the  mind  for  scientific  observa- 
tion. For  these  reasons,  until  fifteen  or  twenty  years  ago,  the  state  of 
knowledge  on  this  subject  was  much  the  same  as  it  was  2,000  years 
ago.  And  yet  now,  we  think,  our  knowledge  of  earthquakes  is  even 
more  advanced  than  that  of  volcanoes. 

Frequency. — Mallet,  in  his  earthquake  catalogue,  has  collected  the 
records  of  6,830  earthquakes  as  occurring  in  3,456  years  previous  to 
1850 ;  but,  of  that  number,  3,240,  or  nearly  one-half,  occurred  in  the 
last  fifty  years  ;  not  because  earthquakes  were  more  numerous,  but  be- 
cause the  records  were  more  perfect.  If  the  records  had  been  equally 
complete  throughout  the  whole  time,  the  number  would  have  been  over 
200,000.  Taking  the  last  four  years  of  his  record,  the  number  was  about 
two  a  week.  According  to  the  more  complete  catalogue  of  Alexis 
Perry,  from  1843  to  1872,  inclusive,  there  were  17,249,  or  575  per  an- 
num. It  seems  probable,  therefore,  that,  considering  the  fact  that  even 
now  the  larger  number  of  earthquakes  are  not  recorded,  occurring  in 
mid-ocean  or  in  uncivilized  regions,  the  earth  is  constantly  quaking  in 
some  portion  of  its  surface. 

Connection  with  other  Forms  of  Igneous  Agency.— The  close  connec- 
tion of  earthquakes  with  volcanoes  is  undoubted :  1.  Volcanic  eruptions, 
especially  those  of  the  explosive  type,  are  always  preceded  and  accom- 
panied by  earthquakes.  2.  Earthquake-shocks  which  have  continued  to 


EARTHQUAKES.  105 

trouble  a  particular  region  for  a  long  time,  often  suddenly  cease  when 
an  outburst  takes  place  in  a  neighboring  volcano,  showing  that  the  lat- 
ter are  safety-vents  for  the  interior  forces  which  produce  earthquakes. 
Also,  the  sudden  cessation  of  accustomed  volcanic  activity  will  often 
bring  on  earthquakes.  Thus,  when  the  wreath  of  smoke  disappears 
from  Cotopaxi,  the  inhabitants  of  Quito  expect  earthquakes.  During 
the  great  Calabrian  earthquake  of  1783,  Stromboli,  for  the  first  time  in 
the  memory  of  man,  ceased  erupting.  The  great  earthquake  which  de- 
stroyed Riobamba  in  1797,  and  in  which  40,000  persons  perished,  took 
place  immediately  after  the  stopping  of  activity  in  a  neighboring  vol- 
cano. The  earthquake-shocks  which  destroyed  Caracas  in  1812  ceased 
as  soon  as  St.  Vincent,  500  miles  distant,  commenced  erupting.  3.  Ex- 
amination of  Prof.  Mallet's  earthquake-map  shows  that  the  distribution 
of  earthquake-centres  is  much  the  same  as  that  of  volcanoes  already 
given  (page  81).  It  may  be  regarded  as  almost  certain,  therefore,  that 
the  forces  which  generate  earthquakes  are  closely  allied,  if  not  identical, 
with  those  which  produce  volcanic  eruptions. 

Again,  the  connection  of  earthquakes  with  bodily  movements  of 
great  areas  of  the  earth's  crust,  by  elevation  or  depression,  is  equally 
close.  In  1835,  after  a  great  earthquake,  which  shook  the  coast  of 
South  America  over  an  area  of  600,000  square  miles,  the  whole  coast- 
line of  Chili  and  Patagonia  was  found  elevated  from  two  to  ten  feet 
above  sea-level.  Again,  in  1822,  after  a  similar  earthquake  in  the  same 
region,  the  coast-line  was  found  elevated  from  two  to  seven  feet.  Now, 
in  this  very  region,  old  beach-marks,  100  feet  to  1,300  feet  above  the 
sea-level,  and  extending  1,200  miles  along  the  coast  on  each  side  of  the 
southern  end  of  this  continent,  plainly  show  that,  in  very  recent  geo- 
logical times,  the  whole  southern  end  of  South  America  has  been  bodily 
raised  out  of  the  sea  to  that  extent.  It  is  impossible  to  doubt  that  the 
force  which  produced  this  continental  elevation  was  also  the  cause  of 
the  accompanying  earthquakes.  Again,  in  1819,  after  a  severe  earth- 
quake, which  shook  the  whole  region  about  the  mouth  of  the  Indus,  a 
large  tract  of  land  of  2,000  square  miles  was  sunk  and  became  a  salt 
lagoon;  while  another  area,  fifty  miles  long  and  ten  to  sixteen  miles 
wide,  was  elevated  ten  feet.  In  commemoration  of  this  wonderful 
event,  the  raised  portion  was  called  Ullah  Bund,  or  the  Mound  of  God. 
Again,  in  1811,  a  severe  earthquake  shook  the  valley  of  the  Missis- 
sippi. In  the  region  about  the  mouth  of  the  Ohio,  where  it  was  se- 
verest, large  tracts  of  land  were  sunk  bodily  several  feet  below  their  for- 
mer level,  and  have  been  covered  with  water  ever  since.  It  is  now 
called  the  "  Sunk  Country"  In  the  two  cases  last  mentioned  there 
was  evidently  formed  &  fault  or  dislocation,  i.  e.,  there  was  a  fissure  in 
the  earth's  crust,  and  one  side  dropped  down  lower  than  the  other. 
Such  fissures  and  faults  are  found  intersecting  the  earth  in  all  direc- 


106  IGNEOUS  AGENCIES. 

tions.  We  see  them,  in  these  cases,  formed  under  our  eyes,  and  in 
connection  with  earthquakes. 

Ultimate  Cause  of  Earthquakes. — The  connection  of  earthquakes 
with  the  two  other  forms  of  igneous  agency  suggests  each  a  possible 
cause.  Preceding  and  accompanying  volcanic  eruptions,  especially  of 
the  explosive  type,  occur  subterranean  explosions,  which  are  often  heard 
hundreds  of  miles.  Such  eruptions  are  also  accompanied  with  escape  of 
immense  quantities  of  steam  and  gas.  These  facts,  together  with  the 
association  of  earthquakes  with  volcanoes,  have  suggested  the  idea  that 
the  sudden  formation  or  the  sudden  collapse  of  vapor  is  the  cause  of 
earthquakes.  According  to  this  view,  an  earthquake  is,  on  a  grand 
scale,  a  phenomenon  similar  to  the  jar  produced  by  the  explosion  of  a 
keg  of  gunpowder  buried  in  the  earth. 

But  the  association  of  earthquakes  with  bodily  movements  of  large 
areas  of  the  earth's  crust  suggests  another  and  a  far  more  probable 
cause.  The  earth's  crust,  as  is  well  known,  is  in  gradual  movement  by 
elevation  or  depression  almost  everywhere.  These  movements,  as  we 
shall  show  hereafter,  are  probably  due  to  the  greater  interior  contrac- 
tion of  the  earth  thrusting  the  crust  upon  itself,  by  horizontal  pressure. 
If  the  yielding  is  constant  like  the  force,  the  movement  will  be  grad- 
ual; but  if  the  crust  resists,  and  the  force  still  accumulates,  the  yielding 
must  take  place  suddenly  by  fissure  or  crushing.  Such  assuring  or 
crushing  of  the  rocky  crust  would  certainly  produce  a  concussion  or 
jar,  which,  propagating  itself,  would  finally  reach  the  surface  and 
spread  outward  from  the  point  of  first  emergence.  Furthermore, 
when  we  remember  that  these  fissures  often  break  through  thousands 
of  feet  and  even  miles  in  thickness  of  solid  rock,  we  easily  perceive 
that  the  resulting  concussion  would  be  fully  adequate  to  produce  all 
the  dreadful  effects  of  earthquakes. 

Proximate  Cause. — But  whatever  be  our  view  of  the  ultimate  cause 
of  earthquakes,  there  can  be  no  doubt  that  the  proximate  or  immediate 
cause  of  the  observed  effects  is  the  arrival  of  an  earth-jar — the  emer- 
gence, on  the  earth-surface,  of  a  succession  of  elastic  earth-waves,  pro- 
duced by  a  violent  concussion  of  some  kind  in  the  interior.  Evidently, 
therefore,  the  discussion  of  earthquake  phenomena  is  nothing  more 
than  the  discussion  of  the  laws  of  propagation  and  the  effects  of  elastic 
waves  occurring  under  peculiar  and  very  complex  conditions.  It  is 
impossible  to  understand  the  subject  without  some  preliminary  knowl- 
edge of  the  nature  and  properties  of  waves.  For  the  sake  of  greater 
clearness  we  will  state  some  principles  which  we  will  make  use  of  in 
this  discussion. 

Waves— their  Kinds  and  Properties. — Waves  may  be  classified  in 
several  ways,  according  to  the  point  of  view  from  which  we  regard 
them.  Regarding  only  the  force  of  propagation,  they  are  divided  into 


EARTHQUAKES.  107 

waves  of  gravity  and  waves  of  elasticity.  Regarding  the  direction  of 
oscillation,  they  are  divided  into  waves  of  transverse  and  waves  of 
longitudinal  oscillation ;  regarding  the  form,  into  circular  and  spheri- 
cal waves. 

(  of  elasticity _^_z^._longitudinal  oscillation— <^ — «K- spherical. 
Waves  -j  ^^v 

(  of  gravity    -•»  ^>  -transverse  oscillation     ••<-..  ••-circular. 

A  wave  of  elasticity  may  have  either  longitudinal  or  transverse  oscil- 
lation, as  shown  in  the  diagram,  but  those  of  which  we  shall  speak  will 
be  principally  the  former.  Waves  of  gravity  are  always  of  transverse 
vibration.  Spherical  waves  are  of  longitudinal  vibration,  and  circular 
waves  are  transverse. 

If  a  stone  be  thrown  into  still  water  a  series  of  waves  run  in  every 
direction  from  the  point  of  disturbance,  becoming  lower  and  lower  as 
the  distance  increases,  until  the}'  become  insensible.  These  are  circular 
waves  of  transverse  oscillation  propagated  by  gravity  alone.  The  direc- 
tion of  propagation  is  along  the  surface  of  the  water  in  direction  of  the 
radius  of  the  circle ;  the  direction  of  oscillation  is  up  and  down,  or  trans- 
verse to  the  direction  of  propagation.  Water-waves  are,  therefore, 
transverse  waves  of  gravity,  and,  if  propagated  from  a  central  point, 
are  circular.  They  move  with  uniform  velocity  ;  their  height  decreases 
as  they  pass  outward.  If,  on  the  other  hand,  an  impulse  like  an  ex- 
plosion originate  in  the  interior  of  a  medium,  as,  for  example,  in  the 
air  or  in  the  interior  of  the  earth,  the  impulse  acting  in  every  direction 
compresses  a  spherical  shell  of  matter  all  around  itself,  while  the  point 
of  impulse  itself  passes  into  a  state  of  rarefaction  ;  this  compressed  shell 
in  expanding  by  its  elastic  force  compresses  the  next  outer  shell  of  mat- 
ter, itself  becoming  rarefied  in  the  act,  and  this  last  in  its  turn  propa- 
gates the  impulse  to  the  next,  and  so  on.  Thus,  if  only  a  single  wave 
were  formed,  there  would  run  outward  from  the  focal  point  an  ever- 
widening  spherical  shell  of  compressed  matter,  followed  closely  by  a 
similar  shell  of  rarefied  matter.  But  in  every  case  of  impulse  or  con- 
cussion there  is  always  a  series  of  such  alternate  compressed  and  rare- 
fied shells  following  one  another.  The  alternate  compression  and 
rarefaction  causes  each  particle  in  succession  to  move  forth  and  back. 
This  oscillatory  motion  is  in  the  direction  of  propagation  of  the  wave, 
and  therefore  longitudinal.  All  waves  propagated  from  a  point  within 
a  medium,  such  as  sound-waves^  are  elastic  spherical  waves  of  longi- 
tudinal oscillation. 

Definition  Of  Terms. — In  transverse  waves,  such  as  water-waves,  the 
distance  from  wave-crest  to  wave-crest,  or  from  wave-trough  to  wave- 
trough,  is  called  the  wave-length,  and  the  perpendicular  distance  from 
trough  to  crest  is  called  the  wave-height.  Similar  terms  are  used  in 
speaking  of  waves  of  longitudinal  vibrations.  The  sense  in  which  they 


108  IGNEOUS  AGENCIES. 

are  used  and  their  propriety  are  shown  in  the  accompanying  figure  (Fig. 
93).  Let  the  bar  A  B  represent  a  prism  cut  from  a  vibrating  sphere  in 
the  direction  of  the  radius,  i.  e.,  the  direction  of  propagation  of  the 
wave,  and  let  the  dark  and  light  portions  represent  condensation  and 
rarefaction.  Now,  on  the  line  a  b,  representing  the  natural  state  of  the 


bar,  draw  ordinates  above,  to  represent  the  degrees  of  compression,  and 
below,  to  represent  degrees  of  rarefaction ;  then  the  undulating  line 
will  correctly  represent  the  state  of  the  bar  during  the  transmission  of 
elastic  longitudinal  waves.  Thus  longitudinal  waves  may  be  repre- 
sented in  the  same  way  as  transverse  waves.  The  most  compressed 
portions  are  called  crests,  and  the  most  rarefied  troughs  ;  from  crest  to 
crest  is  the  length,  and  the  amount  of  oscillation  of  the  particles  back 
and  forth  in  compression  and  rarefaction  is  the  height  of  the  wave. 
We  shall  be  compelled  to  use  these  terms  in  speaking  of  earthquake- 
waves. 

Thus,  then,  there  are  two  very  distinct  kinds  of  waves,  both  of 
which  are  common — viz.,  circular  waves  of  gravity,  of  which  water- 
waves  are  the  type,  and  spherical  elastic  waves,  of  which  sound-waves 
are  the  type.  We  will  have  much  to  do  with  both  of  these  in  the  ex- 
planation of  earthquake  phenomena. 

The  velocity  of  water-waves  depends  wholly  on  the  wave-length, 
and  not  at  all  on  the  wave-height.  Therefore,  water-waves  run  with 
uniform  motion,  since,  although  their  height  diminishes,  their  length 
remains  the  same.  But  there  is  one  important  exception  to  this  law, 
and  one  which  peculiarly  concerns  us  in  this  discussion — viz.,  when  the 
length  of  waves  is  great  in  proportion  to  the  depth  of  the  water,  then 
they  drag  bottom,  and  their  velocity  is  a  function  of  the  depth  of  the 
water  as  well  as  of  the  length  of  the  wave. 

The  velocity  of  elastic  waves,  on  the  other  hand,  is  not  affected 
either  by  the  height  or  the  length  of  the  wave,  but  only  by  the  elasticity 
of  the  medium.  Thus  the  harmony  of  a  full  band  of  music  is  perfect 
even  at  a  great  distance ;  but  this  would  be  impossible  unless  loud 
sounds  (high  waves)  and  soft  sounds  (low  waves),  deep  sounds  (long 
waves)  and  sharp  sounds  (short  waves),  all  run  with  the  same  velocity. 
But  there  is  one  exception  here  also  which  especially.concerns  us  in  the 


EARTHQUAKES.  109 

discussion  of  earth-waves.  It  is  this  :  When  the  medium  is  very  im- 
perfectly elastic,  and  the  waves  are  high,  then  the  medium  is  broken  by 
the  passage  of  the  waves  at  every  step,  its  elasticity  is  diminished,  and 
the  waves  retarded. 

In  order  to  understand  clearly  what  follows,  it  is  necessary  to  bear 
well  in  mind  the  distinction  between  velocity  of  oscillation  and  velocity 
of  transmission  or  transit.  These  bear  no  relation  to  one  another. 
Thus  we  may  have  a  long,  low  water-wave  moving  with  immense 
velocity  along  the  surface,  and  yet  communicating  only  a  slow  oscillat- 
ing motion  up  and  down  to  a  boat  resting  on  its  surface.  In  the  case 
of  water-waves  the  velocity  of  transit  depends  on  the  length  of  the 
wave  only,  the  amount  of  vibration  on  the  height  of  the  wave  only, 
while  the  velocity  of  vibration  depends  on  the  relation  of  the  height  to 
the  length.  In  elastic  longitudinal  waves  the  velocity  of  transit  de- 
pends on  the  elasticity  of  the  medium  only;  the  amount  of  vibration,  as 
in  the  last  case,  on  the  height  of  the  wave,  and  the  velocity  of  vibration 
upon  the  relation  of  height  to  length  of  wave. 

Application  to  Earthquakes. — Suppose,  then,  a  concussion  of  any 
kind  to  occur  at  a  considerable  depth  (x,  Fig.  94),  say  ten  or  twenty  miles, 
beneath  the  earth-surface,  S  S.  A  series  of  elastic  spherical  waves  will 
be  generated,  consisting  of  alternate  compressed  and  rarefied  shells,  the 
whole  expanding  with  great  rapidity  in  all  directions  until  they  reach 


FIG.  94. 

the  surface  at  a.  From  this  point  of  first  emergence  immediately  above 
the  focus  a,  the  still-enlarging  spherical  shells  would  outcrop  in  rapidly- 
expanding  circular  waves  similar  in  form  to  water-waves,  but  very  dif- 
ferent in  character.  This  we  will  call  the  surface-wave.  Fig.  94  is  a 
vertical  section  through  the  focus  x  and  the  point  of  first  emergence 
(epicentrum)  a,  showing  the  series  of  spherical  waves  outcropping  at 
a,  J,  c,  d,  etc.  The  circles  here  drawn  would  equally  represent  a  series 
of  waves,  or  the  same  wave  in  successive  degrees  of  enlargement. 

This  surface-wave  would  not  be  similar  to  any  wave  classified  above. 
It  would  not  be  a  normal  wave  of  any  kind.  It  would  be  only  the  out- 
cropping or  emergence  of  the  ever-widening  spherical  wave  on  the 
earth-surface.  Both  its  velocity  of  transit  along  the  surface,  and  the 
direction  of  its  vibration  in  relation  to  the  surface,  will  vary  constantly 


110  IGNEOUS  AGENCIES. 

according  to  a  simple  law.  The  direction  of  vibration,  being  along 
the  radii  x  a,  x  b,  x  c,  etc.,  will  be  perpendicular  to  the  surface  at  a, 
and  become  more  inclined  until  it  finally  becomes  parallel  with  the 
surface  at  an  infinite  distance.  The  velocity  of  its  transit  will  be  in- 
finite at  a,  and  then  gradually  decrease  until,  if  we  regard  the  surface 
as  a  plane  surface,  at  an  infinite  distance  it  reaches  its  limit,  which  is 
the  velocity  of  the  spherical  wave.  Between  these  two  extremes  of 
infinity  at  a,  and  the  velocity  of  the  spherical  wave  at  infinite  distance, 
the  velocity  of  the  surface-wave  varies  inversely  as  the  cosine  of  the 
angle  of  emergence  x  b  a,  x  c  a,  etc. 

For,  if  a  a,  bb,  cc,  del,  etc.,  be  successive  positions  of  the  spherical 
wave,  then  the  radii  x  a,  x  b,  x  c,  would  be  the  direction  both  of  propaga- 
tion and  of  vibration.  Now,  when  the  wave-front  is  at  b  while  the 
spherical  wave  moves  from  b'  to  c,  the  surface-wave  would  move  from  b 
to  c ;  when  the  spherical  wave  moves  from  c'  to  d,  the  surface-wave 
moves  from  c  to  d,  etc.  If,  therefore,  be,  cd,  etc.,  be  taken  very  small, 
so  that  b  b'  c,  c  c'  d,  may  be  considered  right-angled  triangles,  then  in 
every  position  the  surface-wave  moves  along  the  hypothenuse,  while 
the  spherical  wave  moves  along  the  cosine  of  the  angle  of  emergence 
x  ca,x  da,  etc.  Letting  v  =  velocity  of  the  spherical  wave,  and  vr 
that  of  the  surface-wave,  and  E  the  angle  of  emergence,  we  have  the 

proportion — v'  :  v'.'.Rad.  I  cos.  .Z?,  and  v'  = =,,  or  if  v  is  constant 

cos.  J? 

v'  oc j-.    Therefore,  at  a,  the  point  of  first  emergence,  E  being  a 

right  angle,  and  the  cos.  E  =  0,  v'  =  —  =  infinity.  At  an  infinite 
distance  from  a  the  angle  E  becomes  0,  and  the  cosine  =  1,  and 
v'  =  —  =  v.  That  is,  at  the  point  of  first  emergence  the  velocity  of  the 

surface-wave  is  infinite ;  from  this  point  it  decreases  as  the  cosine  of  the 
angle  of  emergence  increases,  until  finally  at  an  infinite  distance  it  be- 
comes equal  to  the  velocity  of  the  spherical  wave. 


On  a  spherical  surface  (Fig.  95)  it  is  evident  that  E  never  becomes 
0,  and  therefore  v'  never  reaches  the  limit  v.  If  we  conceived  the 
wave  to  pass  through  the  whole  earth  (Fig.  96),  then  the  velocity  of 
the  surface-wave  would  decrease  to  a  certain  point  where  E  is  a  mini- 


EARTHQUAKES.  HI 

mum,  say  about  c,  and  then  would  again  in- 
crease to  infinity  on  the  opposite  side  of  the 
earth,£>,  where  E  becomes  again  a  right  angle. 
If  x  be  near  the  surface,  v'  would  become  nearly 
equal  to  v  at  some  point  of  its  course ;  but 
as  x  approaches  the  centre,  (7,  the  limit  of  v' 
would  be  greater  and  greater,  until,  if  x  is  at 
the  centre,  v'  would  become  infinite  every- 
where :  i.  e.,  a  shock  at  the  centre  of  the 
earth  would  reach  the  surface  everywhere  at 
the  same  moment. 

Experimental  Determination  of  the  Velocity  of  the  Spherical  Wave. 

— On  the  supposition  that  earthquakes  are  really  produced  by  the  emer- 
gence on  the  surface  of  a  series  of  elastic  earth-waves,  Mallet  under- 
took to  determine  experimentally  the  velocity  of  such  waves.  Two 
stations  were  taken  about  a  mile  or  more  apart,  and  connected  by  tele- 
graphic apparatus  ;  a  keg  of  gunpowder  was  buried  at  one,  and  at  the 
other  was  placed  an  observatory,  in  which  was  a  clock,  a  mercury  mir- 
ror, and  a  light,  the  image  of  which  reflected  from  the  mercury  mirror 
was  thrown  on  a  screen.  The  slightest  tremor  communicated  to  the 
mercury  surface  of  course  caused  the  image  to  dance.  The  moment 
of  explosion  was  telegraphed ;  the  moment  of  arrival  of  the  earth- 
tremor  was  observed.  The  difference  gave  the  time  of  transit ;  the 
distance,  divided  by  the  time,  gave  the  velocity  per  second.  In  this 
manner  Mallet  found  the  velocity  in  sand  825  feet  per  second,  or  nearly 
nine  and  one-half  miles  per  minute  ;  in  slate,  1,225  feet  per  second,  or 
fourteen  miles  per  minute ;  and  in  granite  1,665  feet  per  second,  or 
nineteen  miles  per  minute.  As  an  earthquake-focus  is  always  several 
miles  beneath  the  surface,  and  as  rocks  at  that  depth  are  probably  as 
hard  as  granite,  nineteen  miles  per  minute  may  be  taken  as  the  aver- 
age velocity  of  earth-waves  as  determined  by  these  experiments.  It 
agrees  well  with  the  observed  velocity  of  many  earthquakes. 

This  result  was  unexpected,  considering  the  law  that  all  elastic 
waves  in  the  same  medium  run  with  the  same  velocity,  for  the  velocity 
of  sound  in  granite  or  slate  is  probably  not  less  than  10,000  or  12,000 
feet  per  second.  The  explanation  is  to  be  found  in  the  very  imperfect 
coherence  and  elasticity  of  rocks.  The  medium  is  broken  by  the  pas- 
sage of  large  and  high  waves  of  the  explosion,  but  carries  successfully 
the  small  waves  of  sound. 

Explanation  of  Earthquake-Phenomena— Earthquakes  have  been 
divided  into  three  kinds,  viz.,  the  explosive,  the  horizontally  progres- 
sive, and  the  vorticose.  The  first  kind  is  described  by  Humboldt  as 
a  violent  motion  directly  upward,  by  which  the  earth-crust  is  broken 
up,  and  bodies  on  the  surface  are  thrown  high  in  the  air.  The  shock 


112  IGNEOUS  AGENCIES. 

is  extremely  violent,  but  does  not  extend  very  far.  In  the  second,  the 
shock  spreads  on  the  surface  like  the  waves  on  water  to  a  great  dis- 
tance. In  the  third  there  is  a  whirling  motion  of  the  earth  entirely 
different  from  ordinary  wave-motion.  These  three  kinds  are  sometimes 
supposed  to  be  essentially  distinct,  and  possibly  produced  by  different 
causes  ;  but  we  will  attempt  to  show  that  the  difference  is  wholly  due 
to  the  different  conditions  under  which  the  waves  emerge  on  the  sur- 
face. The  three  kinds  are,  iu  fact,  often  united  in  the  same  earth- 
quake. 

The  most  remarkable  example  of  explosive  earthquake  is  that  which 
destroyed  Riobamba  in  1797.  In  this  dreadful  earthquake  the  shock 
came  suddenly,  like  the  explosion  of  a  mine.  Not  only  was  the  earth 
broken  up  and  rent  in  various  places,  but  objects  lying  on  the  surface 
of  the  earth  were  thrown  violently  upward  ;  bodies  of  men  were  hurled 
several  hundred  feet  in  the  air,  and  afterward  were  found  across  a 
river  and  on  the  top  of  a  hill.  In  earthquakes  of  this  kind — 1.  The 
impulse  is  very  powerful  and  sudden,  so  as  to  make  a  high  but  not 
a  long  wave,  or,  in  other  words,  the  velocity  of  vibration  or  of  the 
shock  is  very  great ;  and,  2.  The  focus  is  not  deep,  so  that  the 
velocity  of  the  shock  (height  of  the  wave)  does  not  become  small  be- 
fore it  reaches  the  surface.  At  Riobamba  the  velocity  of  the  shock 
was  still  very  great  when  the  wave  reached  the  surface.  From  the 
distance  bodies  were  thrown,  Mallet  supposes  the  velocity  of  the  shock 
could  not  have  been  less  than  eighty  feet  per  second  (Jukes). 

The  horizontally  progressive  kind  may  be  regarded  as  the  true 
type  of  an  earthquake ;  it  is  in  fact  the  spreading  surface-wave  al- 
ready explained.  If  the  elasticity  of  the  earth,  and  therefore  the 
velocity  of  the  waves,  is  the  same  in  all  directions,  the  surface-wave 
will  spread  in  concentric  circles;  but  if  the  elasticity,  and  therefore 
the  velocity  of  the  waves,  be  greater  in  one  direction  than  in  another, 
as,  for  example,  north  and  south  than  east  and  west,  or  the  converse, 
then  the  form  of  the  outcrop  will  be  elliptical.  In  some  rare  cases  the 
shock  seems  to  run  along  a  line.  Thus  progressive  earthquakes  have 
been  subdivided  into  circular,  elliptical,  and  linear  progressive.  We 
have  already  given  the  simple  explanation  of  the  first  two  ;  the  last 
may  be  briefly  explained  as  follows  : 

Let  it  be  borne  in  mind  :  1.  That  these  linear  earthquakes  usually 
run  along  mountain-chains  ;  2.  That  most  great  mountain-chains  consist 
of  a  granite  axis  (appearing  along  the  crest  and  evidently  connected  be- 
neath with  the  great  interior  rocky  mass  of  the  earth),  flanked  on  each 
side  with  stratified  rocks  consisting  of  many  different  kinds  ;  3.  When 
elastic  waves  pass  from  one  medium  to  another  of  different  elasticity, 
in  all  cases  a  part  of  the  waves  passes  through,  but  a  part  is  always 
reflected.  For  every  such  change — for  every  layer — a  reflection  oc- 


EARTHQUAKES. 


113 


curs;  and,  therefore,  if  there  are  many  such  layers,  the  waves  are 
quickly  quenched.  If,  now,  Fig.  97  represent  a  transverse  section 
across  such  a  mountain,  and  X  the  focus  of  an  earthquake,  it  is  evident 
that  portion  of  the  enlarging  spherical  wave  which  emerged  along  the 
axis  a  would  reach  the  surface  successfully;  while  those  portions  which 


FIG.  97.—  Diagram  illustrating  Linear  Earthquakes.  FIG.  9a 

struck  against  the  strata  of  the  flanks  would  be  partially  or  wholly 
quenched.  The  mode  of  outcrop  on  the  surface  is  shown  in  the  map- 
view,  Fig.  98,  in  which  a  is  the  epicentrum,  b  b  the  granite  axis,  and 
c  c  the  stratified  flanks. 

The  velocity  of  the  surface-waves,  as  observed  in  many  cases  of 
severe  earthquakes,  is  about  twenty  miles  a  minute.  This  accords 
well  with  Mallet's  experiments  in  granite.  In  some  earthquakes  the 
velocity  has  been  found  to  be  twelve  to  fifteen  miles  (Mallet's  results  in 
slate),  and  in  some  as  high  as  thirty  to  thirty-five  miles  per  minute.  In 
no  great  earthquake  has  the  velocity  been  found  higher  than  the  last 
mentioned.  In  some  slight  shocks,  however,  occurring  recently  in 
New  England,  the  velocity,  as  determined  by  telegraph,  is  estimated 
as  high  as  one  hundred  and  forty  miles  per  minute,  or  12,000  feet  per 
second. 

This  amazing  difference  may  be  fully  explained :  It  will  be  re- 
membered that  the  velocity  of  the  surface- wave  is  infinite  at  the  epi- 
centrum, and  diminishes,  according  to  a  law  already  discussed,  until  it 
reaches,  or  nearly  reaches,  the  velocity  of  the  spherical  wave.  Now,  if 
the  earthquake-focus  be  comparatively  shallow,  the  initial  velocity  of 
the  surface-wave  very  rapidly  approaches  its  minimum,  and  therefore 
the  observed  velocity  of  the  surface-wave  may  be  taken  as  nearly  the 
same  as  that  of  the  spherical  wave ;  but,  if  the  earthquake  be  very 
deep,  the  diminution,  even  on  a  plane  surface,  is  far  less  rapid ;  and 
when  we  take  into  consideration  the  curvature  of  the  earth-surface,  it 
is  evident  that  the  velocity  of  the  surface-wave  is  always  and  for  all 
distances  much  greater  than  that  of  the  spherical  wave.  This  would 
well  account  for  velocities  of  thirty  to  thirty-five  miles,  but  not  for  one 
8 


114  IGNEOUS  AGENCIES. 

hundred  and  forty  miles.  This  latter  is  accounted  for  by  another 
principle. 

We  have  seen  that  these  high  velocities  occur  only  in  slight  shocks. 
Now,  while  heavy  shocks  (large  and  high  waves)  break  the  medium  at 
every  step  of  their  passage,  and  are  therefore  retarded,  as  already 
explained,  slight  tremors  (small  and  low. waves)  are  successfully  trans- 
mitted without  rupture,  and  therefore  run  with  the  natural  velocity 
belonging  to  the  medium,  i.  e.,  the  velocity  of  sound.  Now,  the 
velocity  of  sound  in  granite  is  probably  about  12,000  feet  per  second, 
or  one  hundred  and  forty  miles  per  minute. 

Vorticose  Earthquakes. — In  these  cases  the  ground  is  twisted  or 
whirled  round  and  back,  or  sometimes  ruptured  and  left  in  a  twisted 
condition.  The  most  conspicuous  examples  of  this  kind  of  motion 
occurred  in  the  earthquake  of  Riobamba,  and  in  the  great  Calabrian 
earthquake  of  1783.  In  this  latter  earthquake  the  blocks  of  stone 
forming  obelisks  were  twisted  one  on  another  ;  the  earth  was  broken 
and  twisted,  so  that  straight  rows  of  trees  were  left  in  interrupted  zig- 
zags. Phenomena  similar  to  some  of  these  were  observed  also  in  the 
California  earthquake  of  1868.  Chimney-tops  were  separated  at  their 
junction  with  roofs,  and  twisted  around  without  overthrow  ;  wardrobes 
and  bureaus  turned  about  at  right  angles  to  the  wall,  or  even  with 
their  faces  to  the  wall. 

Explanation. — Some  of  these  effects — such  as  twisting  of  obelisks 
and  chimney-tops,  and  turning  about  of  bureaus,  etc. — may  be  ex- 
plained, as  Lyell  has  shown,  without  any  twisting  motion  of  the  earth 
at  all  or  any  other  than  the  backward  and  forward  motion  common  to 
all  earthquakes.  Thus,  if  we  place  one  brick  on  another,  and  shake 
them  back  and  forth,  holding  only  the  lower  one,  they  are  almost  cer- 
tain to  be  left  twisted  one  on  the  other.  The  reason  is,  that  the  adhe- 
sion is  almost  certain  to  be  greater  toward  one  end  than  the  other — the 
centre  of  friction  does  not  coincide  with  the  centre  of  gravity.  This 
is  the  probable  explanation  of  twisted  obelisks  and  chimney -tops,  etc. 
Also,  the  simple  back-and-forth  shaking  of  a  wardrobe  in  a  diagonal 
direction  would  almost  certainly  lift  up  one  end  and  swing  it  around. 
The  vorticose  motion  in  such  cases  is  probably  not  real,  but  only 
apparent. 

But  there  are  other  cases  of  undoubtedly  real  vorticose  motion ;  as, 
for  example,  straight  rows  of  trees  changed  into  interrupted  zigzags  by 
fissures  and  displacement.  All  such  cases  of  real  twisting  are  prob- 
ably explicable  on  the  principle  of  concurrence  and  interference  of 
waves.  If  two  systems  of  waves  of  any  kind  meet  each  other,  there 
will  be  points  of  concurrence  where  they  reenforce  each  other,  and 
points  of  interference  where  they  destroy  each  other.  Suppose,  for 
instance,  a  system  of  water-waves,  represented  by  the  double  lines  i,  i 


EARTHQUAKES. 


115 


(Fig.  99),  running  in  the  direction  b  £,  strike  against  a  wall,  w  w: 
the  waves  would  be  reflected  in  the  direction  c  c,  and  are  represented 
by  the  single  lines  r,  r.  Then,  if  the  lines  represent  crests,  and  the  inter- 
vening space  the  troughs,  at  the  places  marked  with  crosses  and  dots 
there  would  be  concurrence,  and  therefore  higher  crests  and  deeper 


FIG.  99.— Diagram  illustrating  Reflection  of  Waves. 

troughs,  while  at  the  points  indicated  by  a  dash  there  would  be  inter- 
ference and  mutual  destruction,  and  therefore  smooth  water.  The  same 
takes  place  in  earth- waves.  If  two  systems  of  earth-waves  meet  and 
cross  each  other,  we  must  have  points  of  concurrence  and  interference 
in  close  proximity.  The  ground,  therefore,  will  be  thrown  into  violent 
agitation — points  in  close  proximity  moving  in  opposite  directions 
(twisting).  If  the  motion  be  sufficient  to  rupture  the  earth,  restoration 
is  not  made  by  counter-twisting,  and  the  earth  is  left  in  a  displaced 
condition. 

The  causes  of  interference  may  be  various — sometimes  difference 
of  velocity  of  waves,  already  explained,  by  which  some  overrun  others, 
concurring  and  interfering;  more  often  it  is  the  result  of  reflection 
from  surfaces  of  different  elasticity.  For  example,  it  is  well  known 
that  the  most  violent  effects  of  earthquakes,  especially  twisting  of  the 
ground,  usually  occur  near  the  junction  of  the  softer  strata  of  the 
plains  with  the  harder  and  more  elastic  strata  of  the  mountains.  Now, 
suppose  from  a  shock  at  X  (Fig.  100)  a  system  of  earth-waves  should 
emerge  at  a,  and  run  as  a  surface-wave  toward  the  mountain  m.  The 


116  IGNEOUS  AGENCIES. 

waves,  striking  the  hard,  elastic  material  m,  would  be  partly  trans- 
mitted and  partly  reflected.  The  reflected  waves,  running  in  the  direc- 
tion of  the  arrow  r, 
would  meet  the  ad- 
vancing incident  waves 
moving  in  the  direc- 
tion of  the  arrow  i,  and 
N/  •  concurrence  and  inter- 
'.  ference  would  be  in- 
evitable. 

Minor  Phenomena. — 


kinds  of  earthquakes,  but  many  of  the  minor  phenomena,  are  explained 
by  the  wave-theory. 

1.  Sounds. — These  are  usually  described  as  a  hollow  rumbling,  roll- 
ing, or  grinding  /  sometimes  as  clashing,  thundering,  or  cannonading. 
They  are  probably  produced  by  rupture  of  the  earth  at  the  origin,  and 
by  the  passage  of  the  wave   through  the   imperfectly  elastic  rocky 
medium,  breaking  the  medium  and  grinding  the  broken  parts  together. 
But  what  is  especially  noteworthy  is,  that  these  sounds  precede  as  well 
as  accompany  the  shocks.     In  every  earthquake  there  are  transmitted 
waves  of  every  variety  of  size.     The  great  waves  are  sensible  as  shocks, 
or  jars,  or  tremors  ;  the  very  small  waves,  too  small  to  be  appreciated 
as  tremors,  are  heard  as  sounds.     But,  as  already  explained,  these 
last  run  with  greater  velocity  in  an  imperfectly  coherent  and  elastic 
medium  like  the  earth,  and  therefore  arrive   sooner  than  the  great 
waves,  which  constitute  the  shock.     The  same  was  observed  in  Mallet's 
experiments. 

2.  Motion. — As  to  direction,  the  observed  motion  is  sometimes  verti- 
cally up  and  down,  sometimes  horizontally  back  and  forth,  and  some- 
times oblique  to  the  horizon.     Almost  always  a  rocking  motion,  i.  e.,  a 
leaning  of  tall  objects  first  in  one  direction  and  then  in  the  other,  is 
observed.     As  to  violence  or  velocity  of  motion,  this  is  sometimes  so 
great  that  objects  are  thrown  into  the  air,  and  whole  cities  are  shaken 
down  as  if  they  were  a  mere  collection  of  card-houses,  while  in  other 
cases  only  a  slow  swinging,  or  heaving,  or  gentle  rocking,  is  observed. 

The  difference  in  direction  is  wholly  due  to  the  position  of  the  ob- 
server. At  the  epicentrum  it  is  of  course  vertical,  and  thence  it  be- 
comes more  and  more  oblique,  until  at  great  distances  it  is  usually 
horizontal.  The  violence  of  the  shock  or  velocity  of  ground-motion  de- 
pends partly  upon  the  violence  of  the  original  concussion,  and  partly  on 
the  distance  from  the  origin  or  focus.  This  velocity  of  the  ground- 
motion  must  not  be  confounded  with  the  velocity  of  the  wave  already 
discussed.  The  latter  is  the  velocity  of  transit  from  place  to  place ;  the 


EARTHQUAKES. 


117 


former  is  the  velocity  of  oscillation  up  and  down,  or  back  and  forth. 
The  velocity  of  oscillation  has  no  relation  to  the  velocity  of  transit,  but 
depends  only  on  the  height  of  the  wave,  which  constantly  diminishes 
and  becomes  finally  very  small,  though  the  velocity  of  transit  remains 
the  same,  and  always  enormously  great.  The  rocking  motion  is  also 
easily  explained.  A  series  of  waves,  somewhat  similar  in  form  to  water- 
waves  (though  differing  in  nature),  actually  passes  beneath  the  observer. 
Of  course,  when  an  object  is  on  the  front-slope,  it  will  lean  in  the  direc- 
tion of  transit ;  and,  when  on  the  hind-slope,  in  the  contrary  direction. 
3.  Circle  of  Principal  Destruction. — In  some  earthquakes  a  certain 
zone  at  considerable  distance  from  the  point  of  first  emergence  (epicen- 
trum)  has  been  observed,  in  which  the  destruction  by  overthrow  is  very 
great,  and  beyond  which  it  speedily  diminishes.  This  has  been  called 
the  circle  of  principal  destruction  or  overthrow.  It  is  thus  explained  : 
The  overthrow  of  buildings  depends  not  so  much  on  the  amount  of  oscil- 
lation as  upon  the  horizontal  element  of  the  oscillation.  Now,  the  whole 
amount  of  oscillation  is  greatest  at  the  point  of  first  emergence,  and 
decreases  outward ;  but 
the  horizontal  element 
is  nothing  at  «,  and  in- 
creases as  the  cosine  E. 
Therefore,  under  the  in- 
fluence of  these  two  con- 
ditions, one  decreasing 
the  whole  oscillation,  the 
other  increasing  the  hor- 
izontal element  of  that 

Oscillation,  it  is  evident       FlG'  101-Diagram  illustrating  Circle  of  Principal  Disturbance. 

that  there  will  be  a  point  on  every  side,  or,  in  other  words,  a  circle,  where 

a  maximum.      This  is  shown  in  Fig. 


\ 


the  horizontal  element  will  be 


101,  in  which  a  a',  b  £',  c  c',  etc.,  are  the  decreasing  oscilation,  and 
b  b\  c  c",  are  the  horizontal  element.  This  reaches  a  maximum  at  c.  It 
has  been  found  by  mathematical  calculation,  based  upon  the  supposition 
that  the  whole  oscillation  varies  inversely  as  the  square  of  the  distance 
from  JT,  that  the  horizontal  element  will  be  a  maximum  when  the  angle 
of  emergence  is  54°  44'.  By  determining  by  observation  the  circle  of 
principal  disturbance,  it  is  easy  to  calculate  the  depth  a  X  of  the  focus, 
for  it  will  be  the  apex  of  a  cone  whose  base  is  that  circle,  and  whose 
apical  angle  is  70°  32'. 

4.  Shocks  more  severely  felt  in  Mines. — It  has  been  sometimes  ob- 
served that  shocks  are  distinctly  felt  in  mines  which  are  insensible  at 
the  surface.  This  is  probably  explained  as  follows  :  Let  8  S  (Fig.  102) 
be  the  surface  of  the  ground  ;  and  let  a  b  represent  hard,  elastic  strata, 
covered  with  loose,  inelastic  materials,  c  c.  Now,  if  a  series  of  waves 


118 


IGNEOUS   AGENCIES. 


come  in  the  direction  of  the  arrows  d  <#,  and,  passing  through  a  b  on 
their  way  to  the  surface,  strike  upon  the  lower  surface  of  c  c,  a  portion 
would  reach  the  surface  by  refraction,  but  a  portion  would  be  reflected 
and  return  into  a  5,  concurring  and  interfering  with  the  advancing 
waves,  and  producing  great  commotion  in  these  strata. 

5.  Shocks  less  severe  in  Mines. — This  case  is  probably  more  common 
than  the  last.     It  was  notably  the  case  in  the  earthquake  of  1872  in 


FIG.  102.—  Shocks  in  Mines. 


Inyo  County,  California.  While  the  surface  was  severely  shaken,  and 
many  houses  destroyed,  and  large  fissures  formed  in  the  earth,  the 
miners,  several  hundred  feet  below  the  surface  in  the  hard  rock,  scarcely 
felt  it  at  all. 

This  fact  has  scarcely  been  noticed,  and  no  attempt  has  been  made 
to  explain  it. 

6.  Bridges.  —  In  a  somewhat  similar  manner  are  to  be  accounted  for 
the  phenomena  of  bridges.  In  the  earthquake-regions  of  South  America 
there  are  certain  favored  spots,  often  of  small  extent,  which  are  partially 
exempt  from  the  shocks  which  infest  the  surrounding  country.  The 


FIG.  108. 

earthquake-wave  seems  to  pass  under  them  as  under  a  bridge  to  reap- 
pear again  on  the  other  side.  The  mere  inspection  of  Fig.  103  will 
explain  the  probable  cause  of  this  exemption,  viz. :  reflection  from  the 
under  surface  of  an  isolated  mass  of  soft,  inelastic  strata,  c  c. 

7.  Fissures. — The  ground-fissures,  so  commonly  produced  by  earth- 
quakes, are  sometimes  of  the  nature  of  the  great  fissures  of  the  crust, 
which  are  the  probable  cause  of  earthquakes.  Such  great  fissures  are 


EARTHQUAKES  ORIGINATING  BENEATH   THE  OCEAN.  H9 

usually  wholly  beneath  the  surface  at  great  depth,  but  sometimes  may 
break  through  and  appear  on  the  surface.  This  is  certainly  the  case 
when  decided  faults  occur  with  elevation  or  depression  of  large  tracts 
of  land.  But  the  surface-fissures  so  frequently  described,  small  in  size, 
very  numerous,  and  running  in  all  directions,  have  an  entirely  different 
origin.  They  are  evidently  produced  by  the  shattering  of  the  softer, 
more  incoherent,  and  inelastic  surface-soil,  and  by  the  passage  of  the 
earth-wave.  Even  the  more  elastic  underlying  rock  is  broken  by  the 
same  cause,  but  to  a  much  less  extent. 

Earthquakes  originating  beneath  the  Ocean. 

We  have  thus  far  spoken  of  earthquakes  originating  beneath  the 
land-surface.  But  three-fourths  of  the  earth-surface  is  covered  by  the 
sea ;  and  we  have  already  seen  that  other  forms  of  igneous  agency  are 
most  abundant  in  and  about  the  sea.  As  we  might  expect,  therefore, 
the  greater  number  of  earthquake-shocks  occur  beneath  the  sea-bed. 
In  such,  the  phenomena  already  described  are  complicated  by  the  ad- 
dition of  the  "  Great  Sea  -  Wave." 

Suppose,  then,  an  earthquake-shock  to  occur  beneath  the  sea-bed : 
the  following  waves  will  be  formed :  1.  As  before,  a  series  of  elastic 
spherical  waves  will  spread  from  the  focus,  until  they  emerge  on  the 
sea-bed.  2.  As  before,  a  series  of  circular  surface-waves,  the  outcrop  of 
the  spherical  waves,  will  spread  on  the  sea-bottom  until  they  reach  the 
nearest  shore,  and  perhaps  produce  destructive  effects  there.  3.  On  the 
back  of  this  submarine  earth-wave  is  carried  a  corresponding  sea-wave. 
This  is  called  the  "forced  sea-wave,"  since  it  is  not  a  free  wave,  but  a 
forced  accompaniment  of  the  ground-wave  beneath.  It  reaches  the 
shore  at  the  same  time  as  the  earth-wave.  It  is  of  little  importance. 
4.  In  addition  to  all  these  is  formed  the  great  sea-wave  or  tidal  wave. 

Gfreat  Sea- Wave. — This  common  and  often  very  destructive  accom- 
paniment of  earthquakes  is  formed  as  follows  :  The  sudden  upheaval  of 
the  sea-bad  lifts  the  whole  mass  of  superincumbent  water  to  an  equal 
extent,  forming  a  huge  mound.  The  falling  again  of  this  water  as  far 
beloio  as  it  was  before  above  its  natural  level  generates  a  circular  wave 
of  gravity,  which  spreads  like  other  water-waves,  maintaining  its  origi- 
nal wave-length,  but  gradually  diminishing  its  wave-height  until  it  be- 
comes insensible.  Usually,  a  series  of  such  waves  is  formed  by  the 
motion  of  the  sea-bottom  up  and  down  several  times.  These  waves  are 
often  100  to  200  miles  across  their  base  (wave-length)  and  fifty  to  sixty 
feet  high  at  their  origin.  Their  destructive  effects  may  be  inferred  from 
the  enormous  quantity  of  water  they  contain.  In  the  open  sea  they 
create  no  current,  and  are  not  even  perceived,  but,  when  they  touch 
bottom  near  shore,  they  rush  forward  as  great  breakers  fifty  or  sixty 
feet  high,  sweeping  away  everything  in  their  course. 


120  IGNEOUS  AGENCIES. 

Being  waves  of  gravity,  their  velocity,  though  very  great  on  account 
of  their  size,  is  far  less  than  that  of  the  earth-waves,  and  they  reach  the 
neighboring  shore,  therefore,  some  time  later,  and  often  complete  the 
destruction  commenced  by  the  earth-waves. 

Examples  of  the  Sea-Wave. — In  the  great  earthquake  which  de- 
stroyed Lisbon  in  1755,  the  epicentrum  was  on  the  sea-bed  fifty  or  more 
miles  off  the  coast  of  Portugal.  From  this  point  the  surface  earth- 
waves  spread  along  the  sea-bottom  until  they  reached  shore.  It  was 
the  arrival  of  these  waves  which  destroyed  Lisbon.  About  a  half-hour 
later,  when  all  had  become  quiet,  several  great  sea-waves,  one  of  them 
sixty  feet  high,  came  rushing  in,  deluging  the  whole  coast  and  com- 
pleting the  destruction  commenced  by  the  earth-waves.  This  wave  was 
thirty  feet  high  at  Cadiz,  eighteen  feet  at  Madeira,  and  five  feet  on  the 
coast  of  Ireland.  It  was  sensible  on  the  coast  of  Norway,  and  even  on 
the  coast  of  the  West  Indies,  after  having  crossed  the  whole  breadth 
of  the  Atlantic. 

In  1854  a  great  earthquake  shook  the  coast  of  Japan.  Its  focus  was 
evidently  beneath  the  sea-bed  some  distance  off  the  coast,  for,  in  about 
a  half-hour,  a  series  of  water-waves  thirty  feet  high  rushed  upcn  shore 
and  completely  swept  away  the  town  of  Simoda.  From  the  same  cen- 
tre the  waves,  of  course,  spread  in  the  contrary  direction,  traversed  the 
whole  breadth  of  the  Pacific,  and  in  about  twelve  and  a  quarter  hours 
struck  on  the  coast  of  California  at  San  Francisco,  and  swept  down  the 
coast  to  San  Diego.  These  waves  were  thirty  feet  high  at  Simoda,  fif- 
teen feet  high  at  Peel's  Island,  about  1,000  miles  off  the  coast  of  Japan, 
0.65  feet,  or  eight  inches,  high  at  San  Francisco,  and  six  inches  at  San 
Diego.1 

On  the  13th  of  August,  1868,  a  great  earthquake  desolated  the  coast 
of  Peru.  Its  focus  was  evidently  but  a  little  way  off  shore,  for  in  less 
than  a  half-hour  a  series  of  water-waves  fifty  or  sixty  feet  high  rushed 
in  and  greatly  increased  the  devastation  commenced  by  the  earth-waves. 
These  waves  reached  Coquiinbo,  800  miles  distant,  in  three  hours; 
Honolulu,  Sandwich  Islands,  5,580  miles,  in  twelve  hours  ;  the  Japan 
coast,  over  10,000  miles,  the  next  day.  They  were  also  observed  on 
the  coast  of  California,  Oregon,  and  Alaska,  over  6,000  miles  in  one 
direction,  and  on  the  Australian  coast,  nearly  8,000  miles  in  another 
direction.  This  series  of  waves  was  distinctly  sensible  at  a  distance  of 
nearly  half  the  circumference  of  the  earth.  Had  it  not  been  for  the 
barrier  of  the  South  American  Continent,  it  would  have  encircled  the 
globe.9 

There  are  several  points  in  the  above  description  which  we  must 
very  briefly  explain : 

1.  The  velocity  of  these  great  sea-waves,  though  less  than  that  of 

1  "  Report  of  Coast  Survey  for  1862."  2  "  Report  of  Coast  Survey  for  1869." 


EARTHQUAKES   ORIGINATING  BENEATH  THE  OCEAN.  J21 

the  earth-waves,  is  still  very  great  in  comparison  with  ordinary  sea- 
waves.  The  waves  of  the  Japan  earthquake  crossed  the  Pacific  to  San 
Francisco,  a  distance  of  4,525  miles,  in  a  little  more  than  twelve  hours, 
and  therefore  at  a  rate  of  370  miles  per  hour,  or  over  six  miles  per 
minute.  The  waves  of  the  South  American  earthquake  of  1868  ran  to 
the  Hawaiian  Islands  at  a  rate  of  454  miles  per  hour.  This  amazing 
velocity  is  the  result  of  the  great  size  of  these  waves. 

2.  The  size  of  these  great  waves  is  determined  by  multiplying  the 
time  of  oscillation  by  the  velocity,  on  the  well-known  principle  that 
every  kind  of  wave  runs  its  own  length  during  the  time  of  one  com- 
plete oscillation.     The  velocity  is  obtained  by  observing  the  time  at 
different  points.     The  time  of  oscillation  is  determined  by  means  of 
tidal  gauges.     The  tidal  gauges  established  by  the  coast  survey  on  the 
Pacific  coast  showed  that  the  time  of  oscillation  of  the  larger  waves  of 
the  Japan  earthquake  was  about  thirty-three  (thirty  one  to  thirty-five) 
minutes.     This  would  give  a  wave-length  of  a  little  over  200  miles.     It 
is  probable  that  the  wave-length  in  the  case  of  the  South  American 
earthquake  was  at  least  equally  great. 

3.  The  distance  to  which  the  sea-waves  run  is  far  greater  than  that 
of  the  earth-waves.     The  former  is  distinctly  sensible  for  10,000  miles  ; 
the  latter  very  rarely  more  than  a  few  hundreds.     There  are  two  rea- 
sons for  this  :  1.  All  waves  diminish  in  oscillation  (wave-height)  as  they 
spread  from  the  origin,  because  the  quantity  of  matter  successively 
involved  in  the  oscillation  constantly  increases.     But  in  the  one  case 
the  matter  involved  lies  in  the  circumference  of  a  fcircle  ;  in  the  other, 
in  the  surface  of  a  sphere  ;  therefore,  the  one  increases  as  the  distance, 
the  other  as  the  square  of  the  distance.     Therefore,  the  decrease  of  os- 
cillation (height  of  wave)  is  far  less  rapid  for  water-waves  than  for  elas- 
tic spherical  waves.     2.  A  still  more  effective  reason  is  this:  Water- 
waves  run  in  a  perfectly  homogeneous  medium,  and  therefore  diminish 
only  according  to  the  regular  law  just  stated  ;  but  the  earth-waves  run 
in  an  heterogeneous,  imperfectlv  elastic,  and  imperfectly  coherent  me- 
dium, and  therefore  they  are  rapidly  quenched  and  dissipated  by  re- 
peated refractions  and  reflections,  and  by  repeated  fractures  of  the 
medium,  and  thus  changed  into  other  forms  of  force,  as  heat,  electrici- 
ty, etc.     Were  it  not  for  this,  the  destructive  effects  of  earthquakes 
would  be  far  more  extensive. 

4.  We  have  said  the  wave-length  remains  unchanged.    This  length, 
therefore,  represents  the  diameter  of  the  original  water-mound,  and 
therefore  of  the  original  sea-bottom  upheaval.      In  the  Japan  earth- 
quake this  was  200  miles  across.     This  shows  the  grand  scale  upon 
which  earthquake-movements  take  place. 

5.  As  already  explained,  earthquake  sea-waves  differ  from  all  other 
sea-waves  in  that  their  great  size  makes  them  drag  bottom  even  in  open 


122  IGNEOUS  AGENCIES. 

deep  sea.  In  their  case,  therefore,  the  velocity  depends  not  only  on 
the  wave-length,  but  also  on  the  depth  of  the  sea.  Knowing  the  size 
(wave-length)  of  these  waves,  and  therefore  what  ought  to  be  their 
free  velocity,  and  also  knowing  their  actual  velocity  by  observation, 
the  difference  gives  the  retardation  by  dragging ;  and  by  the  retarda- 
tion may  be  calculated  the  mean  depth  of  the  ocean  traversed.  In  this 
Avay  it  has  been  determined  that  the  mean  depth  of  the  Pacific  between 
Japan  and  San  Francisco  is  12,000  feet,  and  between  Peru  and  Hono- 
lulu, Sandwich  Islands,  18,500  feet.  The  great  importance  of  such 
results  is  obvious. 

Depth  of  Earthquake-Focus. 

The  great  obscurity  which  hangs  about  the  subject  of  the  interior 
condition  of  the  earth  and  the  ultimate  cause  of  igneous  agencies  ren- 
ders any  positive  knowledge  on  these  subjects  of  peculiar  interest.  There 
can  be  little  doubt  that  the  phenomena  of  earthquake-waves,  their  form, 
their  velocity,  their  angle  of  emergence,  etc.,  if  once  thoroughly  under- 
stood, would  be  a  most  delicate  index  of  this  condition,  and  a  powerful 
means  of  solving  many  problems  which  now  seem  beyond  the  reach  of 
science.  Among  problems  of  this  kind  none  is  more  important,  and  at 
the  same  time  more  capable  of  solution,  than  the  depth  of  the  origin  of 
earthquakes,  and  therefore  presumably  of  volcanoes. 

Seismometers.— The  most  direct  way  of  determining  the  depth  of 
an  earthquake-focus  is  by  means  of  well-constructed  seismometers. 
These  are  instruments  for  measuring  and  recording  earthquake-phenom- 
ena. They  are  of  infinite  variety  of  forms,  depending  partly  upon  the 
facts  desired  to  be  recorded,  and  partly  upon  the  mode  of  record.  As 
examples  we  will  mention  only  two  : 

An  excellent  instrument  for  recording  slight  tremors  is  one  invented 
and  used  by  Prof.  Palmieri,  of  the  Vesuvian  Observatory.  It  consists 
of  a  telegraphic  apparatus  with  the  usual  paper-slip  and  stile.  The 
paper-slip,  accurately  divided  into  hours,  minutes,  and  seconds,  travels 
at  a  uniform  rate  by  means  of  clock-work.  The  battery-circuit  is  closed 
and  opened,  and  the  recording  stile  worked  by  the  shaking  of  a  metallic 
bob,  hung  by  a  delicate  spiral  spring  above  a  mercury-cup  ;  the  shak- 
ing of  the  bob  being  determined  by  the  tremor  of  the  earth.  Such  an 
instrument  records  the  exact  moment  of  occurrence  of  earthquake-shocks, 
however  slight ;  also,  the  moment  of  passage  of  every  wave  and  its  time 
of  oscillation ;  and  if  there  be  more  than  one  such  instrument,  the  mo- 
ment of  occurrence  at  different  places  gives  the  velocity  of  the  surface- 
wave  v'.  It  records,  however,  rather  than  measures  earthquake-phe- 
nomena ;  it  is  a  seismograph  rather  than  a  seismometer. 

The  best  form  of  seismometer  which  we  have  seen  described — that 
which  gives  the  most  important  information — is  that  of  Prof.  Cavalleri, 
of  Monza.  It  consists  essentially  of  two  pendulums,  one  horizontally 


DEPTH   OF  EARTHQUAKE-FOCUS. 


123 


and  the  other  vertically  oscillating  (Figs.  104  and  105).  The  former 
(Fig.  104)  is  an  ordinary  pendulum,  with  a  heavy  bob,  b,  armed  with  a 
stile  which  touches  a  bed  of  sand,  s  s.  The  sharp  point  of  the  stile 
rests  loosely  in  a  slight  depression  in  a  small  flat  cylinder  or  button,  c, 
resting  lightly  on  the  top  of  the  firm  column  d.  When  the  earthquake- 
shock  arrives,  the  whole  building,  and  therefore  the  attachment  a,  above, 
and  the  bed  of  sand,  s  s,  on  the  floor,  will  move  in  the  direction  of  the 
shock.  This  direction  will  generally  be  partly  horizontal  and  partly 
vertical  (x  b,xc,x  d,  Fig.  94).  We  will  consider  now  only  the  hori- 

V 


d  FIGS.  104  and  105.— Cavalleri's  Seismometer. 

zontal  element.  The  pendulum,  b,  will  tend  to  retain  its  position,  and 
the  bed  of  sand  will  move  beneath  it,  first  in  one  direction  and  then  in 
the  other,  and  the  stile  will  thus  mark  the  sand  back  and  forth  to  a  dis- 
tance equal  to  the  back-and-forth  motion  of  the  earth.  The  direction 
from  which  the  impulse  came  is  determined  by  the  side  on  which  the 
little  cylinder  falls.  It  is  easy  to  connect  the  pendulum  with  a  clock 
set  at  twelve,  in  such  wise  that  the  motion  of  the  former  will  in- 
stantly set  agoing  the  latter.  The  difference  between  this  clock-time 
and  the  real  time  will  give  the  instant  of  transit.  It  is  clear  that  this 
pendulum  does  not  give  the  whole  amount  of  the  vibration  or  motion 
of  the  shock,  but  only  the  horizontal  element.  If  a  b  (Fig.  106)  repre- 
sent the  direction  and  amount  of  vibration,  then 
a  c  is  the  horizontal  element  measured  by  the 
pendulum.  This  instrument,  therefore,  gives 
the  moment  of  transit,  the  direction  of  transit, 
and  the  horizontal  element  of  vibration. 

The  vertical  element,  b  c,  of  the  vibration  is 


FIG.  106. 


124 


IGNEOUS  AGENCIES. 


given  bv  a  vertically  oscillating  pendulum  (Fig.  105),  the  point  of 
which  rests  lightly  on  one  arm,  a,  of  a  very  easily-moved  lever,  the  other 
arm,  b,  of  which  acts  as  an  index  by  means  of  a  graduated  quadrant. 
When  the  shock  moves  the  floor  of  the  building  upward,  the  heavy 
weight  of  the  pendulum  retaining  its  position  by  stretching  of  the  wire 
spring,  the  arm  a  is  pressed  against  the  stile,  and  the  arm  b  is  elevated ; 
when  the  floor  descends  again,  b  is  retained  in  its  elevated  position  by  a 
ratchet  at  <?,  and  thus  records  the  amount  of  elevation  of  the  floor.  This 
pendulum,  therefore,  gives  the  upward  movement  or  one-half  the  whole 
vertical  element.  Having  now  the  horizontal  and  vertical  element,  i.  e., 
the  base  and  perpendicular  of  a  right-angled  triangle,  the  hypothenuse,  or 
whole  oscillation,  and  the  direction  of  oscillation,  or  angle  of  emergence 
(a,  Fig.  106),  are  gotten  by  simple  calculation  (a  b  —  \/  a  c*  +  b  ca, 
and  a  b  :  R  ::  be:  sine  a),  or  by  accurate  plotting. 

The  important  facts  recorded  by  this  instrument  are :  1.  The  in- 
stant of  transit ;  2.  The  direction  of  transit ;  3.  The  direction  of  os- 
cillation, or  angle  of  emergence  ;  4.  The  amount  of  oscillation.  From 
these  elements  (if  we  have  several  seismometers  scattered  about  the 
country)  may  be  calculated  :  1.  The  velocity  of  transit ;  2.  The  posi- 
tion of  the  focus  ;  3.  The  form  of  the  focus,  whether  point  or  fissure; 
4.  The  force  of  the  original  concussion.  The  most  important  of  these 
are  the  position  and  depth  of  the  focus. 

The  Determination  of  the  Epicentnim.— Cavalleri's  seismometer 
gives  the  direction  of  transit  of  the  surface-wave.  If,  by  the  use  of 
many  such  seismometers,  or  even  by  rougher  methods,  we  get  a  number 
of  these  surface-lines  of  transit,  by  following  these  back  we  get  the 

epicentrum  at  their   intersec- 
tion.    Or  if,  by  means  of  many 
seismographs  giving  time  of 
transit,  or   even    by  observa- 
tories or  stations  of  any  kind 
_  with  accurate  clocks,  we  get 
several  points  of  simultaneous 
arrival  of  the  wave,  then  by 
drawing  a  curve  through  these 
points    we   have   a   coseismal 
curve.    A  perpendicular  drawn 
from  the  middle  point  of  the 
line  joining  any  two  of  these 
points  will  pass  through  the 
epicentrum,  and  two  such  per- 
pendiculars would  determine  its  position.    Fig.  107  represents  coseismal 
curves,  and  b,  c,  <7,  three  points  on  the  curve ;  a  is  the  epicentrum. 
Determination  of  the  FOCUS. — The  spherical  wave  is  a  wave  of  longi- 


Fio.  107.— Coseismal  Lines. 


DEPTH  OF  EARTHQUAKE-FOCUS.  125 

tudinal  oscillation.  The  direction  of  oscillation,  therefore,  is  the  same 
as  the  direction  of  transmission  (wave-path),  which  is  the  radius  of 
the  agitated  sphere.  If,  therefore,  the  direction  of  the  ground-motion 
(the  line  a  £,  Fig.  106)  be  followed  into  the  earth,  it  carries  us  back 
along  the  wave-path  to  its  origin,  the  focus.  Two  such  wave-paths 
by  their  intersection  would  determine  its  position.  Thus,  in  Fig.  108, 
if  c  and  b  be  the  position  of  two  seismometric  observatories,  the  angles 


of  emergence,  x  c  a  and  x  b  «,  being  given  by  observation,  and  the  dis- 
tance, c  b,  being  known,  we  have  all  the  elements  necessary  to  deter- 
mine either  by  calculation  or  by  accurate  plotting  the  wave-paths  c  x 
and  b  x,  and  their  point  of  intersection  x,  and  therefore  of  the  depth  a  x. 

Although  seismometers,  such  as  we  have  described,  are  necessary 
for  accurate  results  from  few  observations,  yet  by  multiplying  the  ob- 
servations, even  by  rough  methods,  approximative  results  may  be  ob- 
tained. We  will  mention  only  two  examples  : 

In  1857  a  terrible  earthquake  shook  the  territory  of  Naples,  de- 
stroying many  towns  and  villages,  and  killing  about  10,000  people. 
The  scene  of  destruction  was  visited  soon  after  by  Mr.  Mallet.  By  care- 
ful examination  of  overthrown  objects,  many  lines  of  transit  of  the  sur- 
face-wave were  determined,  which,  protracted,  carried  him  with  consider- 
able certainty  to  the  epicentrum  ;  similarly  many  lines  of  emergence,  or 
paths  of  the  spherical  wave,  protracted  back,  conducted  to  the  focus. 
This  focus  was  determined  to  be  not  a  point,  but  a  fissure,  nine  miles 
long  and  through  three  miles  of  solid  rock.  The  centre  of  this  rent 
was  about  six  miles  beneath  the  surface. 

In  1874  a  not  very  severe  earthquake  shook  Central  Germany.  It 
has  been  thoroughly  investigated  by  Seebach.  The  epicentrum  was 
determined  with  great  precision  by  erecting  perpendiculars  to  the  bi- 
sec,ted  chords  of  the  coseismal  curves.  The  focus  was  determined  as 
a  rent  through  four  miles  of  rock,  the  centre  of  the  rent  being  nine  or 
ten  miles  in  depth. 

The  velocity  of  transit  of  the  waves  of  the  Naples  earthquake  was 
860  feet  per  second,  or  between  nine  and  ten  miles  per  minute ;  that 


126  IGNEOUS  AGENCIES. 

of  the  earthquake  of  Middle  Germany  was  about  twenty-eight  miles 
per  minute. 

There  have  been  many  attempts  to  determine  the  depth  of  earth- 
quakes by  other  methods,  especially  by  using  the  relative  velocities  of 
the  spherical  and  the  surface  waves  as  a  means  of  getting  the  angle  of 

emergence   (cos.  E  =— H  ;  but  such  a  method  is  evidently  valueless, 

because  the  velocity  of  the  spherical  wave  (v)  is  not  constant. 

Effect  of  the  Moon  on  Earthquake  -  Occurrence. — By  an  extensive 
comparison  of  the  times  of  occurrence  of  several  thousand  earthquakes 
with  the  positions  of  the  moon,  Alexis  Perry  has  made  out  with  some 
probability  the  following  laws :  1.  Earthquakes  are  a  little  more  fre- 
quent when  the  moon  is  on  the  meridian  than  when  she  is  on  the 
horizon.  2.  They  are  a  little  more  frequent  at  new  and  full  moon 
(syzygies)  than  at  half-moon  (quadratures).  3.  They  are  a  little  more 
frequent  when  the  moon  is  nearest  the  earth  (perigee)  than  when  she 
is  farthest  off  (apogee).  Now,  if  these  laws  are  really  true,  it  would 
seem  that  there  is  a  slight  tendency  for  earthquakes  to  follow  the  law 
of  tides  :  for  the  first  law  gives  the  time  of  flood-tide,  and  the  sec- 
ond and  third  the  times  of  highest  flood-tide.  It  would  seem,  there- 
fore, that  the  attraction  of  the  sun  and  moon  has  a  perceptible  effect 
in  determining  the  time  of  occurrence  of  earthquakes.  Many  geolo- 
gists regard  these  laws,  if  established,  as  conclusive  proof  of  the  gen- 
eral fluid  condition  of  the  earth  beneath  a  comparatively  thin  crust. 
This  interior  liquid  they  suppose  to  be  influenced  by  the  tide-generat- 
ing forces  of  the  sun  and  moon ;  but,  if  this  were  true,  the  effect  ought 
to  be  far  greater  than  we  find  it.  Whatever  be  the  interior  condition 
of  the  earth,  the  effect  of  the  moon  on  the  meridian  would  be  to  assist, 
and  on  the  horizon  to  repress,  any  force  whatsoever  tending  to  break 
up  the  crust  of  the  ea%rth  and  to  produce  an  earthquake. 

Relation  of  Earthquake-Occurrence  to  Seasons  and  Atmospheric  Con- 
ditions.— By  extensive  comparison  of  earthquake-occurrence  with  the 
seasons,  it  has  been  shown  that  they  are  a  trifle  more  frequent  in  win- 
ter than  in  summer.  Constructing  a  curve  representing  the  annual 
variation  of  earthquake-intensity,  this  curve  rises  to  its  maximum  in 
January  and  sinks  to  its  minimum  in  July.  But  the  difference  is  small. 
There  has  been  no  satisfactory  explanation  of  this  fact. 

There  is  an  almost  universal  popular  belief  in  earthquake-regions 
that  the  occurrence  is  preceded  by  a  still,  oppressive  state  of  the  air. 
Although  no  scientific  investigations  have  confirmed  this  impression, 
yet  it  seems  quite  possible  and  even  probable  that  diminished  atmos- 
pheric pressure,  indicated  by  a  slow  state  of  the  barometer,  may  act  as 
a  determining  cause  of  earthquake-occurrence,  precisely  as  the  position 
of  the  moon  on  the  meridian.  In  both  cases,  however,  we  must  regard 


ELEVATION  AND   DEPRESSION  OF  EARTH'S   CRUST.  127 

these  not  as  true  causes  of  earthquakes,  but  only  as  causes  determin- 
ing the  moment  of  occurrence. 

SECTION  4. — GRADUAL  ELEVATION  AND  DEPRESSION  OF  THE  EARTH'S 
CRUST. 

Of  all  the  effects  of  igneous  agencies  these  are  by  far  the  most  impor- 
tant. Although  not  violent  and  destructive  like  volcanoes  and  earth- 
quakes, although  indeed  so  little  conspicuous  as  to  be  generally  unob- 
servable  except  to  the  eye  of  science,  yet,  acting  not  paroxysmally  but 
constantly,  not  in  isolated  spots  but  over  wide  areas,  and  affecting 
whole  continents,  their  final  result  in  modifying  the  crust  of  the  earth 
and  making  history  is  far  greater  than  that  of  all  other  igneous  agen- 
cies put  together.  It  is  probable  that  the  same  causes  which  are  now 
at  work  gradually  raising  or  depressing  the  earth's  crust  have  during 
geological  times  formed  the  continents  and  the  seas. 

Elevation  or  Depression  during  Earthquakes.— We  have  already 

spoken  (page  105)  of  sudden  elevations  or  depressions  of  very  great 
areas  of  country  at  the  time  of  earthquake-occurrence  in  Hindostan,  in 
the  valley  of  the  Mississippi  River,  and  especially  of  the  southern  part  of 
South  America.  It  is  not  probable,  however,  that  much  is  accomplished 
in  this  paroxysmal  way.  These  cases  are  referred  to  in  order  to  show 
the  close  connection  of  such  sudden  bodily  movements,  and  therefore 
presumably,  also,  of  the  slower  movements  about  to  be  described,  with 
the  causes  and  forces  which  produce  earthquakes. 

Movements  not  connected  with  Earthquakes — South  America. — Be- 
sides the  sudden  elevation  of  Chili  and  Patagonia  by  earthquakes,  the 
same  countries  show  evidences  of  gradual  elevation  on  a  stupendous 
scale.  The  evidences  are  old  sea-beaches,  full  of  shells  of  species  now 
living  in  the  adjacent  sea,  far  above  the  present  water-level.  These 
"  raised  beaches  "  have  been  traced  1,180  miles  on  the  eastern  shore 
and  2,075  miles  on  the  western,  and  at  different  levels  from  100  to  1,300 
feet  above  the  sea.  More  recently  Alexander  Agassiz  has  traced  them 
by  means  of  corals  still  sticking  to  the  rocks  to  the  height  of  3,000  feet. 
It  is  not  probable  that  all  this  movement  took  place  during  the  present 
geological  epoch,  but  it  is  the  more  instructive  on  that  very  account, 
since  it  shows  the  identity  of  geological  causes  with  causes  now  in 
operation. 

Italy. — The  most  carefully-observed  instance  of  gradual  depression 
and  elevation  is  that  of  the  coast  of  Naples.  Fig.  109  is  a  map 
and  Fig.  110  a  section  of  the  coast  of  the  bay  of  Baiae,  near  Naples. 
Between  a  a  a,  the  present  coast-line,  and  the  cliff  b  b  b,  which  marks 
the  position  of  the  former  coast-line,  there  is  a  nearly  level  plain  called 
the  Starza.  Now,  there  is  perfect  evidence  that  at  one  time  the  land 
was  depressed  until  the  sea  beat  against  the  cliff  b  5,  and  that  both 


128 


IGNEOUS  AGENCIES. 


the  depression  and  the  reelevation  to  its  present  condition  took  place 
since  the  period  of  Roman  greatness.    The  evidence  is  as  follows : 


Sta, 


1.  There  are  certain  shells  abundant  in  the  Mediterranean  and  in 
many  other  seas,  called  lithodomus  (/U0oc,  a  stone;  domus,  a  house), 

from  their  habit  of  boring  for  them- 
selves holes  in  the  rocks  near  the 
water-line.  Such  borings,  often 
with  the  dead  shells  in  them,  are 
found  all  along  the  base  of  the  cliff 
b  b,  twenty  feet  above  the  present 
sea-level.  2.  The  level  plain  called 
Starza  is  composed  of  strata  containing  shells  cf  the  Mediterranean 
and  Roman  works  of  art.  3.  On  this  plain,  near  the  present  sea- 
margin,  are  the  ruins  of  a  Roman  temple  dedicated  to  Jupiter  Serapis. 
The  floor  and  three  of  the  columns  of  this  beautiful  work  are  still  almost 
perfect  (Fig.  110).  When  first  discovered  the  floor  and  the  lower  part 
of  the  columns  were  covered  by  the  materials  of  the  plain.  Above  the 
part  thus  covered  the  columns  were  bored  with  lithodomi  to  a  height 
of  twenty  feet.  This  temple  was,  of  course,  above  the  sea-level  during 
the  Roman,  period.  After  that  period  it  sank  until  the  sea-level  stood 
at  s'  (Fig.  110),  twenty  feet  above  the  base.  Now,  the  floor  of  the 
temple  is  again  on  a  level  with  the  sea.  These  changes  were  so  gradual 
that  they  were  entirely  insensible,  and,  in  fact,  unknown  to  the  inhab- 
itants. The  upright  position  of  the  columns  also  shows  that  it  could 
not  have  been  produced  by  convulsive  action.  4.  Italian  historians 
state  that  in  1530  the  sea  beat  against  the  cliff  b  b.  5.  Evidences  of 
similar  changes,  in  some  cases  depression  and  in  others  elevation,  are 
seen  in  many  places  along  the  coast  of  Italy,  Candia,  and  Greece. 


ELEVATION  AND   DEPRESSION  OF  EARTH'S   CRUST.  129 

In  all  the  cases  thus  far  mentioned,  but  especially  that  of  the  tem- 
ple of  Serapis,  the  near  vicinity  of  volcanoes  (Fig.  109)  suggests  that 
these  effects  were  probably  in  some  way  connected  with  volcanic  ac- 
tion. But  there  are  many  instances  in  which  no  such  connection  can 
be  traced. 

Scandinavia. — The  best-observed  instance  of  this  kind  is  that  of  the 
coasts  of  Norway  and  Sweden.  Careful  observations  on  the  coasts  of 
the  Baltic  and  Polar  Seas  have  proved  that  nearly  the  whole  of  Norway 
and  Sweden  is  rising  slowly,  and  has  been  rising  for  thousands  of  years. 
South  of  Stockholm  there  is  no  elevation,  but,  on  the  contrary,  slight  de- 
pression; but  north  of  Stockholm  the  whole  coast  is  rising  at  a  rate  which 
increases  as  we  go  north  until  it  attains  a  maximum  at  the  North 
Cape  of  five  to  six  feet  per  century.  These  observations  were  made 
under  the  direction  of  the  Swedish  Government  by  means  of  permanent 
marks  made  at  the  sea-level,  and  examined  from  year  to  year.  That 
similar  changes  have  been  in  progress  for  thousands  of  years,  and  have 
greatly  increased  both  the  height  and  the  extent  of  these  countries,  is 
proved  by  the  fact  that  old  sea-beaches,  full  of  shells  of  species  now 
living  in  the  neighboring  seas,  are  found  fifty  to  seventy  miles  inland, 
and  100,  200,  and  even  600  feet  above  the  present  sea-level.  In  some 
places,  the  country  rock,  when  uncovered  by  removing  superficial  de- 
posit of  beach-shells,  is  found  studded  with  barnacles  like  those  which 
mark  the  present  shore-line  (Jukes). 

The  rising  area  is  about  1,000  miles  long  north  and  south,  and  of 
unknown  breadth.  It  may  embrace  a  considerable  portion  of  Russia. 
Lyell  estimates  the  average  rate  as  not  more  than  two  and  a  half 
feet  per  century.  At  this  rate,  to  rise  600  feet  would  require  24,000 
years.1  Similar  raised  beaches  are  found  in  nearly  all  countries.  We 
give  these  as  examples  of  an  almost  universal  phenomenon,  which  will 
be  again  more  perfectly  described  in  the  chapter  on  the  Quaternary. 

Greenland. — For  obvious  reasons,  evidences  of  elevation  are  much 
more  conspicuous  than  evidences  of  depression.  One  of  the  best-ob- 
served instances  of  the  latter  is  that  of  the  coast  of  Greenland.  This 
coast  is  now  sinking  along  a  space  of  600  miles.  Ancient  buildings  on 
low  rock-islands  have  been  gradually  submerged,  and  experience  has 
taught  the  native  Greenlander  never  to  build  his  hut  near  the  water's 
edge. 

Deltas  of  Large  Rivers. — In  the  deltas  of  the  Mississippi,  the 
Ganges,  the  Po,  and  many  other  large  rivers,  there  are  unmistakable 
evidences  of  gradual  depression.  These  evidences  are  fresh-water 
shells,  and  planes  of  vegetation,  or  dirt-beds,  far  below  the  present  level 
of  the  sea.  A  section  of  the  delta  deposits  of  the  Mississippi  River  re- 
veals the  fact  that  these  deposits  consist  of  river  sands  and  clays,  s,  cl 

1  Lyell'a  "  Antiquity  of  Man,"  p.  58. 
9 


130 


IGNEOUS  AGENCIES. 


(Fig.  Ill),  containing  fresh-water  shells,  with  now  and  then  an  inter- 
calated stratum  of  marine  origin,  I,  containing  marine  shells,  and  at 
uncertain  intervals  distinct  lines  of  turf  or  vegetable  soil,  g',  g",  each 
with  the  stumps  and  roots  of  cypress-trees  as  they  originally  grew. 
Each  one  of  these  turf-lines  is  a  submerged  forest-ground,  except  the 

uppermost,  which  is  the  pres- 
ent forest-ground.  Precisely 
similar  phenomena  have  been 
observed  in  other  large  deltas. 
The  deltas  of  the  Ganges  and 
the  Po  have  been  penetrated 
more  than  400  feet  without 
reaching  bottom.  In  both 
the  deposit  is  made  up  '  of 
fresh -water  strata  alternat- 
ing with  dirt-beds  or  forest- 
grounds.  These  facts  prove  that  these  great  deltas  have  been  at 
intervals  during  the  whole  period  of  their  formation,  as  they  are  now, 
fresh-water  swamps,  overgrown  in  parts  with  trees,  etc. ;  that  they  have 
steadily  subsided  to  a  depth  indicated  by  the  thickness  of  the  deposit 
containing  the  old  forest-levels;  that  the  upbuilding  by  river-deposit 
has  gone  on  pari  passu,  so  as  to  maintain  nearly  the  same  level  all  the 
time  ;  but  that  from  time  to  time  the  subsidence  was  more  rapid,  so 
that  the  sea  gained  possession  for  a  while  until  it  was  again  reclaimed 
by  river-deposit,  and  again  more  slow,  so  that  the  area  was  again  thor- 
oughly covered  with  forests,  and  so  on.  These  facts  are  of  great  im- 
portance in  geology,  and  will  be  frequently  referred  to  in  the  following 
pages. 

Southern  Atlantic  States. — Evidence  of  a  similar  kind  proves  that  a 
'large  portion  of  the  coasts  of  our  Southern  Atlantic  States  is  slowly  sub- 
siding at  the  present  time,  though  there  are  also  evidences,  in  the  form 
of  raised  beaches,  of  elevation  immediately  preceding  the  present  sub- 
sidence. The  evidences  of  subsidence  are  most  conspicuous  along  the 
coast  of  South  Carolina  and  Georgia.  They  consist  of  cypress-stumps 
in  situ  below  the  present  tide-level. 

These  facts  seem  to  point  to  the  conclusion  that  subsidence  is  going 
on  in  nearly  all  places  where  large  deposits  of  sediment  are  accumu- 
lating. 

Pacific  Ocean. — But  by  far  the  grandest  example  of  subsidence 
known  is  that  which  has  been  going  on  for  thousands,  probably  hundreds 
of  thousands,  of  years,  and  is  still  going  on  in  the  mid-Pacific  Ocean. 
The  subsiding  area  is  situated  under  the  equator,  and  is  about  6,000 
miles  long,  by  about  2,000  to  3,000  miles  wide.  The  evidence  of  the 
.subsidence  and  its  rate  is  entirely  derived  from  the  study  of  coral-reefs 


THEORIES  OF  ELEVATION  AND   DEPRESSION.  131 

in  this  region.     The  further  discussion  of  the  subject  will  be  deferred 
until  we  take  up  coral-reefs. 

Our  examples,  be  it  observed,  are  all  taken  from  the  vicinity  of 
coast-lines,  the  sea-level  being  used  as  term  of  comparison.  In  the 
interior  of  continents,  and  in  the  midst,  of  the  sea  where  there  are  no 
islands,  we  have  no  such  means  of  detecting  changes,  yet  it  is  precisely 
there,  i.  e.,  in  the  middle  of  the  rising  or  subsiding  area,  that  the 
changes  are  probably  the  greatest. 

Theories  of  Elevation  and  Depression, 

It  is  evident  that  observation  only  determines  changes  of  relative 
position  of  sea  and  land.  These  changes  may  be  the  result  of  rise  and 
fall  of  sea,  or  rise  and  fall  of  land.  The  popular  mind  naturally  at- 
tributes them  to  the  rise  and  fall  of  the  sea,  as  the  more  unstable  ele- 
ment. But,  by  the  principle  of  hydrostatic  level,  it  is  clearly  impossi- 
ble that  the  ocean  should  rise  or  fall  permanently  at  one  place  without 
being  similarly  affected  everywhere.  It  is  certain,  therefore,  that  the 
changes  we  have  described  above,  being  in  different  directions  in  dif- 
ferent places,  must  be  due  to  movements  of  the  solid  crust.  Neverthe- 
less, it  is  also  true  that  any  increase  in  the  height  and  extent  of  the 
whole  amount  of  land  on  the  globe  must  be  attended  with  a  correspond- 
ing depression  of  the  sea-bottoms,  and  therefore  an  actual  subsidence 
of  the  sea-level  everywhere.  Hence,  if  it  be  true,  as  is  generally  be- 
lieved, that  the  continents  have  been,  on  the  whole,  increasing  in  ex- 
tent and  in  height,  in  the  course  of  geological  history,  then  it  is  true 
also  that  the  seas  have  been  subsiding,  and  that  therefore  the  relative 
changes  are  the  sum  of  these  two. 

Admitting,  however,  that  the  actual  increase  of  land  at  the  present 
time  is  imperceptible,  or  at  least  very  small  in  comparison  with  the 
oscillatory  movements  described  above,  we  may  look  upon  the  sea-level 
as  fixed ;  this  statement  being  sufficiently  correct  when  regarding  the 
subject  from  the  physical  point  of  view,  though  untenable  when  re- 
garded from  the  geological  point  of  view.  Admitting,  then,  the  fixed- 
ness of  the  sea-level,  what  are  the  causes  of  the  gradual  movements  of 
the  solid  crust  ? 

Babbage's  Theory. — Babbage  believed  that,  in  the  vicinity  of  volca- 
noes, the  rise  and  fall  of  ground  were  due  to  the  expansion  and  contrac- 
tion of  rocks  by  heating  and  cooling.  The  reelevation  of  the  temple 
of  Serapis  occurred  apparently  soon  after  the  eruption  which  formed 
Monte  Nuovo  (Fig.  109).  It  is  not  improbable  that  this  reelevation 
was  the  result  of  the  heating  and  vertical  expansion  of  the  rocks  to 
great  depth,  caused  by  the  eruption  of  the  interior  heat  at  this  point. 
A  very  small  elevation  of  temperature  of  rocks  several  miles  thick 
would  be  sufficient  to  produce  a  vertical  expansion  of  twenty  feet. 


132  IGNEOUS  AGENCIES. 

Other  cases,  such  as  the  rise  of  sea-margins  at  a  distance  from 
volcanic  action,  Babbage  explains  as  follows :  Large  accumulations  of 
sediments,  such  as  occur  generally  on  coasts,  would  cause  a  rise  toward 
the  surface  of  all  the  subjacent  isogeotherms.  This  increase  of  tem- 
perature of  the  crust  would  cause  a  vertical  elongation  or  swelling  of 
the  crust  at  that  point,  and  a  consequent  rise  above  the  sea-level. 

The  great  objection  to  this  theory,  as  applied  to  these  latter  cases, 
is,  that  the  places  where  the  greatest  quantities  of  sediments  are  depos- 
iting (as,  for  instance,  the  deltas  of  great  rivers)  are  places  of  subsi- 
dence, instead  of  elevation. 

Herschel's  Theory. — Sir  John  Herschel  assumes,  as  a  general  law — 
what  has  been  proved  in  a  great  number  of  instances — that  areas  of 
great  accumulation  of  sediment  are  areas  of  subsidence.  He  agrees 
with  Babbage,  that  accumulation  of  sediments  must  cause  an  upward 
movement  of  the  isogeotherms,  but  he  differs  from  Babbage  in  believ- 
ing that  this  invasion  of  sediments  by  the  interior  heat  would  produce 
subsidence  instead  of  elevation.  For,  according  to  Herschel,  the  inva- 
sion of  sediments  by  the  interior  heat  would  produce  chemical  changes, 
and  sometimes  even  aqueo-igneous  fusion.  These  chemical  changes, 
whatever  other  effects  they  produce,  would  certainly  change  sediments 
into  crystalline  rocks  (metamorphism).  The  accumulating  sediment 
meanwhile  would  subside,  by  the  pressure  of  its  own  weight,  on  the 
liquid  or  semi-liquid  thus  formed. 

General  Theory. — The  theory  of  Babbage  accounts  with  great  prob- 
ability for  the  rise  of  ground  in  the  vicinity  of  volcanoes,  and  Herschel's 
theory  accounts,  perhaps,  for  the  subsidence  of  deltas  and  other  places 
where  great  accumulation  of  sediments  occurs ;  and  this  latter  theory 
has  the  additional  advantage  of  accounting  for  metamorphism,  and 
perhaps,  also,  for  volcanic  phenomena.  But  it  is  evident  that  some 
other  and  more  general  theory  is  necessary  to  account  for  those  great 
inequalities  of  the  earth's  crust  which  form  land  and  sea-bottom.  The 
formation  of  these  must  be  a  phenomenon  somewhat  different  from 
those  local  oscillations  which  alone  have  been  the  subject  of  direct  ob- 
servation. Such  general  changes  can  only  be  the  result  of  gradual  un- 
equal contraction  of  the  whole  earth  consequent  upon  its  secular  cool- 
ing. The  full  discussion  of  this  theory,  however,  belongs  properly  to 
the  second  part  of  this  work. 


PEAT-BOGS  AND  PEAT-SWAMPS.  133 


CHAPTER    IV. 

ORGANIC    AGENCIES. 

As  agents  modifying  the  crust  of  the  earth,  organisms  are,  per- 
haps, inferior  to  the  agents  already  mentioned  (although  the  immense 
thickness  and  extent  of  limestone  strata  are  a  monument  of  their  power 
in  this  respect)  ;  nevertheless,  they  are  peculiarly  interesting  to  the 
geologist  as  delicate  indicators  of  climate,  and  recorders  of  the  events 
of  the  earth's  history.  We  will  take  up  the  subject  of  their  agency 
under  four  heads,  each  having  a  separate  application  in  interpreting  the 
structure  and  history  of  the  earth,  viz. :  1.  Vegetable  Accumulations,  to 
account  for  coal ;  2.  Bog-iron  Ore,  to  account  for  iron-ores  inclosed  in 
the  strata  ;  3.  Lime  Accumulations,  to  account  for  limestones,  etc. ;  4. 
Geographical  Distribution  of  Organisms,  to  explain  their  distribution 
in  former  epochs. 

SECTION  1. — VEGETABLE  ACCUMULATIONS. 
Peat-Bogs  and  Peat- Swamps. 

Description. — In  humid  climates,  in  certain  places,  badly  drained 
and  overgrown  with  moss  and  shrubs,  a  black  carbonaceous  mud  accu- 
mulates often  to  great  depths.  This  substance  is  called  peat  or  turf, 
and  such  localities  peat-bogs.  The  thick  mass  of  vegetation  which 
covers  their  surface,  with  its  interlaced  roots,  often  forms  a  crust, 
upon  which  a  precarious  footing  may  be  found,  but  beneath  this  is  a 
tremulous,  semi-fluid  quagmire,  sometimes  twenty  to  forty  feet  deep,  in 
which  men  and  animals,  venturing  in  search  of  food,  are  often  lost. 
These  bogs  are  most  numerous  in  northern  climates.  One-tenth  of  the 
whole  surface  of  Ireland,  and  large  portions  of  Scotland,  England,  and 
France,  are  covered  with  peat.  The  bog  of  the  Shannon  River  is  fifty 
miles  long  and  three  miles  wide ;  that  of  the  Loire  in  France  is  150 
miles  in  circumference.  Extensive  bogs  exist  also  in  the  northern  por- 
tions of  our  own  country.  The  amount  of  peat  in  Massachusetts  alone 
has  been  estimated  at  more  than  15,000,000,000  cubit  feet  (Dana).  In. 
California,  an  imperfect  peat  covers  large  areas  about  the  mouth  of  the 
San  Joaquin  River  and  elsewhere  (tule-lands).  In  more  southern  cli- 
mates, where  the  condition  of  humidity  is  present,  immense  accumula- 
tions of  peat  also  occur — not,  however,  in  bogs  overgrown  with  moss 
and  shrubs,  but  in  extensive  swamps  covered  with  large  trees. 

Composition    and  Properties  of  Peat. — Peat  is  disintegrated  and 


134:  ORGANIC  AGENCIES. 

partially  decomposed  vegetable  matter.  It  is  composed  of  carbon,  with 
small  and  variable  quantities  of  hydrogen,  oxygen,  and  nitrogen.  It  is, 
therefore,  vegetable  matter  which  has  lost  a  part  of  its  gaseous  con- 
stituents, and  in  which,  therefore,  the  carbon  is  greatly  in  excess.  In 
more  recent  peat,  the  vegetable  nature  and  structure  are  plainly  detect- 
able by  the  eye,  but  in  older  peat  only  by  the  microscope.  In  all  coun- 
tries where  it  occurs,  it  is  dried  and  used  as  a  valuable  domestic  fuel. 
By  powerful  pressure  it  may  be  converted  into  a  substance  scarcely 
distinguishable  from  some  varieties  of  coal,  and,  thus  changed,  is  now 
extensively  used  for  all  purposes  for  which  coal  is  used,  and  has  there- 
fore become  an  important  article  of  commerce. 

Peat  possesses  a  remarkable  antiseptic  property.  This  property  is 
probably  due  to  the  presence  of  humic  acid  and  of  hydrocarbons  anal- 
ogous to  bitumen,  which  are  formed  only  when  vegetable  matter  is 
decomposed  in  presence  of  excess  of  water.  The  bodies  of  men  and 
animals  have  been  found  in  bogs  in  a  good  state  of  preservation,  which 
must  have  been  buried  many  hundred  years.  In  1747,  in  an  English 
bog,  the  body  of  a  woman  was  found,  with  skin,  nails,  and  hair,  almost 
perfect,  and  with  sandals  on  her  feet.  In  Ireland,  under  eleven  feet  of 
peat,  the  body  of  a  man  was  found  clothed  in  coarse  hair-cloth.  Several 
other  instances  of  bodies  of  men  and  animals,  and  innumerable  instances 
of  skeletons  of  animals,  preserved  in  bogs  where  they  have  perished, 
might  be  mentioned.  Large  trunks  of  trees  are  often  so  perfectly  pre- 
served that  they  are  used  as  timber,  and  stumps  similarly  preserved 
are  found  with  their  roots  firmly  fixed  in  the  under-soil  of  the  bog  as 
if  they  had  grown  on  the  original  soil  on  which  the  bog  was  accumu- 
lated. 

Mode  of  Growth. — Plants  take  the  greater  portion  of  their  food  from 
the  air,  and  give  it,  by  the  annual  fall  of  leaf  and  finally  by  their  own 
death,  to  the  soil.  Thus  is  formed  the  humus  or  vegetable  mould  found 
in  all  forests.  This  substance  would  increase  without  limit,  were  it  not 
that  its  decay  went  onparipassu  with  its  formation.  But  in  peat  bogs 
and  swamps  the  excess  of  water,  and,  still  more,  the  antiseptic  property 
of  the  peat  itself,  prevent  complete  decay.  Thus  each  generation  takes 
from  the  air  and  adds  to  the  soil  continually  and  without  limit.  The 
soil  which  is  made  up  entirely  of  this  ancestral  accumulation  continues 
to  rise  higher  and  higher,  until  the  bog  often  becomes  higher  than  the 
surrounding  country,  and,  when  swollen  by  unusual  rains,  bursts  and 
floods  the  country  with  black  mud.  A  bog  is  therefore  composed  of 
the  vegetable  matter  of  thousands  of  generations  of  plants.  It  repre- 
sents so  much  matter  withdrawn  from  the  atmosphere  and  added  to 
the  soil.  In  some  cases,  besides  the  material  deposited  from  the 
growth  of  vegetation  in  situ,  the  accumulation  may  be  partly  also  the 
result  of  organic  matter  drifted  from  the  surrounding  surface-soil. 


PEAT-BOGS  AND   PEAT-SWAMPS.  135 

Rate  of  Growth. — The  rate  of  peat-growth  must  be  very  variable, 
since  it  depends  upon  the  vigor  of  the  vegetation  and  upon  the  manner 
of  accumulation,  whether  entirely  by  growth  of  plants  in  situ,  or  partly 
by  driftage.  Many  of  the  European  bogs  are  evidently  the  growth  of  not 
more  than  eighteen  hundred  years,  for  they  were  forests  in  the  time  of 
the  Romans,  or  even  later.  The  felling  of  these  forests,  as  a  military 
measure  to  complete  the  subjugation  of  the  country,  and  the  consequent 
impediments  to  drainage  thus  produced,  have  changed  them  into  bogs. 
At  their  bottoms,  and  covered  with  eight  to  ten  feet  of  peat,  are  found 
the  trunks  and  the  stumps  of  the  original  forests,  the  axe£  and  coins  of 
the  Roman  soldiers,  and  the  roads  of  the  Roman  army.  The  rate  of 
accumulation  has  been  variously  estimated,  from  one  or  two  inches  to 
several  feet  per  century.  In  all  cases  of  simple  growth  in  situ,  however, 
and  therefore  always  in  great  peat-swamps,  the  increase  is  very  slow. 

Conditions  of  Growth. — The  conditions  usually  considered  necessary 
for  the  formation  of  peat  are  cold  and  moisture;  and  of  these  the 
former  is  considered  the  more  important,  as  without  cold  it  is  supposed 
vegetable  matter  would  be  destroyed  by  decay.  In  proof  of  this  it  is 
stated  that  peat-bogs  are  more  numerous  in  cold  climates.  But  it  is 
more  probable  that  excess  of  moisture  is  the  only  important  condition. 
This  condition  may  be  rarer  in  warm  climates  on  account  of  the  greater 
capacity  of  the  air  for  moisture  in  these  climates ;  but  when  it  is  pres- 
ent, immense  accumulations  of  peat  occur  in  extensive  swamps.  The 
Great  Dismal  Swamp  is  a  good  illustration.  This  swamp,  situated 
partly  in  North  Carolina  and  partly  in  Virginia,  is  forty  miles  long  by 
twenty-five  miles  wide.  It  is  covered  with  a  dense  forest  of  cypress 
and  other  swamp  trees,  by  the  annual  fall  of  whose  leaves  the  peat  is 
formed.  These  trees,  by  means  of  their  long  tap-roots  and  their  wide- 
spreading  lateral  roots,  maintain  a  footing  in  the  insecure  soil,  but  are 


FIG.  112. 

often  overthrown,  and  add  their  trunks  and  branches  to  the  vegetable 
accumulation.  The  original  soil,  upon  which  the  accumulation  was 
formed,  must  have  been  lower  in  the  centre,  but  the  surface  of  the  peat 
rises  very  gently  toward  the  centre,  which  is  twelve  feet  higher  than 
the  circumference  (Fig.  112).  Near  the  centre  there  is  a  lake  of  clear, 
wine-colored  water,  seven  miles  across  and  fifteen  feet  deep,  the  banks 
and  bottom  of  which  are  composed  of  pure  peat. 

In  the  Mississippi  River  swamps  there  are  also  large  areas  where  pure 
peat  has  been  accumulating  for  ages,  and  is  still  accumulating,  by 
growth  of  trees  in  situ,  though  subject  to  the  annual  floods  of  the  river. 


136  ORGANIC   AGENCIES. 

The  pureness  of  the  peat  in  these  cases  is  due  to  the  fact  that  the  mud- 
dy waters  of  the  river  are  strained  of  all  their  sedimentary  matter  by 
passing  through  the  dense  jungle-growth  of  cane  and  herbage  which 
surrounds  these  favored  spots.  Thus  only  pure  water  reaches  them.1 
Similar  peat-swamps  are  found  at  the  mouths  of  the  Ganges,  the  Niger, 
and  other  great  rivers. 

Alternation  Of  Peat  With  Sediments. — We  have  already  stated  (page 
129)  that  a  section  of  the  delta-deposit  of  many  great  rivers,  such  as  the 
Mississippi,  Ganges,  and  Po,  reveals  alternate  layers  of  fresh-water  and 
marine  sediments,  with  thin  layers  of  vegetable  mould  containing 
stumps.  In  some  cases  these  layers  of  vegetable  mould  amount  to  con- 
siderable thickness  of  turf  or  peat.  Layers  of  peat  two  feet  thick  have 
been  found  between  layers  of  river-mud  in  the  delta  of  the  Ganges 
(Lyell's  "Principles  of  Geology").  Similar  layers  have  been  found  in 
the  delta  of  the  Po.  They  are  evidently  submerged  peat-swamps. 
These  facts  are  of  great  importance  in  the  explanation  of  the  accumu- 
lation of  coal. 

Drift-Timber. 

Great  rivers  in  wooded  countries  always  bring  down  in  large  num- 
bers the  trunks  of  trees  torn  from  the  soil  of  their  banks.  These  trunks 
lodging  near  their  mouths,  where  the  current  is  less  swift,  and  accumu- 
lating from  year  to  year,  form  rafts  of  great  extent.  The  great  raft  of 
the  Atchafalaya,  which  was  removed  in  1835  by  the  State  of  Louisiana, 
was  a  mass  of  timber  ten  miles  long,  seven  hundred  feet  wide,  and  eight 
feet  thick.  It  had  been  accumulating  for  more  than  fifty  years,  and  at  the 
time  of  its  removal  was  covered  with  vegetation,  and  even  with  trees 
sixty  feet  high.  Similar  accumulations  of  drift-wood  are  described  as 
occurring  in  the  Red  River,  the  Mackenzie  River,  and  in  Slave  Lake. 
Such  rafts  become  finally  imbedded  in  river-mud,  and  undergo  a  slow 
change  into  lignite  or  imperfect  coal.  Beds  of  partially -formed  lignite 
are  therefore  found  in  sections  of  the  delta-deposit  of  almost  all  great 
rivers.  "We  will  use  these  facts  in  speaking  of  the  theories  of  the  coal. 

SECTION  2. — BOG-!RON  ORE. 

At  the  bottom  of  peat-bogs  is  often  found  a  "  hard  pan  "  of  iron- 
ore,  sometimes  one  to  two  feet  thick.  The  same  material  often  collects 
in  low  spots,  even  when  there  is  no  decided  bog.  The  manner  in  which 
this  iron-ore  accumulates  is  very  interesting,  and  in  a  geological  point 
of  view  very  important. 

Peroxide  of  iron  exists  very  generally  diffused  as  the  red  coloring- 
matter  of  soils  and  rocks.  In  this  form,  however,  it  is  insoluble,  and 
therefore  cannot  be  washed  out  by  percolating  waters.  For  this  pur- 
1  Lyell's  "Elements  of  Geology,"  fifth  edition,  p.  385. 


BOG-IRON  ORE.  13f 

pose  the  agency  of  decomposing  organic  matter,  present  in  all  percolat- 
ing waters,  is  necessary.  Decomposition  of  organic  matter  is  a  process 
of  oxidation.  In  contact  with  peroxide  of  iron  (ferric  oxide)  it  deoxi- 
dizes, and  reduces  it  to  protoxide  (ferrous  oxide).  The  acids,  especially 
carbonic  acid,  produced  by  decomposition  of  the  organic  matter,  then 
unite  with  the  protoxide,  forming  carbonate  of  iron.  The  carbonate, 
being  soluble  in  water  containing  excess  of  carbonic  acid,  is  washed  out, 
leaving  the  soils  or  rocks  decolorized,  and  the  iron-charged  waters  come 
up  as  chalybeate  springs.  But  the  ferrous  carbonate  rapidly  oxidizes 
again  in  the  presence  of  air,  by  exchanging  its  carbonic  acid  for  oxygen, 
and  returns  to  its  former  condition  of  ferric  oxide,  and  is  deposited. 
Thus  all  about  iron-springs,  and  in  the  course  of  the  streams  which 
flow  from  them,  and  in  low  places  where  their  waters  accumulate,  we 
find  reddish  deposits  of  ferric  oxide.  This  is  the  most  common  but  not 
the  only  form.  For  if  the  iron-waters  accumulate,  and  the  iron  be  de- 
posited in  the  presence  of  excess  of  organic  matter,  as  peat,  then  the 
iron  is  not  (for  in  the  presence  of  this  reducing  agent  it  cannot  be) 
reoxidized,  but  remains  in  the  form  of  ferrous  carbonate. 

Thus  there  are  two  forms  in  which  iron  leached  out  from  the  soils 
and  rocks  may  accumulate,  viz.,  ferric  oxide  and  ferrous  carbonate :  the 
former  is  accumulated  where  the  organic  matter  is  in  small  quantities, 
and  consumes  itself  in  doing  the  work  of  dissolving  and  carrying ;  the 
latter  where  the  organic  matter  is  in  excess. 

Many  familiar  phenomena  may  be  explained  by  the  principles  given 
above :  1.  Clay  containing  both  iron  and  organic  matter  is  never  red, 
but  always  blue  or  slate-colored,  because  the  iron  is  in  the  form  of  fer- 
rous carbonate ;  but  the  same  clay  will  make  good  red  brick,  because 
by  burning  the  organic  matter  is  destroyed  and  the  iron  peroxidized. 
2.  In  red-clay  soils,  such  as  those  of  our  primary  regions,  the  surface- 
soil,  especially  in  forests,  is  always  decolorized,  the  coloring  of  peroxide 
of  iron  being  washed  out  and  carried  deeper  by  water  containing  or- 
ganic matter  derived  from  the  vegetable  mould.  3.  In  sections  of  red 
clay,  as  the  sides  of  gullies  or  railroad-cuttings,  along  every  fissure  or 
crevice  through  which  superficial  waters  percolate,  the  clay  is  bleached. 
The  marbled  appearance  of  red  clays  is  also  probably  due,  in  a  great 
measure,  to  the  irregular  percolation  of  superficial  waters  containing  or- 
ganic matter.  4.  The  under  clay  or  sand  of  peat-bogs  is  usually  de- 
colorized. 

We  will  hereafter  make  use  of  these  facts  and  principles  in  the  ex- 
planation of  beds  of  iron-ore. 


138  ORGANIC  AGENCIES. 

SECTION  3. — LIME  ACCUMULATIONS. 
Coral  Reefs  and  Islands. 

Interest  and  Importance. — The  subject  of  corals  and  coral  reefs  is 
one  of  much  popular  as  well  as  scientific  interest.  The  strange  forms 
and  often  splendid  colors  of  the  living  animals  ;  the  number  and  ex- 
treme beauty  of  the  coral  islands  which  gem  the  surface  of  certain 
seas ;  the  large  amount  of  habitable  land  which  owes  its  existence  to 
the  agency  of  these  minute  animals ;  the  fact  that  a  large  area,  prob- 
ably several  thousand  square  miles,  has  been  thus  added  to  our  own 
territory ;  the  great  dangers  connected  with  the  navigation  of  coral  seas, 
strikingly  displayed  on  our  own  coast  by  the  fact  that  the  considerable 
town,  of  Key  West  is  almost  wholly  dependent  on  the  wrecking  busi- 
ness for  its  existence — these  and  many  other  facts  invest  the  subject 
with  popular  interest,  while  the  great  importance  of  corals  as  a  geo- 
logical agent  gives  the  subject  a  scientific  interest  no  less  strong. 

Coral  Polyp. — The  animal  which  secretes  coralline  stone  is  no  insect, 
as  generally  supposed,  but  belongs  to  one  of  the  lowest  divisions  of 
the  animal  kingdom,  viz.,  the  class  of  polyps.  Like  most  of  the  lowest 
animals,  it  is  composed  of  soft,  gelatinous,  and  almost  transparent 
tissue.  The  animal,  however,  has  the  power  of  extracting  carbonate  of 
lime  from  sea-water,  and  depositing  it  within  its  own  body.  The  lime 
carbonate  is  deposited  only  in  the  lower  portion  of  the  animal,  leaving 
thus  the  upper  part  and  the  tentacles  free  to  move.  The  radiated 
structure  of  the  polyp  is  perfectly  reproduced  in  the  coralline  axis. 
This  is  a  purely  vital  function,  having  no  more  connection  with  voli- 
tion than  the  secretion  of  the  shell  of  an  oyster  or  the  bones  of  the 
higher  animals.  The  limestone  thus  deposited  within  the  animal  con- 
stitutes 90  to  95  per  cent,  of  its  whole  weight. 

Compound  Coral,  or  Corallum. — A  single  coral  polyp  is  very  small, 
but,  like  many  of  the  lower  animals,  it  has  the  power  of  multiplying 
indefinitely  by  buds  and  branches.  Thus  are  formed  compound  corals. 
These  may  branch  profusely,  and  then  may  be  called  coral-trees  ;  or  may 
grow  in  hemispherical  masses,  and  are  then  called  coral-heads.  Coral- 
trees  are  sometimes  six  or  eight  feet  high,  and  coral-heads  fifteen  to 
twenty  feet  in  diameter.  They  consist  of  millions  of  individual  coral 
polyps.  Only  the  upper  and  outer  portions  of  a  coral-tree,  and  outer 
portion  of  a  coral-head,  are  living ;  the  lower  and  interior  portions  con- 
sist only  of  coralline  limestone  without  life. 

Coral  Forests. — Coral  polyps,  however,  reproduce  not  only  by  bud- 
ding, but  also  by  eggs.  These  eggs  have  the  power  of  locomotion. 
As  soon  as  they  are  extruded,  they  swim  and  float  away,  and,  if  they 
fall  on  sea-bottom  favorable  for  their  growth,  they  soon  form  first  a 


CORAL   REEFS  AND   ISLANDS.  139 

coral  polyp,  and  finally  a  coral-tree  or  coral-head.  Thus  from  one  coral- 
tree  other  coral-trees  spring  up  all  around  and  form  a  coral  forest,  which 
spreads  in  every  direction  where  they  find  conditions  favorable. 

Coral  Reef. — Finally,  the  limestone  accumulation  of  thousands  and 
millions  of  coral  forests  growing  and  dying  on  the  same  spot,  to- 
gether with  the  shells  of  mollusks  and  the  bones  of  fishes  which  live  in 
swarms  preying  on  the  corals,  the  whole,  of  course,  crowned  with  the 
living  forest  of  the  present  generation,  constitute  the  coral  reef.  It  is 
evident,  then,  that  a  reef  is  formed  somewhat  after  the  manner  of  a 
peat-bog.  As  a  peat-bog  represents  so  much  matter  taken  from  the 
air,  so  a  coral  reef  represents  so  much  matter  taken  from  the  sea-water. 
As  each  generation  adds  itself  to  the  ancestral  funeral-pile,  the  ground 
upon  which  the  corals  grow  steadily  rises  until  it  becomes  elevated  far 
above  the  surrounding  sea-bottom. 

Coral  Islands. — These  are  due  to  the  action  of  waves  upon  the 
coral  reefs.  We  have  already  seen  how  low  islands  are  formed  on  sub- 
marine banks  by  this  agency.  Now,  reefs  are  also  a  kind  of  submarine 


FIG.  113 

bank.  On  these,  therefore,  islands  are  also  formed  by  waves.  Fig.  113 
represents  an  ideal  section  across  a  reef,  as  it  would  be  if  no  wave- 
action  interfered,  II  being  the  sea-level.  But  by  the  action  of  the 
beating  waves  during  storms  large  masses  of  reef-rock,  often  six  or  eight 
feet  in  diameter,  or  great  coral-heads,  are  broken  off  from  the  outer  or 


— I 


FIG.  114. 

seaward  side  of  the  reef  and  rolled  over  to  the  leeward  side.  These 
form  a  nucleus  about  which  collect  similar  or  smaller  fragments,  and 
among  these  still  smaller  fragments,  and  these  again  are  filled  in  and 
made  firm  with  coral-sand,  and  the  whole  cemented  into  solid  lime- 
stone rock  (breccia)  by  the  carbonate  of  lime  in  the  sea-water. 

Islands  thus  formed,  like  all  wave-formed  islands,  are  low  (twelve  to 
fifteen  feet  high)  and  narrow  (one-quarter  to  one-half  mile  wide),  but 
long  in  the  direction  of  the  reef.  They  are  at  first  perfectly  bare,  but 


140  ORGANIC  AGENCIES. 

become  in  time  covered  with  vegetation,  and  even  teeming  with 
population.  They  are  celebrated  for  their  gem-like  beauty.  The  final 
result  is  shown  in  ideal  section  in  Fig.  114,  in  which  the  dotted  portion 
is  reef-rock,  the  strong  waving  line  the  surface  of  the  living  reef,  and 
the  shaded  portion  the  island. 

Conditions  of  Coral-Growth. — Reef-building  corals  do  not  grow  in 
all  seas,  nor  over  the  whole  bottom  of  the  sea  indiscriminately,  but  are 
confined  to  certain  seas,  and  in  these  to  certain  spots  and  lines.  The 
conditions  of  their  growth  are  : 

1.  A  Winter-Temperature  0/68°. — This  condition  confines  them  al- 
most entirely  to  the  torrid  zone.     The  most  marked  exception  to  this  is 
on  the  Florida  coast  and  the  Bahamas,  where  corals  extend  to  28°  north 
latitude,  and  in  the  Bermudas  to  32°  north  latitude.     This  extension  of 
the  usual  limits  of  reef-building  corals  is  due  to  the  warm  tropical 
waters  carried  northward  by  the  Gulf  Stream. 

2.  A  Depth  of  not  more  than  One  Hundred  Feet.—rY\\\s,  condition 
confines  them  to  submarine  banks,  and  especially  to  the  shores  of  con- 
tinents and  islands. 

3.  Clearness  and  Saltness  of  the  Water. — On  account  of  this  con- 
dition corals  will  not  grow  on  muddy    shores,  nor  off  the  mouths  of 
rivers,  being  destroyed  by  the  fresh  and  muddy  water. 

4.  Free  Exposure  to  Waves. — Some  species  of  corals  grow  in  still 
water,  but  the  strongest  reef-building  species  delight  in  the  dash  of  the 
surf.     They  will  even  flourish  and  build  an  almost  perpendicular  wall 
in  breakers  which  would  wear  away  the  hardest  rock.     The  reason  is, 
that  the  immense  profusion  of  life  on  a  reef  rapidly  exhausts  the  water 
of  the  oxygen  necessary  for  respiration,  and  of  the  carbenate  of  lime 
necessary  for  their  stony  structure,  and  therefore  constant  change  of 
water  is  necessary.  t 

All  the  conditions  mentioned  above  apply  only  to  reef-building 
species.  Some  corals  live  in  temperate  regions,  some  in  very  deep 
water,  and  some  in  sheltered  places. 

Pacific  Reefs.— The  reefs  of  the  Pacific  Ocean  are  of  three  general 
kinds,  viz.,  fringing  reefs,  barrier  reefs,  and  circular  reefs  or  atolls. 
We  will  describe  these  in  the  order  mentioned. 

Fringing  Reefs. — In  the  tropical  Pacific  every  high  island  or  previ- 
ously-existing land  of  any  kind  is  surrounded  by  a  reef  which  attaches 
itself  to  the  shore-line,  and  extends  outward  on  every  side  just  beneath 
the  water-level,  as  far  as  the  condition  of  depth  will  allow,  thus  forming 
a  submarine  platform  bordering  the  island  or  other  land.  At  the  outer 
margin  of  this  platform  the  bottom  drops  off  very  suddenly,  forming 
a  slope  of  50°  to  60°,  and  sometimes  almost  perpendicularly.  The  po- 
sition and  extent  of  the  coral  platform  is  indicated  to  the  eye  of  the 
observer  by  a  white  sheet  of  breakers  which  surrounds  the  island  like 


CORAL   REEFS   AND   ISLANDS. 


141 


a  snowy  girdle,  and  extends  some  distance  from  the  shore-line  (Fig. 
115.)  The  section  Fig.  116  will  give  a  clear  idea  of  the  contour  of 
land  and  sea  bottom.  In  this  and  the  following  sections  the  dotted 
parts  represent  coral  formation.  If  the  island  is  large,  and  considerable 


rivers  flow  into  the  sea,  breaks  in  the  reef  platform  will  occur  opposite 
the  mouths  of  the  rivers,  the  corals  in  these  places  being  destroyed  by 
the  fresh,  muddy  waters.  In  the  case  of  fringing  reefs  no  islands  are 


formed  by  the  action  of  waves,  but  only  a  shore-addition  to  the  orig- 
inal island,  as  shown  at  a  a  in  the  section. 

Barrier  Reefs  — In  many  cases  besides  the  fringing  reef  there  is  an- 
other reef  surrounding  the  island  like  a  submarine  rampart  at  the  dis- 
tance of  from  ten  to  fifty  miles.  As  the  reef  rises  nearly  to  the  surface  of 
the  sea,  its  position  is  indicated  by  a  snowy  girdle  of  breakers  surround- 
ing the  island  at  a  distance,  and  this  snowy  girdle  is  gemmed  with  wave- 


formed  green  islets.  Within  this  girdle,  and  between  the  rampart  and 
the  island,  there  is  a  ship-channel  twenty  or  thirty  fathoms  deep  (Fig. 
117).  Through  breaks  in  the  coral  rampart  ships  enter  this  channel 
and  find  secure  harbor  in  a  stormy  sea.  The  section  Fig.  118  wih  give 
a  clear  idea  of  the  conformation  of  bottom.  On  the  landward  side  of  the 


142 


ORGANIC  AGENCIES. 


coral  rampart  the  slope  of  the  bottom  is  gentle,  but  on  the  seaward  side 
it  is  very  steep,  so  that  it  is  almost  unfathomable  at  a  short  distance 
from  the  reef. 


Circular  Reefs,  or  Atolls. — These  are  the  most  wonderful  of  the 
reefs  of  the  Pacific.     In  a  circular  reef  there  is  no  volcanic  island  or 


other  visible  land  to  which  the  reef  is  attached.     Imagine  a  circular 
line  of  breakers  like  a  snow-wreath  on  the  sea,  indicating  a  circular 


FIG.  120. 


submarine  ridge  (the  coral  reef)  gemmed  as  before  with  wave-formed 


FIG.  121.— View  of  Whitsunday  Island. 


CORAL   REEFS   AND   ISLANDS. 


143 


islets  ;  and  within  the  circle  a  lagoon  of  placid  water  twenty  or  thirty 
fathoms  deep  (Fig.  119).  It  is  a  submarine  urn  standing  in  unfath- 
omable water,  as  seen  in  the  section  Fig.  120.  Through  breaks  in  the 
reef  ships  enter  the  charmed  circle  and  find  safe  harbor.  By  means  of 
sounding  it  is  found  that  on  the  interior  or  lagoon  side  the  slope  of  the 
bottom  is  very  gentle,  but  on  the  outer  or  seaward  side  is  very  steep, 
often  50°  to  60°,  and  sometimes  in  places  almost  perpendicular  to  al- 
most unfathomable  depth.  Fig.  121  gives  a  perspective  view,  and 
Fig.  122,  a,  a  map  view,  of  an  atoll,  showing  their  regular  circular  form 
of  the  reef  and  the  little  islands  which  gem  its  surface. 

Small  Atolls  and  LagOOnless  Islands. — Besides  the  atolls  already  de- 
scribed, there  are  others,  evi- 
dently of  similar  origin,  but 
much  smaller,  in  which  the 
land  is  continuous.  Some- 
times the  continuous  line 
is  open  on  one  side  (Fig. 
122,  b),  and  the  lagoon  is 
still  in  connection  with  the 
open  sea.  Sometimes  the 
circle  of  land  is  complete, 
and  the  lagoon  is  isolated 
from  the  sea  (Fig.  122,  c). 

Sometimes  the  lagoon  closes  FIG  122 

up,   and    a    lagoonless   isl- 
and is  the  result  (Fig.  122,  (?).    These  different  forms  graduate  into  one 
another  and  into  the  typical  atoll. 

Theories  of  JBarrier  and  Circular  Reefs. 

Fringing  reefs  require  no  theory.  Corals  attach  themselves  to  the 
shore-line  because  they  find  there  the  depth  necessary  for  their  growth, 
and  they  extend  outward  until  they  are  limited  by  the  increasing  depth. 
But  there  is  a  real  difficulty  in  explaining  barriers,  for  they  seem  to  rise 
from  water  too  deep  for  coral-growth;  and  the  difficulty  becomes  still 
greater  in  the  case  of  circular  reefs  or  atolls,  for  these  seem  to  have  no 
connection  with  any  preexisting  land,  but  seem  to  grow  up  from  an 
unfathomable  bottom.  These  latter,  by  their  singularity  and  extreme 
beauty,  have  always  attracted  the  attention  and  excited  the  wonder  of 
Pacific  travelers ;  and  to  their  explanation  theories  have  been  princi- 
pally addressed. 

Crater  Theory. — This  theory  supposes  that  an  atoll  is  an  extinct 
submarine  volcano,  the  lagoon  being  the  crater  and  the  reef  the  lip  or 
margin  of  the  crater ;  that  corals,  finding  on  this  circular  rim  the  con- 
ditions of  depth  necessary  for  their  growth,  occupy  and  build  upon  it  to 


144  ORGANIC  AGENCIES. 

the  surface  of  the  water,  after  which,  of  course,  waves  finish  the  work 
by  beating  up  the  islets.  The  incredible  supposition  that  thousands  of 
these  volcanoes  should  have  come  within  100  feet  of  the  surface,  and 
yet  none  of  them  appear  above  the  surface,  is  not  necessary ;  for  we 
may  suppose  that  many  of  them  were  originally  above  the  surface, 
but,  being  composed  of  ashes  and  cinders,  have  been  washed  down  by 
the  waves.  In  1831  a  volcano  burst  forth  in  the  Mediterranean  and 
quickly  formed  an  island  of  cinders  and  ashes,  called  Graham's  Island. 
In  a  few  months  this  island  was  entirely  washed  away  by  the  waves, 
and  only  a  circular  submarine  bank  remained.  If  corals  grew  in  the 
Mediterranean,  there  seems  no  reason  why  a  circular  reef  should  not 
have  been  formed. 

Objections. — Even  in  its  most  plausible  form,  however,  this  theory 
is  very  improbable  as  a  general  explanation  of  atolls.  1.  The  great 
size  of  some  of  these  atolls — thirty,  sixty,  and  even  ninety  miles  in 
diameter  ;  and,  2.  The  high  angle  of  the  slope  of  these  submarine  moun- 
tains— 50°  to  60°  or  more — seem  inconsistent  with  their  volcanic  origin. 
3.  This  theory  offers  no  explanation  of  the  barrier  reefs,  and  yet  it  is 
possible  to  trace  every  stage  of  gradation  between  barriers  and  atolls, 
showing  that  they  are  due  to  similar  causes. 

Subsidence  Theory. — There  can  be  little  doubt  that  this  is  the  true 
theory.  It  explains  not  only  atolls,  but  also  barriers,  and  connects  both 
in  a  satisfactory  manner  with  fringing  reefs.  It  supposes  that  the  sea- 
bottom,  where  atolls  and  barriers  occur,  has  been  for  ages  subsiding, 
but  at  a  rate  not  greater  than  the  upward  building  of  the  coral-ground  ; 
that  every  reef  commences  as  a  fringing  reef,  but,  in  the  progress  of 
subsidence,  was  converted  first  into  a  barrier  and  finally  into  an  atoll. 
For,  as  the  volcanic  island  went  down,  the  corals  would  build  upward 
on  the  same  spot ;  and  as  the  island  would  become  smaller  and  smaller, 
and  the  corals  would  grow  fastest  on  the  outer  side  of  the  reef,  where 
they  are  exposed  to  the  breakers,  it  is  evident  that  the  reef  would  be- 
come separated  from  the  island  by  a  ship-channel,  and  thus  become  a 
barrier.  Finally,  when  the  island  disappears  entirely,  the  reef,  still 
building  upward,  would  become  an  atoll.  These  changes  are  represented 
in  the  accompanying  section  (Fig.  123).  As  the  changes  are  relative, 
they  may  be  represented  either  by  the  land  sinking  or  the  sea-level  ris- 
ing ;  for  the  sake  of  convenience  we  use  the  latter.  In  the  figure,  I"  I" 
represents  the  sea-level  when  the  reef  was  a.  fringe,  I'  I'  when  it  was  a 
barrier,  and  1 1  the  present  sea-level,  when  it  has  become  an  atoll.  The 
ship-channel  and  the  lagoon,  though  always  lower,  rise  pari  passu  with 
the  reef  proper.  This  is  the  result  partly  of  the  growth  of  placid-water 
species  of  corals,  and  partly  of  the  drifting  of  coral  debris  from  the 
reef,  and  detritus  from  the  volcanic  island.  It  is  seen  that  the  corals 
do  not  build  a  vertical  wall,  and  therefore  that  the  atoll  is  always 


THEORIES  OF   BARRIER  AND   CIRCULAR  REEFS.  14.5 

smaller  than  the  coast-line  of  the  original  island.  Consequently,  if  the 
subsidence  continues,  a  typical  atoll  is  changed  into  a  small  closed 
lagoon,  and,  finally,  into  a  lagoonless  island.  These,  therefore,  indicate 
the  deepest  subsidence. 


Proofs. — 1.  This  theory  accounts  for  all  the  more  obvious  phenomena 
of  atolls,  such  as  their  irregular  circular  form,  their  size,  the  steepness 
of  their  outer  slopes,  etc.  2.  Every  stage  of  gradation  between  the 
fringing  reef  on  the  one  hand,  and  the  atoll  on  the  other,  has  been 
traced  by  Dana,  showing  that  they  are  all  different  stages  of  develop- 
ment of  the  same  thing.  We  have  in  the  Pacific  some  high  islands, 
which  are  surrounded  by  a  pure  fringing  reef;  others  in  which  the  reef 
is  a  fringe  on  one  side  and  a  barrier  on  the  other ;  others  in  which  the 
barrier  is  one  mile,  two  miles,  five  miles,  ten  miles,  twenty,  or  thirty 
miles  distant ;  others  which  are  called  atolls,  but  the  point  of  the  origi- 
nal volcanic  island  is  still  visible  in  the  middle  of  the  lagoon ;  others 
which  are  perfect  atolls,  but,  by  sounding,  the  head  of  drowned  volcanic 
island  is  still  detectible.  The  next  step  in  the  series  is  the  perfect  atoll, 
then  the  small  atoll,  and,  finally,  the  lagoonless  coral  island.  These  last 
kinds  show  that  the  original  island  has  gone  down  deeply.  3.  By  grap- 
pling-hooks  dead  coral-trees  have  been  broken  off  and  brought  up  from 
the  ground  where  they  once  grew,  now  far  below  the  limiting  depth  of 
coral-growth.  The  evidence  of  subsidence  in  this  case  is  of  the  same 
kind  arid  force  as  that  derived  from  submerged  forest-ground  (page 
129).  The  corals  have  been  carried  below  their  depth  and  drowned. 
4.  The  remarkable  distribution  of  the  various  kinds  of  reefs  brought  to 
light  by  Dana  is  satisfactorily  explained  by  this  theory,  and  therefore  is 
an  argument  in  its  favor.  In  the  middle  of  the  atoll  region  of  the  Pa- 
cific there  is  a  blank  area,  2,000  miles  long  and  1,000  or  more  miles 
wide,  where  there  are  no  islands.  Next  about  this  is  an  area  in  which 
small  atolls  predominate  ;  about  this  again  the  region  of  ordinary  atolls; 
beyond  this  the  region  mostly  of  barriers,  and  finally  of  fringes.  Now, 
by  this  theory  this  distribution  is  thus  explained :  The  sea-bottom  in 
the  blank  area  has  gone  down  so  fast  that  the  corals  have  not  been  able 
to  keep  pace,  and  have  therefore  been  drowned,  and  left  no  monu- 
10 


146  ORGANIC  AGENCIES. 

ment  of  their  existence.  In  the  next  region  the  corals  have  been  able 
to  keep  within  living  distance  of  the  surface,  but  the  original  islands 
have  not  only  disappeared,  but  gone  down  to  great  depths.  In  the 
next  the  original  high  islands  have  disappeared,  but  not  gone  down  so 
deep ;  in  the  next  they  have  sunk  only  to  the  middle.  The  fringing  reefs 
stand  on  the  margin  of  the  sinking  area.  Outside  of  this  again  there  is 
in  some  places  even  evidence  of  upheaval  instead  of  subsidence.  Raised 
beaches  in  the  form  of  fringing-reef  rocks  are  found  clinging  to  the 
sides  of  high  islands  many  feet  above  the  present  sea-level.  5.  In  some 
places  this  subsidence  seems  to  be  still  in  progress.  On  certain  coral 
islands  sacred  structures  of  stone  made  by  the  natives  are  now  stand- 
ing in  water,  and  the  paths  worn  by  the  feet  of  devotees  are  now  pas- 
sages for  canoes  (Dana). 

It  may  be  regarded  as  certain,  therefore,  that  every  atoll  marks  the 
site  of  a  sunken  volcanic  island. 

Area  Of  Land  lost. — Probably  several  hundred  thousand  square 
miles  of  habitable  high  land  has  been  lost  by  this  subsidence.  The 
actual  extent  of  atolls  known  is  at  least  50,000  square  miles.  But  this 
is  far  less  than  the  loss  of  high  land.  For — 1.  It  is  certain  that  the 
area  of  an  atoll  is  always  less  than  that  of  the  original  fringe  or  base  of 
the  original  high  island,  for  the  outer  wall  of  an  atoll  is  not  perpendicu- 
lar. The  contraction  continues  as  the  subsidence  progresses,  until 
small  atolls  or  only  lagoonless  islands  remain.  2.  An  immense  lost 
area  is  represented  by  the  space  between  barriers  and  their  high  isl- 
ands. The  great  Australian  barrier  extends  along  that  coast  1,100 
miles,  at  an  average  distance  of  thirty  miles,  with  a  ship-channel  be- 
tween of  thirty  to  sixty  fathoms  deep.  This  single  barrier,  therefore, 
represents  a  lost  land-area  of  33,000  square  miles.  3.  In  the  blank 
area  already  spoken  of,  probably  many  islands  went  down,  and  left  no 
record  behind. 

The  large  amount  of  high  land  thus  lost  has  been  replaced  only  to  a 
small  extent  by  the  wave-formed  coral  islets  on  the  reefs. 

Amount  of  Vertical  Subsidence. — The  amount  of  subsidence  may  be 
estimated  by  the  distance  of  barriers  from  their  high  islands,  or  by 


soundings  off  the  reefs,  to  ascertain  the  height  of  these  coral  mounds, 
or  by  the  average  height  of  the  high  islands  of  the  Pacific.  1.  The 
average  slope  of  the  high  islands  of  the  Pacific  is  about  8°.  Now,  as- 
suming this  slope  (Fig.  124),  a  barrier,  d,  at  the  distance  of  five  miles 


THEORIES   OF   BARRIER   AND   CIRCULAR  REEFS.  14.7 

would  be  3,700  feet  thick,  and  would  represent  a  subsidence  nearly  to 
that  extent  (Rad.  :  tan.  8°  ::  a  d  :  d  b);  a  distance  of  ten  miles  would 
represent  a  vertical  subsidence  of  7,400  feet.  Many  barriers  are  at  much 
greater  distance.  2.  Off  Keeling  atoll  6,600  feet,  a  line  of  7,200  feet 
found  no  bottom  (Darwin).  Near  other  atolls  a  depth  of  3,000  feet  has 
been  found  (Dana).  3.  The  average  height  of  the  high  islands  of  the 
Pacific  cannot  be  less  than  9,000  feet  (Dana) ;  some  of  them  reach 
nearly  14,000  feet.  It  is  very  improbable  that  among  the  hundreds 
of  atolls  known,  not  one  of  their  high  islands  should  have  reached 
the  average  elevation  of  9,000  feet.  Yet  these  have  entirely  disap- 
peared, and  not  only  so,  but  the  small  atolls  and  lagoonless  islands, 
and  more  especially  the  blank  area,  would  seem  to  indicate  that  they 
have  disappeared  to  great  depths.  For  these  reasons,  it  is  almost  cer- 
tain that  the  extreme  subsidence  has  been  at  least  9,000  feet.  We  will 
take  10,000  feet  as  the  most  probable  extreme  subsidence. 

Rate  of  Subsidence. — The  rate  of  subsidence  may  have  been  to  any 
degree  less,  but  cannot  have  been  greater,  than  the  rate  of  coral  ground- 
rising^'  for  otherwise  the  corals  would  have  been  carried  below  their 
depth  and  drowned.  It  is  difficult  to  estimate  the  rate  of  coral  ground- 
rising,  but  the  only  basis  of  such  estimate  is  the  rate  of  coral-growth. 
Of  the  observations  on  this  point  we  select  two,  one  of  them  on  the 
head-coral  (meandrina),  the  other  on  the  staghorn-coral  (madrepore)  : 

1.  On  the  walls  of  the  fort  at  the  Tortugas,  Florida,  meandrina  com- 
menced to  grow,  and  in  fourteen  years  the  crust  had  become  only  one 
inch  thick.     Agassiz  takes  one  inch  in  eight  years  as  a  probable  rate 
under  favorable  circumstances.     This  would  be  one  foot  in  a  century. 
As  this  is  a  head-coral,  the  coral-growth  may  be  taken  as  the  measure 
of  the  reef  ground-rising. 

2.  In  examining  the  reefs  about  the  Tortugas  in  the  winter  of  1851, 
an  extensive  grove  of  madrepore  was  found  in  the  comparatively  still 
water  on  the  inside  of  the  outer  reef,  in  which  the  thick-set  prongs  had 
grown,  year  after  year,  to  the  same  level,  and  were  successively  killed. 
The  mean  level  of  the  water  here  is  lower  during  the  winter,  by  about 
a  foot,  than  during  the  summer.     The  falling  of  the  water  annually 
clips  this  grove  at  the  same  level.     Now  all  the  prongs  at  this  level 
were  dead  for  about  three  inches.     Evidently,  therefore,  this  is  the  an- 
nual growth  of  madrepore-prongs.1     But  in  branching  corals  the  rate 
of  point-growth  is  very  different  from  the  rate  of  ground-rising.     If  all 
the  points  of  a  madrepore  be  cut  off  three  inches,  then  ground  into 
powder,  and  the  powder  strewed  evenly  over  the  ground  shaded  by 
the  coral-tree,  the  elevation  thus  produced  would  correctly  represent 
the  annual  rate  of  reef  ground-rising  for  this  species.     A  quarter  of  an 

1  See  full  account  of  these  observations  in  American  Journal  of  Science  and  Arts,  vol. 
x.,  p.  34. 


148  ORGANIC  AGENCIES. 

inch  would  probably  be  a  full  estimate.  This  would  make  two  feet  for 
a  century.  One  foot  to  two  feet  per  century  is,  therefore,  probably 
about  the  rate  at  which  coral  ground  rises.  As  already  stated,  the  rate 
of  subsidence  may  be  less,  but  cannot  be  greater,  than  this. 

Time  involved. — At  this  rate  10,000  feet  of  vertical  subsidence  would 
require  500,000  to  1,000,000  years.  How  much  of  this  belongs  to  the 
present  geological  epoch,  it  is  impossible  to  say.  Dead  corals,  identical 
with  those  still  living  on  the  reefs,  have  been  brought  up  from  a  depth 
of  250  feet,  but,  as  this  is  only  150  feet  below  the  limit  of  coral-growth, 
it  would  require  only  75  to  150  centuries.  The  process  probably  com- 
menced in  previous  geological  epochs,  and  has  continued  to  the  present 
time.  This  is,  therefore,  an  admirable  example  of  geological  agencies 
still  at  work. 

Geological  Application. — The  facts  brought  out  in  the  preceding 
pages  are  of  great -importance  in  geology. 

1.  We  have  here  the  most  magnificent  example  of  subsidence  still 
in  progress.     The  subsiding  area  has  not  been  accurately  defined,  but 
it  probably  covers  nearly  the  whole  of  the  intertropical  Pacific.     Ac- 
cording to  Dana,  estimated  by  the  atolls  alone,  it  is  6,000  miles  long 
and  2,000  miles  wide ;  but  if  we  take  into  account  also  barriers,  which 
are  equally  certain  evidences  of  subsidence,  it  extends  east  and  west 
from  the  extreme  of  the  Paumotu  group  on  the  one  side  to  the  Pelews 
on  the  other,  and  north  and  south  from  the  Hawaiian  group  to  the  Fee- 
jees,  making  an  area  of  not  less  than  20,000,000  square  miles.    Now,  it  is 
evident  that  there  must  have  been,  as  a  correlative  of  this  extensive  and 
permanent  downward  movement,  an  equally  extensive  permanent  ele- 
vation of  the  earth's  crust  somewhere  else.     Dana  thinks  its  correlative 
is  found  in  the  extensive  elevations  of  the  Glacial  epoch,  and  there- 
fore   that    the   whole   work   was    accomplished    since   the    Tertiary. 
But  it  is  more   probable  that  its  correlative  is  found  in  the  gradual 
bodily  upheaval  of  the  whole  western  side  of  the  continent,  especially 
in  the  Rocky  Mountain  region,  which  commenced  after  the  Cretaceous. 

2.  We  have  here  the  formation  of  limestone  rocks  of  various  kinds 
going  on  before  our  eyes  over  immense  areas  and  several  thousand  feet 
in  thickness,  and  we  learn  thus  that  limestones  are  of  organic  origin. 

3.  The  character  of  the  rocks  thus  formed  is  very  interesting  to 
the  geologist.     In  some  places,  as  we  have  already  seen,  it  is  a  coarse 
conglomerate,  or  breccia,  composed  of  fragments  of  all  sizes  cemented 
together  ;  in  some  places  it  is  made  up  entirely  of  rounded  granules  of 
coralline  limestone  (coral-sand},  cemented  together,  and  forming  a  pe- 
culiar oOlitic  rock  (wov  kiOog,  egg-stone).      But  the  larger  portion  of 
the  reef  ground  is  a  fine  compact  limestone,  made  up  of  comminuted 
coralline  matter  (coral  mud),  cemented  together.     This  fine  coral  mud 
is  carried  by  waves  and  tides  into  the  lagoon,  and  serves  to  raise  its 


REEFS   OF  FLORIDA. 


149 


bottom:  it  is  also  carried  by  currents  and  distributed  widely  over  the 
neighboring  sea-bottoms.  Soundings  in  coral  seas  bring  up  everywhere 
this  fine  coral  mud,  showing  that  compact  limestone  is  now  forming  over 
wide  areas  in  coral  seas.  The  reef-rock,  as  already  stated,  has  been 
found  clinging  to  the  sides  of  high  islands,  having  been  elevated  many 
feet  above  sea-level;  in  other  cases  atolls  have  been  elevated  250  feet 
above  the  sea-level.  The  structure  of  the  reef-rock  has  thus  been  ex- 
posed to  view.  In  some  places  it  contains  imbedded  remains  of  corals 
and  shells,  but  in  other  parts  it  is  entirely  destitute  of  these  remains. 

Reefs  of  Florida. 

The  reefs  of  Florida  deserve  a  brief  separate  notice,  both  because 
they  are  different  from  those  of  the  Pacific,  having  been  formed  under 
different  conditions,  and  because  they  are  much  more  efficient  agents  in 
land-making,  and  illustrate  in  a  striking  manner  how  different  agencies 
cooperate  for  this  purpose.  The  process  has  been  accurately  observed. 

Description  of  Florida.— Fig.  125  is  a  map  of  Florida,  with  its 
reefs  and  keys,  and  Fig.  126  is  a  section  along  the  line  p  p.  The 
southern  coast  (a  a)  is  ridge,  elevated  twelve  to  fifteen  feet  above  the 


FIG  125.— Gulf  Stream  and  the  Eeefs  and  Keys  of  Florida. 


150  OKGANIC  AGENCIES. 

sea-level,  within  which  is  the  Everglades  (e),  an  extensive  fresh-water 
swamp  only  two  or  three  feet  above  sea-level,  and  dotted  over  with 
small  islands,  called  hummocks.  Between  the  southern  coast  (a  a)  and 
the  line  of  keys  (b  b)  the  water  (e')  is  very  shallow,  only  navigable  to 
smallest  fishing-craft,  and  dotted  over  with  small  low  mangrove  islands. 
A  considerable  portion  of  this  area,  in  fact,  forms  mud-flats  at  low  tide. 
Between  the  line  of  keys  (b  b)  and  the  living  reef  (c  c)  there  is  a  ship- 
channel  (e")  five  to  six  fathoms  deep.  Outside  the  reef  (c  c)  the  bot- 


FIG.  126.— In  both  figures  a  =  Southern  coast;  6,  Keys  •  c,  Reef;  e,  Everglades ;  ef,  Shoal  water  ;  e", 
Ship-channel ;  ff  S  S,  Gulf  Stream. 

torn  slopes  rapidly  into  the  almost  unfathomable  abyss  of  the  Gulf  Stream 
(GSS). 

General  Process  of  Formation. — Now,  Agassiz  has  proved  that  not 

only  the  living  reef  but  the  keys,  the  southern  coast,  and  the  peninsula, 
certainly  as  far  north  as  the  north  shore  of  the  Everglades  (d  d),  and 
probably  as  far  north  as  St.  Augustine  (c?'),  have  been  formed  by  coral 
agency.  The  evidence  of  this  important  conclusion  is  that  the  rock  in 
all  these  parts  is  identical  with  the  reef-rock  already  described,  and  with 
what  is  even  now  forming  under  our  eyes  on  the  living  reef  (c  c).  It  is, 
moreover,  almost  certain  that  the  peninsula  of  Florida  has  been  progres- 
sively elongated  by  the  formation  of  successive  barrier  reefs,  one  outside 
of  the  other,  from  the  north  toward  the  south,  and  the  successive  filling 
up  of  the  intervening  ship-channels,  probably  by  coral  d&bris  from  the 
reef  and  sediments  from  the  mainland. 

History  of  Changes. — The  history  of  changes  was  as  follows:  There 
was  a  time  when  the  north  shore  of  the  Everglades  (d  d)  was  the  south- 
ern limit  of  the  peninsula.  At  that  time  the  ridge  (a  a)  which  now 
forms  the  south  shore  was  a  reef.  Upon  this  reef  by  the  action  of  waves 
was  gradually  formed  a  line  of  coral  islands,  which  finally  coalesced  into 
a  continuous  line  of  land,  and  by  the  filling  up  of  the  intervening  ship- 
channel  was  added  to  the  peninsula,  the  ship-channel  being  converted 
into  the  present  Everglades.  In  the  mean  time  another  reef  was  formed 


REEFS   OF  FLORIDA.  151 

in  the  position  of  the  present  line  of  keys.  This  has  already  been  con- 
verted into  a  line  of  wave-formed  islands,  and  its  ship-channel  into  shoal 
•water  and  mud-flats.  Eventually  the  peninsula  will  be  extended  to  the 
line  of  keys,  and  the  shoal  water  (er)  will  become  another  Everglades 
and  the  mangrove  islands  its  hummocks.  Already  another  reef  has 
been  again  formed  outside  the  last,  viz.,  the  present  living  reef  (c  c), 
and  upon  it  the  process  of  island-formation  has  commenced.  This  will 
also  be  eventually  converted  into  a  line  of  keys,  into  a  continuous  line 
of  land,  and  be  added  in  its  turn  to  the  peninsula.  It  is  not  probable 
that  another  reef  will  be  formed  outside  of  this,  for  the  bottom  slopes 
rapidly  under  the  Gulf  Stream,  as  seen  in  the  section  Fig.  126.  In 
this  process  each  reef  dies  when  another  is  formed  beyond  it,  for  the 
water  being  protected  by  the  outside  reef  becomes  placid  or  lagoon 
water,  and  the  strong  reef-building  species  no  longer  flourish. 

North  of  the  line  d  d  the  evidence  is  of  the  same  kind,  but  less  com- 
plete. True  reef-rock,  similar  to  that  now  forming  on  the  reef,  has 
been  found  at  various  points  as  far  north  as  St.  Augustine,  on  the  east- 
ern shore.  The  western  shore  is  less  known.  Tuomey  found  the  bluffs 
at  Tampa  Bay  to  consist  of  tertiary.  The  line  d'  d',  therefore,  may  indi- 
cate the  limit  of  the  peninsula  at  the  end  of  the  Tertiary  period.  The 
position  of  the  successive  reefs  in  this  part  has  not  been  determined. 

Mangrove  Islands. — Mangrove-trees  cooperate  in  an  interesting  man- 
ner with  corals  in  the  process  of  land-formation.  These  trees  form 
dense  jungles  on  the  low,  muddy  shores  of  tropical  regions.  They  are 
very  abundant  on  the  southern  and  western  shores  of  Florida.  They 
have  the  remarkable  power  of  throwing  out  aerial  roots  from  their 
trunks  and  branches,  thus  forming  subordinate  connections  with  the 
ground  or  with  the  bottom  of  shallow  water.  From  these  may  spring 
other  trunks,  which  throw  out  similar  roots,  etc.  Thus  an  inextricable 
entanglement  of  roots  and  branches  continues  to  extend  far  beyond  the 
actual  shore-line.  These  form  a  nidus  for  the  detention  of  sediments, 
and  protect  them  from  the  action  of  waves  ;  and  the  shore-line  thus 
steadily  advances. 

The  seeds  of  the  mangrove  have  also  the  faculty  of  shooting  out 
long  roots  and  stems,  even  while  still  attached  to  the  parent  tree.  These 
shoots,  falling  into  the  water,  float  away,  and  if  their  roots  touch  bottom 
immediately  fix  themselves,  grow  into  mangrove-trees,  and  commence 
multiplying  in  the  manner  described.  Thus  in  the  shoal  water  (er)  are 
found  mangrove  islands  in  which  there  is  no  land,  but  only  a  mangrove 
forest,  standing  above  water  by  means  of  their  interlaced  roots.  By 
these,  however,  sediments  are  detained,  and  a  true  island  is  speedily 
formed.  It  is  in  this  way  that  the  small  mangrove  islands  in  the  shoal 
water  on  the  south  and  west  of  Florida  are  formed.  They  are  en- 
tirely different  from  the  wave-formed  coral  islands  or  keys.  The  hum- 


152  ORGANIC  AGENCIES. 

mocks  in  the  Everglades  have  probably  a  similar  origin,  although  some 
of  them  may  possibly  be  of  coral  origin. 

Florida  Reefs  compared  with  other  Reefs.— In  comparing  the  reefs 
just  described  with  other  reefs,  it  will  be  seen  that  the  former  are  en- 
tirely unique.  JVo  other  reefs  continuously  make  land.  In  fringing  reefs 
there  is  a  small  accretion  about  the  shore-line  of  the  previously-existing 
land,  but  this  process  is  quickly  limited.  In  barriers  and  atolls  there 
is  always  loss  of  land,  only  a  small  fraction  of  which  is  recovered  by 
coral  and  wave  agency.  But  under  these  agencies  Florida  has  steadily 
advanced  southward  more  than  200  miles,  and  the  area  thus  added  to 
the  continent  is  at  least  20,000  square  miles.  It  seems  to  us  utterly 
impossible  to  account  for  this,  except  by  supposing  some  other  agency 
at  work  preparing  the  ground  for  the  growth  of  successive  reefs. 

Probable  Agency  of  the  Gulf  Stream. — Since  corals  cannot  grow  in 

water  more  than  sixty  to  one  hundred  feet  deep,  it  is  evident  that,  unless 
subsidence  goes  on  pari  passu  with  the  growth  of  the  corals,  a  coral 
formation  cannot  be  more  than  one  hundred  feet  thick.  But  there  is  no 
evidence  of  subsidence  on  the  coast  or  keys  of  Florida.1  On  the  contrary, 
the  height  of  these  parts  is  precisely  the  usual  height  of  icave-formed 
islands,  although  no  longer  exposed  to  their  action.  It  follows,  there- 
fore, that  the  corals  must  have  built  upon  an  extensive  submarine  bank, 
produced  by  some  other  agency.  Furthermore,  since  the  reefs  were 
formed  successively  one  beyond  another,  it  is  evident  that  there  must 
have  been  a  progressive  formation  of  this  bank  from  the  north  toward 
the  south.  Such  a  progressive  extension  of  a  bank  can  only  be  formed 
by  sedimentary  deposit.  It  is  impossible  to  conceive  how  such  sedi- 
mentary deposit  could  have  been  formed,  except  by  the  Gulf  Stream. 
It  is  to  this  agencv,  therefore,  that  we  attribute  the  formation  and  ex- 
tension of  the  bank  upon  which  the  corals  grew. 

We  have  already  (p.  40)  given  reasons  for  believing  that  the  Gulf 
Stream  carries  sediment  in  its  deeper  parts.  Now,  a  current  bearing 
sediment  and  sweeping  around  a  deep  curve,  like  the  Gulf  Stream 
around  Florida  (Fig.  125),  must,  as  we  have  already  shown  (p.  22),  con- 
tinually deposit  sediment  on  the  interior  of  the  curve,  forming  in  the 
case  of  a  river  a  bank  above  water,  but,  in  the  case  of  an  oceanic  stream, 
a  submarine  bank.  This  bank,  in  the  case  of  the  Gulf  Stream,  has  been 
extending  southward  for  ages  almost  inconceivable.  On  every  part,  as 
soon  as  it  reached  within  100  feet  of  the  surface,  corals  built.  Previous 
positions  of  the  southern  limit  of  the  bank  and  of  the  successive  reefs 
are  shown  in  Fig.  126  in  dotted  outline. 

1  Evidences  of  subsidence,  in  the  form  of  drowned  corals,  have  been  recently  found 
by  the  Coast  Survey  in  the  course  of  the  Gulf  Stream  oif  the  Florida  reefs,  but  this  sub- 
sidence cannot  have  extended  to  the  keys  and  peninsula,  for  this  is  inconsistent  with  the 
continual  extension  of  land. 


SHELL-DEPOSITS.  153 

It  is  probable,  therefore,  that  the  peninsula  of  Florida  is  due  to  the 
cooperation  of  four  or  five  different  agencies,  viz.  :  1.  The  Gulf  Stream 
building  up  a  submarine  bank  to  the  dotted  line  n  w,  Fig.  126,  within 
100  feet  of  the  surface ;  2.  Then  corals  building  up  to  the  surface ;  3. 
Then  waves  raising  it  twelve  to  fifteen  feet  above  the  surface ;  4.  And, 
finally,  debris  from  the  peninsula  on  the  one  side,  and  the  reef  and  keys 
on  the  other,  filling  up  the  intervening  channels,  and  afterward  raising 
the  level  of  the  swamps  or  Everglades  thus  formed.  5.  In  this  last 
process  the  mangrove-trees  have  assisted. 

The  reefs  of  Florida  are  barrier  reefs;.  Barriers  are  usually  supposed 
to  indicate  subsidence.  This  is  certainly  true  of  the  Pacific  barriers, 
which  commenced  as  fringes  and  became  barriers  by  subsidence.  But 
in  Florida  there  has  been  no  subsidence.  They  did  not  commence  as 
fringes.  The  probable  explanation  is  this :  Corals  will  not  grow  in 
muddy  water.  On  a  gently-sloping  shore  with  mud  bottom,  such  as 
probably  always  existed  on  the  southern  shore  of  Florida,  a  fringing 
reef  could  not  form,  because  the  bottom  would  be  always  chafed  by  the 
waves  and  the  water  rendered  turbid.  But  at  a  distance  from  shore, 
where  such  a  depth  was  attained  that  the  waves  no  longer  chafed  the 
bottom,  a  barrier  would  form,  limited  on  the  one  side  by  the  muddi- 
ness,  and  on  the  other  by  the  depth,  of  the  water. 

Shell-Deposits. 

Rivers  carry  carbonate  of  lime  in  solution  to  the  sea  (p.  76).  In 
some  bays,  where  large  quantities  of  this  material  are  carried  by  rivers 
running  through  limestone  countries,  the  excess  may  be  deposited  as  a 
chemical  deposit.  But  in  most  cases  sea-water  contains  less  lime-car- 
bonate than  river-water.  The  reason  is,  that  the  lime-carbonate  in  sea- 
water  is  continually  being  drafted  upon  by  organisms  and  deposited  on 
their  death  as  organic  limestones.  We  have  already  shown  how  coral 
limestone  is  thus  formed.  But  there  are  many  other  limestone-forming 
animals,  and  some  species  form  other  kinds  of  deposits  besides  lime- 
stone. 

Molluscous  Shells. — Shallow-water  deposits  of  this  kind  are  made 
principally  by  mollusca  which,  living  in  immense  numbers  near  shore 
and  on  submarine  banks,  leave  their  dead  shells  generation  after  gener- 
ation, and  thus  form  sometimes  pure  shelly  deposits,  and  sometimes 
shells  mingled  with  sediments  due  to  other  agencies.  On  quiet  shores 
the  shells  are  quite  perfect,  whether  imbedded  in  mud  or  forming  shell- 
banks  like  our  oyster-banks ;  but  when  exposed  to  the  action  of  break- 
ers, they  are  broken  into  coarse  fragments,  or  even  comminuted,  worn 
into  rounded  granules,  and  cemented  into  shell-rock  or  oo'litic  rock. 
Such  shell-rock  and  oolitic  rock  are  now  being  formed  on  the  coast  of  the 
Florida  keys  and  of  the  West  Indies.  Similar  rock  is  found  in  every 


154: 


ORGANIC  AGENCIES. 


part  of  the  world  in  the  interior  of  continents.  They  indicate  the  ex- 
istence in  these  places  of  a  shore-line  or  of  shallow  water  in  some  pre- 
vious geological  epoch. 

Microscopic  Shells. — Microscopic  plants  and  animals  are  known  to 
multiply  in  numbers  with  almost  incredible  rapidity.  Many  of  them 
form  no  shell,  and  therefore  are  of  no  geological  importance ;  but  many 
species  form  shells  of  silica  or  of  carbonate  of  lime,  and  these  of  course 
accumulate  generation  after  generation,  until  important  deposits  are 
formed. 

Fresh-water  Deposits. — In  streams,  ponds,  lakes,  and  hot  springs, 
the  beautiful  siliceous  shells  of  diatoms  (uni-celled  plants)  accumulate 
without  limit.  The  ooze  at  the  bottom  of  clear  ponds  or  lakes,  as,  for 
example,  in  the  deepest  parts  of  Lake  Tahoe,  consists  often  wholly  of 


FIG.  127.— Shells  of  livinjr  Fornminifera.  a,  Orbulina  wniversa,  in  its  perfect  condition,  showing  the 
tubular  spines  which  radiate  from  the  surface  of  the  shell ;  b.  Qlobigerina  l/ulloides,  in  its  ordinary 
condition,  the  thin  hollow  spines  which  are  attached  to  the  shell  when  perfect  having  heen  broken 
off;  c,  Tfytnlaria  rariabilis;  d,  Peneroplis  pfanatus;  e,  Kotalia  concamerata  ;  f,  Cristellaria 
subarciuitula.  >"ig.  a  is  after  Wyville  Thomson  ;  the  others  are  after  Williamson.  All  the  figures 
are  greatly  enlarged  (after  Nicholson). 

these  shells.  Diatoms  live  also  in  great  numbers  in  the  hot  springs  of 
California  and  Nevada,  and  the  deposits  of  such  springs  sometimes  con- 
sist wholly  of  these  shells.  Thick  strata,  belonging  to  earlier  geological 
times,  are  found  wholly  composed  of  diatoms.  We  are  thus  able  to 
explain  the  formation  of  these  strata. 


GEOGRAPHICAL  DISTRIBUTION   OF  ORGANISMS.  155 

Deep-sea  Deposits. — Over  nearly  all  the  bottom  of  deep  seas,  be- 
yond the  reach  of  sedimentary  deposits,  we  find  a  white,  sticky  ooze, 
composed  of  the  carbonate-of-lime  shells  of  microscopic  animals  (fora- 
minifers)  and  microscopic  plants  (coccospheres),  Fig.  127.  Some  of  these 
seem  to  be  living,  or  recently  dead ;  some  dead  and  empty,  but  still 
perfect ;  but  most  of  them  completely  disintegrated.  On  account  of  the 
great  abundance  of  the  shells  of  one  form  of  foraminifera,  this  soft, 
white  mud  is  called  globigerina  ooze.  Mingled  in  considerable  numbers 
among  the  calcareous  shells  are  others  of  silica.  These  are  also  partly 
animals  (radiolaria)  and  partly  plants  (diatoms).  The  extraordinary 
resemblance  of  this  deep-sea  ooze,  both  in  chemical  and  microscopic 
character,  to  chalk,  leaves  no  room  for  doubt  that  chalk  was  formed  in 
this  way. 

SECTION  4. — GEOGRAPHICAL  DISTRIBUTION  OF  ORGANISMS. 

Fauna  and  Flora. — The  animals  and  plants  inhabiting  any  country 
are  called  the  fauna  and  flora  of  that  country.  In  a  more  scientific 
sense,  however,  a  natural  fauna  or  flora  is  a  group  of  organisms  in- 
habiting one  locality,  differing  conspicuously  from  other  natural  groups 
inhabiting  other  localities.  All  the  members  of  such  a  natural  group 
must  bear  certain  harmonic  relations  to  one  another,  and  the  whole 
group  to  the  external  physical  conditions.  Moreover,  each  group  is 
circumscribed  and  separated  from  other  neighboring  groups  by  limiting 
physical  conditions. 

Kinds  of  Distribution. — Distribution  of  organisms  is  of  two  general 
kinds,  viz.,  distribution  in  space  and  distribution  in  time,  or  geographi- 
cal distribution  and  geological  distribution.  There  are,  therefore,  geo- 
graphical faunae  and  floras  and  geological  faunae  and  florae.  A  geographi- 
cal fauna  is  the  group  of  animals  inhabiting  any  natural  geographical 
region.  Thus  the  animals  of  Australia  form  a  distinct  fauna,  differing 
entirely  from  any  other  upon  the  earth's  surface.  A  geological  fauna 
is  the  whole  group  of  animals  inhabiting  the  earth  at  one  epoch,  and 
differing  from  that  of  other  epochs.  Thus  the  whole  group  of  animals 
inhabiting  the  earth  during  what  geologists  call  the  secondary  period 
form  a  distinct  fauna,  differing  remarkably  from  all  preceding  or  sub- 
sequent faunae.  The  flora  of  the  coal  period  is  very  distinct  from  all 
others. 

The  organisms  of  every  epoch,  however,  were  distributed  over  the 
earth's  surface  in  separate  faunas  and  florae.  Every  geological  fauna 
and  flora  is,  therefore,  divisible  into  more  or  less  distinct  geographical 
faunae  and  florae.  Geological  faunae  and  florae  will  form  the  principal 
subject  of  Part  III.  We  propose  now  to  study  only  geographical  dis- 
tribution of  organisms  at  the  present  time,  this  portion  of  geology 
being  concerned  only  with  "  causes  now  in  operation."  We  study  the 


156 


ORGANIC  AGENCIES. 


laws  of  geographical  distribution  in  the  present  epoch  because  it  throws 
light  on  the  geographical  distribution  in  previous  epochs,  and  also  on 
the  laws  of  geological  distribution,  or  the  history  of  organisms.  It  also, 
as  will  be  shown  hereafter,  furnishes  a  key  to  former  changes  in  physical 
geography  and  former  migrations  of  species. 

Among  physical  conditions  limiting  the  distribution  of  organisms, 
one  of  the  most  important  is  temperature.  We  will,  therefore,  first 
speak  of  temperature-regions,  confining  ourselves,  for  the  sake  of  greater 
clearness,  to  plants.  The  principles  thus  established  we  will  then  ex- 
tend and  modify.  And,  further,  since  temperature-regions  may  be  either 
vertical  or  horizontal  in  latitude,  we  will  commence  with 

Vertical  Botanical  Temperature-Regions. — To  explain  vertical  dis- 
tribution we  will  take  the  case  of  a  mountain,  at  or  near  the  equator, 
because  all  the  vertical  regions  are  there  represented.  If  we  pass  from 

base  to  summit  of  such  a  moun- 
tain we  will  traverse,  first,  a 
region  of  palms,  so  called  be- 
cause the  vegetation  is  char- 
acterized by  the  abundance  of 
palms  and  palm-like  plants,  such 
as  bananas,  tree-ferns,  etc.  The 
second  region  traversed  is  char- 

l acterized  by  the  prevalence  of 

FlG-  m  evergreens,  such  as  myrtles,  lau- 

rels, etc.,  and  ordinary  deciduous  trees,  such  as  hickory,  oaks,  elms, 
poplars,  etc.  ;  and  therefore  may  be  called  the  region  of  ordinary  forest 
or  hard-wood  trees.  The  third  region  traversed  is  characterized  by 
the  prevalence  of  pines  and  other  conifers,  and  therefore  called  the 
region  of  pines.  The  fourth  region  contains  few  or  no  trees,  but  only 
shrubs  and  Alpine  herbaceous  plants,  and  therefore  may  be  called  the 
Alpine  or  treeless  region.  The  fifth,  being  the  region  of  perpetual  snoic, 
is  plantless,  or  nearly  so. 

Botanical  Temperature-Regions  in  Latitude. — As  the  regions  above 
spoken  of  are  determined  entirely  by  temperature,  it  is  evident  that 
they  must  be  reproduced  in  latitude  in  zones 
where  these  limiting  temperatures  successively 
reach  the  earth's  surface.  Thus,  if  a  (Fig.  129) 
represents  an  equatorial  mountain,  the  temper- 
atures which  limit  the  botanical  regions  will 
approach  the  earth  as  we  go  toward  either  pole, 
as  shown  by  the  dotted  lines,  and  successively 
reach  the  sea-level,  giving  rise  to  similar  zones 
of  temperature,  and  therefore  to  similar  botan- 
ical regions,  extending  all  around  the  earth. 


HARD     WOOD     TRESS 


PALMS     AND    TREE  -FEffAIS 


GEOGRAPHICAL   DISTRIBUTION   OF   ORGANISMS.  157 

These  zones,  being  temperature-zones,  are  not  limited  by  parallels  of 
latitude  as  represented  in  the  figure,  but  by  isothermal  lines.  In  pass- 
ing from  the  equator  to  the  poles,  we  traverse :  1.  A  region  of  palms,  or 
tropical  zone ;  2.  A  region  of  ordinary  forest  or  hard-wood  trees,  ever- 
green and  deciduous,  or  subtropical  and  temperate  zone  ;  3.  A  region 
of  pines  and  birch,  or  cold  temperate  and  subarctic  zone ;  4.  A  treeless 
region,  or  arctic  zone.  The  fifth  or  plantless  region  can  hardly  be  said 
to  exist  at  the  sea-level  in  any  part  of  the  earth. 

Further  Definition  of  Regions. — 1.  The  regions  we  have  given  are 
characterized  by  the  prevalence  of  certain  orders  of  plants ;  but  the  same 
law  of  limitation  applies  with  still  greater  force  to  families,  genera, 
and  species.  These  smaller  classification-groups  are  still  more  limited 
in  range.  Thus  palms  range  over  the  whole  of  region  No.  1  (Fig.  129) ; 
but  one  genus  of  palms  may  occupy  the  warmer  or  equatorial  part,  and 
another  genus  the  higher  or  tropical  parts.  Thus,  generally,  the  range 
of  families  is  more  restricted  than  that  of  orders,  the  range  of  genera 
than  that  of  families,  and  the  range  of  species  than  that  of  genera. 
Thus,  these  regions  may  be  divided  into  subordinate  regions,  and  these 
again  into  still  more  subordinate  regions.  What  we  further  say  will 
have  reference  principally,  though  not  entirely,  to  species.  2.  We 
have  separated  these  regions  by  lines.  It  must  not  be  supposed,  how- 
ever, that  these  limits  are  distinctly  marked.  On  the  contrary,  they 
shade  insensjbly  into  each  other.  Some  species  of  palms,  etc.,  pass 
into  the  region  of  hard- wood  trees,  and  vice  versa.  Many  species  of 
hard- wood  trees  pass  into  the  region  of  pines,  and  vice  versa.  So,  also, 
the  sub-regions  of  families,  genera,  and  species,  cannot  be  separated  by 
hard  lines.  They  shade  insensibly  into,  interpenetrate,  or  over- 
lap one  another  at  their  margins.  Thus  if  a  a'  and  b  b'  (Fig. 
130)  be  the  range,  either  vertical  or  horizontal,  of  two  species, 
then  in  the  zone  b  a'  the  two  species  coexist.  3.  In  any  region 
or  sub-region  the  organic  forms  which  characterize  it  are  most 
abundant  in  the  middle  portion,  and  become  less  and  less 
abundant  toward  the  margin,  whe.re  they  disappear.  If  the 
line  a  a'  (Fig.  130)  represents  the  range  of  any  species,  then 
the  breadth  of  the  elliptical  area  will  represent  the  relative 
abundance  of  individuals  in  different  parts  of  the  range.  4. 
Although,  therefore,  species,  so  far  as  numbers  of  individuals 
are  concerned,  come  in  gradually  on  the  margin  of  their  nat- 
ural region,  reach  their  greatest  abundance  in  the  middle  por-  v  f 
tion,  and  again  gradually  die  out  on  the  other  margin,  yet  in  ^ 

specific  characters  we  see  usually  no  such  gradual  transition. 
In  specific  character  they  seem  to  come  in  suddenly,  to  remain  sub- 
stantially unchanged  throughout  their  range,  and  pass  out  suddenly  on 
the  other  margin.     Thus,  to  take  a  single  instance  :  in  passing  from 


158  ORGANIC  AGENCIES. 

the  equator  to  the  poles,  at  a  certain  latitude,  the  sweet-gum  or  liquid- 
amber  tree  first  appears,  few  in  number;  it  increases  in  number  in  the 
middle  part  of  its  range,  and  finally  again  diminishes  in  number  and 
gradually  disappears.  But  throughout  its  whole  range  this  species  is 
unmistakably  the  same — it  does  not  pass  into  any  other  species.  It  is 
as  if  the  species  had  originated  somehow  (we  will  not  now  discuss 
how)  in  the  area  where  we  find  it,  and  had  extended  its  range  as  far 
as  physical  conditions  and  the  struggle  for  life  with  other  species 
would  permit.  5.  We  have  seen  that  the  botanical  zones  in  elevation 
and  in  latitude  are  similar  to  one  another  in  the  great  orders  which 
characterize  them ;  but  they  are  by  no  means  identical  in  genera  and 
species.  This  follows  from  what  we  have  said  under  4.  The  vertical 
and  horizontal  zones  No.  1  being  in  direct  connection  with  one  another, 
the  species  are  to  a  large  extent  identical.  But  between  zones  No.  2 
communication  is  impossible,  except  through  zone  No.  1,  but  this  is 
forbidden  by  physical  conditions ;  and,  therefore,  although  forest-trees 
may  exist  in  both,  the  species  are  all  different.  The  same  is  true  of 
zones  Nos.  3  and  4,  and  also  of  corresponding  zones  north  and  south  of 
the  equator.  It  is  as  if  the  present  species  had  originated  in  the  areas 
where  we  now  find  them,  and  had  not  been  able  to  mingle  on  account 
of  temperature  barriers  intervening.  6.  Although,  when  no  physical 
obstacle  intervenes,  regions  or  zones  in  latitude  like  those  in  elevation 
shade  insensibly  into  one  another  by  interpenetration,  as  already  ex- 
plained, yet  when  physical  barriers,  such  as  an  east-and-west  mountain- 
chain,  occur,  no  such  shading  is  possible ;  but,  on  the  contrary,  there  is 
an  abrupt  change.  Thus,  north  and  south  of  the  Himalaya  Moun- 
tains, or  north  and  south  of  the  Sahara  Desert,  the  plants  are  entirely 
different,  apparently  because  interpenetration  of  contiguous  flora?  by 
spreading  is  impossible  in  this  case. 

Zoological  Temperature-Regions. — We  have  spoken  first  of  plants, 
because,  being  fixed  to  the  soil,  they  illustrate  more  clearly  the  natural 
laws  of  distribution  as  determined  by  temperature.  Animals,  by  their 
power  of  locomotion  and  migration  with  the  seasons,  interfere  seriously 
with  the  simplicity  of  these  laws.  Although  families,  genera,  and  spe- 
cies of  animals,  like  plants,  are  limited  in  their  range,  particular  form* 
characterizing  certain  zones — as  monkeys,  parrots,  elephants  the  torrid 
zone,  and  walruses  and  white  bears  the  polar  zone — yet  it  is  impossible 
to  divide  the  surface  of  the  earth  into  zones  characterized  by  particular 
orders  in  the  same  broad,  general  way  as  in  the  case  of  plants.  Never- 
theless, all  that  we  have  said  concerning  the  limitation  of  range  of 
families,  genera,  and  species,  and  the  manner  in  which  contiguous 
regions  or  sub-regions  shade  into  one  another,  applies  with  equal  force 
to  animals.  The  apparent  fixity  of  animal  species  within  certain  nar- 
row limits  of  variation  is  even  more  striking  than  in  the  case  of  plants. 


GEOGRAPHICAL   DISTRIBUTION   OF   ORGANISMS.  159 

What  we  shall  further  say  will  apply  to  animals  and  plants  without 
distinction. 

Continental  Fauna  and  Flora. — If  no  physical  barriers  intervened, 
there  seems  to  be  no  reason  why  the  fauna  and  flora  of  each  zone  of 
temperature  should  not  be  continuous  all  around  the  earth.  But  im- 
passable barriers  exist  in  the  form  of  oceans  separating  continents,  and 
on  the  continents  in  the  form  of  north-and-south  ranges  of  mountains. 
Consequently,  on  the  supposition  of  the  local  origin  of  species,  it  fol- 
lows that,  although  the  orders  and  sometimes  families  of  animals  and 
plants  are  similar  on  the  two  continents,  the  species  and  many  of  the 
genera  are  entirely  different.  That  this  diversity  is  the  result  of  impas- 
sable barriers  is  sufficiently  proved  by  the  fact  that  most  species  of 


animals  or  plants,  introduced  from  one  continent  to  the  correspond- 
ing zone  of  another,  flourish  well,  and  soon  become  permanent  mem- 
bers of  the  fauna  or  flora  of  their  adopted  country.  We  will  now  dis- 
cuss the  subject  in  more  detail. 

The  accompanying  figure  (Fig.  131)  is  a  view  of  the  northern  hemi- 
sphere, the  circumference  being  the  equator  and  the  centre  the  north- 
pole,  and  the  circular  zones  being  the  same  as  already  described. 
These,  however,  will  not  be  regular  circles  as  represented  in  the  figure, 
but  will  be  isothermal  zones.  They  really  run  farther  north  in  Europe, 


160  ORGANIC  AGENCIES. 

and  farther  south  in  North  America,  than  represented  in  the  figure. 
Now,  comparing  the  eastern  and  western  continents  with  one  an- 
other, commencing  with  the  arctic  zone  No.  4,  we  find  that  in  this 
zone  the  fauna  and  flora  are  nearly  identical  in  the  two  continents, 
the  reason  being,  apparently,  their  close  approximation  to  one  an- 
other in  this  zone,  and  their  connection  by  means  of  solid  ice.  In  the 
next  zone,  No.  3,  the  species  are  already  quite  different ;  and  in  No. 
2,  to  which  the  United  States  and  Europe  mostly  belong,  nearly  all 
the  species,  and  many  of  the  genera,  and  even  some  families,  are  dif- 
ferent. The  few  exceptions  to  the  universal  diversity  of  the  fauna 
and  flora  of  this  country,  as  compared  with  Europe  and  Asia,  are  prin- 
cipally :  1.  Introduced  species  ;  2.  Species  of  wide  range,  either  by 
reason  of  great  hardihood  or  by  extensive  migration,  and  which,  there- 
fore, belong  to  No.  4  as  well  as  Nos.  3  and  2;  and,  3.  Alpine  spe- 
cies, which  seem  to  have  extended,  during  a  former  cold  epoch  (Glacial 
epoch),  from  No.  4  to  No.  3  in  both  continents,  and  with  the  return  of 
milder  climate  have  retreated,  some  northward,  and  some  up  the  sides 
of  the  mountains  of  No.  3,  to  their  appropriate  zone  of  tempera- 
ture. In  No.  1  the  difference  between  the  two  continents  is  still 
greater,  and  continues  without  abatement  into  corresponding  zones  of 
the  southern  hemisphere,  since  these  do  not  approach  each  other  tow- 
ard the  pole  as  they  do  at  the  north.  Thus,  we  find  the  fauna  and 
flora  of  South  America  and  Africa  as  different  as  possible.  As  an  illus- 
tration of  this,  we  will  only  mention  the  prehensile-tailed  monkeys, 
sloths  and  armadillos,  llamas  and  toucans,  humming-birds,  among  ani- 
mals, and  the  cacti  among  plants,  as  characteristic  of  South  America ; 
and  the  lions,  tigers,  elephants,  rhinoceroses,  hippopotamuses,  giraffes, 
and  the  tailless  monkeys,  of  Africa. 

Subdivisions. — The  continental  faunae  and  florae  are  again  subdi- 
vided in  longitude  by  north-and-south  mountain-chains.  Thus  the  fauna 
and  flora  of  the  United  States  are  divided  by  the  Rocky  Mountain  and 
Appalachian  chains  into  three  sub-faunae  and  sub-floras,  viz.,  an  Atlantic 
slope,  an  interior  continental,  and  a  Pacific  slope  fauna  and  flora.  The 
difference  between  the  Atlantic  slope  and  the  interior  continental 
region  is  not  great,  because  the  mountain-barrier  is  not  so  high  but  it 
may  be  overpassed.  The  Rocky  Mountains  being  a  wider  and  higher 
and  therefore  the  more  impassable  barrier,  the  fauna  and  flora  of  the 
Pacific  slope  are  very  distinct,  almost  all  its  species  being  peculiar  to 
that  region.  The  exceptions  are  mostly  strong-winged  birds.  In  a 
similar  manner  in  South  America  the  Andes  chain  separates  faunae  and 
florae  which  are  very  distinct,  and  in  the  eastern  continent  the  Ural 
Mountains  separate  a  European  from  an  Asiatic  fauna  and  flora.  Sub- 
divisions of  this  kind  are  more  marked  in  the  case  of  plants  and  of  those 
animals  which  are  closely  connected  with  plants,  such  as  insects,  than  in 


GEOGRAPHICAL  DISTRIBUTION  OF  ORGANISMS.  101 

the  case  of  higher  animals  which  have  a  greater  power  of  locomotion, 
and  therefore  of  overcoming  obstacles. 

Special  Cases. — We  might  mention  many  special  cases  of  remarkable 
groups  of  animals  and  plants,  especially  on  isolated  islands.  We  will 
only  mention  a  few  by  way  of  illustration :  1.  The  fauna  and  flora  of 
Australia  are  perhaps  the  most  remarkable  in  the  world.  Not  only  are 
the  species  different,  but  the  genera,  families,  and  orders,  are  peculiar  to 
this  continent.  So  remarkably  and  conspicuously  is  this  the  fact,  that 
even  the  unscientific  traveler  is  at  once  struck  with  the  strange  ap- 
pearance of  the  vegetation  and  the  animals — trees  with  narrow,  rigid 
leaves  twisted  on  their  leaf-stalk  so  as  to  turn  their  edges  to  the  sky; 
the  mammals,  about  200  species,  nearly  all  belonging  to  the  non- 
placentals,  including  marsupials  and  monotremes,  a  great  sub-class  of 
quadrupeds  to  which  the  kangaroos,  the  opossums,  and  the  ornitho- 
rhynchus  belong,  and  which  are  confined  to  Australia,  with  the  excep- 
tion of  several  species  of  opossum  found  in  America.  The  island  of 
Madagascar  is  another  remarkable  zoological  province.  All  the  ani- 
mals on  this  island,  with  one  single  exception,  are  peculiar,  being 
found  nowhere  else.  This  exception  is  that  of  a  small  quadruped  sup- 
posed to  have  been  introduced.  On  the  Galapagos,  a  small  group  of 
islands  at  a  considerable  distance  from  the  west  coast  of  South  Amer- 
ica and  from  all  other  islands,  all  the  animals  are  entirely  different 
from  those  of  any  part  of  the  world.  Reptiles  of  peculiar  species 
abound,  but  no  mammal,  except  one  species  of  mouse,  has  yet  been 
found. 

Thus  we  see  that  species  are  limited  in  one  direction  by  tempera- 
ture, and  in  all  directions  by  physical  barriers.  If  we  now  add  to  these 
limitations  also  peculiar  climates  and  soils  (such,  for  example,  as  the 
dry  plains  of  Utah  and  Arizona),  which  limit  vegetation,  and  there- 
fore animals,  we  easily  perceive  that  all  these  limiting  causes  produce 
groups  of  species  confined  within  certain  areas  differing  from  other 
groups,  sometimes  overlapping  them,  sometimes  trenchantly  separated. 

Taking  all  causes  into  consideration,  the  whole  earth  has  been 
divided  into  six  principal  faunal  regions,  viz. :  1.  JVearctic,  including 
North  America,  exclusive  of  Central  America ;  2.  Neotropic,  including 
Central  and  South  America  ;  3.  Palcearctic,  including  Europe,  North 
Africa,  and  Asia  north  of  the  Himalayas ;  4.  African,  including  Africa 
south  of  the  Sahara ;  5.  Indian  or  Oriental,  including  Asia  south  of 
the  Himalayas,  and  the  adjacent  islands  ;  6.  Australian,  including  Aus- 
tralia, New  Zealand,  New  Guinea,  and  South-Sea  Islands,  etc.  These 
primary  regions  are  subdivided  into  provinces  and  sub-provinces  accord- 
ing to  the  principles  already  explained.  For  example,  the  neartic  has 
been  subdivided  into  four  provinces,  viz.,  (a)  the  Alleghanian,  (b)  the 
Rocky  Mountain,  (c)  the  Californian,  and  (d)  the  Canadian. 
11 


162  ORGANIC  AGENCIES. 

Marine  Fauna.— Distribution  in  Latitude. — In  passing  along  the 
shores  of  Europe  or  of  America,  from  south  to  north,  we  find  that  the 
species  of  marine  animals,  such  as  molluscous  shells  and  fishes,  gradu- 
ally change,  one  species  being  replaced  by  another  in  the  manner  al- 
ready explained.  If  the  change  of  temperature  be  gradual,  the  change 
of  fauna  will  also  be  gradual ;  but  if  the  change,  from  any  cause,  be 
sudden,  the  change  of  species  will  be  correspondingly  sudden.  Thus, 
for  example,  on  the  coast  of  the  United  States,  Cape  Hatteras  and  Cape 
Cod  divide  the  littoral  fauna  into  three  quite  distinct  subdivisions, 
changing  somewhat  suddenly  at  these  points,  viz.,  a  Southern,  a  Middle 
States,  and  a  New  England,  fauna.  The  reason  is  that  the  Gulf  Stream 
hugs  the  shore  as  far  as  Hatteras,  thus  carrying  the  southern  fauna 
northward  beyond  its  natural  limit,  and  then  turns  away  from  the 
coast.  On  the  other  hand,  the  arctic  current  hugs  the  New  England 
coast  as  far  as  Cape  Cod,  bringing  with  it  an  arctic  fauna,  and  then 
leaves  the  surface  and  goes  downward. 

Distribution  in  Longitude. — Both  land  and  deep  sea  are  impassable 
barriers  to  marine  species.  Hence  we  find  that  the  marine  species 
on  the  east  and  west  coasts  of  each  continent,  as  well  as  those  inhabit- 
ing the  east  and  west  shores  of  the  same  ocean,  are  almost  entirely  dif- 
ferent. Thus  the  marine  species  on  our  Atlantic  shores  are  not  only 
different  from  those  of  our  Pacific  shores,  but  also  from  those  on  the 
Atlantic  shores  of  Europe  and  Africa.  The  same  is  true  of  the  species 
on  the  two  shores  of  the  Pacific,  as  compared  with  one  another,  or  with 
those  of  Europe.  The  exceptions  to  the  general  rule,  that  the  marine 
species  of  different  shores  are  different,  are  principally  arctic  species  of 
wide  range,  such  as  whales,  etc. 

Depth  and  Bottom. — It  is  found  that  marine  species  vary  with  the 
depth,  so  that  there  are  littoral  species,  and  deep-water  species,  and  pro- 
found sea-bottom  species.  Also  the  species  on  sand-bottoms  are  differ- 
ent from  those  on  mud-bottoms. 

Special  Cases. — The  marine  fauna  of  Australia,  like  its  land  fauna, 
is  very  peculiar,  differing  from  all  others,  not  only  in  species,  but  in 
genera  and  families.  It  is  also  a  remarkable  fact  that  some  of  its  fishes 
belong  to  families  once  abundant  in  the  seas  everywhere,  but  now  ex- 
tinct except  in  these  waters.  The  marine  shells  of  almost  every  isolated 
island  in  the  ocean  are  peculiar.  This  is  still  more  true  of  land  and 
fresh-water  shells  of  islands  and  even  of  different  rivers  of  the  same 
continent.  A  remarkable  illustration  of  this  is  found  in  the  species  of 
the  common  river-mussels.  Almost  every  large  river  in  the  United 
States  has  some  species  of  shell  peculiar  to  it.  Nearly  all  the  shells  of 
the  Altamaha  River  are  peculiar,  being  found  nowhere  else  on  the  face 
of  the  earth. 

Thus  in  all  cases  species  in  different  localities  are  different  in  pro- 


GEOGRAPHICAL   DISTRIBUTION   OF  ORGANISMS.  163 

portion  to  the  height  or  depth  and  the  width  of  the  intervening  barriers, 
and  (most  important  of  all)  also  in  proportion  to  the  length  of  time 
since  these  barriers  were  established.  These  facts  are  now  so  well  at- 
tested that  they  are  used  as  a  basis  of  reasoning.  If  two  countries  now 
separated  have  species  identical,  we  are  sure  that  they  have  been  only 
very  recently  separated.  The  substantial  identity  of  the  species  of  Eng- 
land and  those  of  contiguous  Europe  shows  that  the  British  Isles  have 
been  connected  with  the  Continent  at  a  period  geologically  very  recent. 
The  general  resemblance,  though  not  identity,  of  some  plants  on  the 
Pacific  coast  and  in  Japan,  produces  a  strong  conviction  that  the  two 
continents  have  been  formerly  connected  in  the  region  of  the  Aleutian 
Isles.  The  great  distinctness  of  the  fauna  of  Australia  indicates  a 
long  period  of  isolation  from  all  other  continents.  It  is  as  if  there 
had  been  a  slow  change  of  species  in  time,  and  after  separation  each 
group  had  taken  its  own  way,  and  thus  become  more  and  more  different. 
This  subject,  however,  cannot  be  further  discussed  at  present. 


PAET  II. 
STRUCTURAL  GEOLOGY. 


CHAPTER  I. 

GENERAL  FORM  AND  STRUCTURE   OF  THE  EARTH. 
I.— Form  of  the  Earth. 

THE  form  of  the  earth  is  that  of  an  oblate  spheroid  flattened  at  the 
poles.  The  polar  diameter  is  less  than  the  equatorial  diameter  by 
about  twenty-six  miles,  or  about  -^  of  the  mean  diameter.  The 
highest  mountains,  being  only  five  miles  high,  do  not  interfere  greatly 
with  the  general  form. 

This  form,  being  precisely  that  which  a  fluid  body  revolving  freely 
would  assume,  has  been  regarded  by  many  of  the  most  distinguished 
physicists  as  conclusive  evidence  of  the  former  fluid  condition  of  the 
earth.  The  argument  may  be  stated  as  follows  :  1.  A  fluid  body  stand- 
ing still,  under  the  influence  only  of  its  own  molecular  or  gravitating 
forces,  would  assume  a  perfectly  spherical  form ;  but,  if  rotating,  the 
form  which  it  would  assume,  as  the  only  form  of  equilibrium,  is  that  of 
an  oblate  spheroid,  with  its  shortest  diameter  coincident  with  the  axis  of 
rotation.  Now,  this  is  precisely  the  form  not  only  of  the  earth,  but,  as 
far  as  known,  of  all  the  planetary  bodies.  2.  In  an  oblate  spheroid  of 
rotation  the  oblateness  increases  with  the  rapidity  of  rotation.  Now 
Jupiter,  which  turns  on  its  axis  in  ten  hours,  is  much  more  oblate  than 
the  earth.  The  flattening  of  the  earth  is  only  about  -^  of  its  diameter, 
while  that  of  Jupiter  is  about  -fa.  3.  The  forms  of  the  earth  and  of 
Jupiter  have  been  calculated  ;  the  data  of  calculation  being  the  former 
fluidity,  the  time  of  rotation,  and  an  assumed  rate  of  increasing  density 
from  surface  to  centre ;  and  the  calculated  form  comes  out  nearly  the 
same  as  the  measured  form. 


FORM   OF  THE  EARTH.  165 

This  argument,  however,  only  proves  that  the  forms  of  the  planets 
have  been  assumed  under  the  influence  of  rotation,  or  that  they  are 
spheroids  of  rotation,  but  not  that  they  have  ever  been  in  a  fluid  condi- 
tion. For  since  a  rotating  body,  whatever  be  its  form,  always  tends  to 
assume  an  oblate  spheroid  form,  and  since  the  materials  on  the  surface 
of  the  earth  are  in  continual  motion,  being  shifted  hither  and  thither 
under  the  influence  of  atmospheric  and  aqueous  agencies,  it  is  evident 
that  the  final  and  total  result  of  such  motions  must  be  in  the  course  of 
infinite  ages  to  bring  the  earth  to  the  only  form  of  equilibrium  of  a  ro- 
tating body,  viz.,  an  oblate  spheroid.  If,  for  example,  we  have  a  solid 
and  perfectly  spherical  earth,  standing  still  and  covered,  as  it  would  be, 
by  a  universal  ocean,  and  then  set  it  rotating,  it  is  evident  that  the 
waters  would  gather  into  an  equatorial  ocean,  and  the  land  be  left  as 
polar  continents.  But  this  condition  would  not  remain;  for  atmospheric 
and  aqueous  agencies,  if  unopposed,  would  eventually  cut  down  the 
polar  continents  and  deposit  them  as  sediments  into  the  equatorial  seas, 
and  the  solid  earth  would  thus  become  an  oblate  spheroid.  This  final 
effect  of  degrading  agencies  would  not  be  opposed  by  igneous  agencies, 
as  the  action  of  these  is  irregular,  and  does  not  tend  to  any  particular 
form  of  the  earth. 

Therefore,  although  there  are  many  reasons,  drawn  both  from  ge- 
ology and  from  that  cosmical  theory  (the  nebular  hypothesis)  which  ac- 
counts for  the  formation  of  worlds,  for  believing  that  the  earth  was 
once  in  an  incandescent  fluid  condition,  and  that  it  then  assumed  an 
oblate  spheroid  form  in  obedience  to  the  laws  of  equilibrium  of  fluids, 
yet  the  form  of  the  earth  alone  is  not  only  not  conclusive  proof  of  this 
former  condition,  as  has  been  generally  believed,  but  adds  nothing  to 
proofs  derived  from  other  sources,  since  this  form  is  the  necessary  result 
of  the  forces  now  in  operation  on  the  earth-surface,  whatever  may  have 
been  its  original  form  or  condition.  Further,  since  the  earth  from  the 
time  of  its  first  consolidation  has  continued  to  cool  and  contract,  and 
therefore  to  increase  its  rate  of  rotation,  the  degree  of  oblateness  which 
it  now  has  is  not  that  which  it  first  assumed  in  its  incandescent  fluid 
condition,  but  much  greater. 

2. — Density  of  the  Earth. 

The  mean  density  of  the  earth,  as  determined  by  several  independent 
methods,  is  about  5.6.  The  density  of  the  materials  of  the  earth-sur- 
face, leaving  out  water,  is  only  about  2  to  2.5.  It  is  evident,  therefore, 
that  the  density  of  the  central  portions  must  be  much  more  than  5.6. 
This  great  interior  density  may  be  the  result — 1.  Of  a  difference  of  ma- 
terial. It  is  not  improbable  that  the  surface  of  the  earth  has  become 
oxidized  by  contact  with  the  atmosphere,  and  that  at  great  depths 
the  earth  may  consist  largely  of  metallic  masses.  Or  the  great  in- 


166 


GENERAL  FORM  AND  STRUCTURE  OF  THE  EARTH. 


terior  density  may  be  the  result  —  2.  Of  condensation  by  the  immense 
pressure  of  the  superincumbent  mass.  In  either  case  the  tendency  of 
increasing  heat  would  be  to  diminish  the  increas- 
ing  density.  But  how  much  of  the  greater  den- 
sity is  due  to  difference  of  material  and  how 
much  to  increasing  pressure,  and  how  much  these 
are  counterbalanced  by  expansion  due  to  increas- 
ing heat,  it  is  impossible  to  determine. 

The  increase  of  density  has  been  somewhat 
arbitrarily  assumed  to  follow  an  arithmetical  law. 
Under  this  condition  a  density  equal  to  the  mean 
density  would  be  found  at  £  radius  from  the  sur- 
face,  and  taking  the  surface  density  at  2,  and  the 
mean  density  at  5.5,  the  central  density  would 

be  16.  In  the  diagram  (Fig.  132),  if  a  c  =  radius,  the  ordinate  a  x  = 
surface  density  =  2,  and  b  y  =  mean  density  =  5.5,  then  c  z,  the  central 
density,  will  be  =  16. 

It  is  needless  to  say  that  this  result  (Plana's)  is  unreliable. 

.  3.—  The  Crust  of  the  Earth. 

The  surface  of  the  earth  undoubtedly  differs  greatly  in  many  respects 
from  its  interior,  and  therefore  the  exterior  portion  may  very  properly 
be  termed  a  crust.  It  is  a  cool  crust,  covering  an  incandescent  interior  ; 
a  stratified  crust,  covering  an  unstratified  interior  ;  probably  an  oxi- 
dized crust,  covering  an  unoxidized  interior;  and  many  suppose  a  solid 
crust,  covering  a  liquid  interior.  This  last  idea,  which,  however,  we 
have  shown  (p.  79)  to  be  very  doubtful,  has  probably  given  rise  to  the 
term  crust.  The  term,  however,  is  used  by  all  geologists,  without  ref- 
erence to  any  theory  of  interior  condition,  and  only  to  express  that  por- 
tion of  the  exterior  which  is  subject  to  human  observation.  The  thick- 
ness which  is  exposed  to  inspection  is  about  ten  to  twenty  miles. 

Means  of  Geological  Observation.  —  The  means  by  which  we  are  en- 
abled to  inspect  the  earth  below  its  immediate  surface  are  :  1.  Artificial 
sections,  such  as  mines,  artesian  wells,  etc.  These,  however,  do  not  pen- 
etrate below  the  insignificant  depth  of  half  a  mile.  2.  Natural  sections, 
such  as  cliffs,  ravines,  canons,  etc.  These,  as  we  have  already  seen 
(p.  17),  sometimes  penetrate  5,000  to  6,000  feet.  3.  Tilting,  and  sub- 
sequent erosion,  of  the  rocks,  by  which  strata  from  great  depths  have 
their  edges  exposed.  Thus,  in  passing  along  the  surface  from  a  to  b 
(Fig.  133),  lower  and  lower  rocks  are  successively  brought  under  in- 
spection. This  is  by  far  the  most  important  means  of  observation; 
without  it  the  study  of  geology  would  be  almost  impossible.  4.  Volca- 
noes bring  up  to  the  surface  materials  from  unknown  but  probably  very 
great  depths. 


GENERAL  SURFACE  CONFIGURATION  OF  THE  EARTH. 


167 


Ten  miles  seem  an  insignificant  fraction  of  the  earth's  radius,  being, 
in  fact,  equivalent  to  less  than  one-thirtieth  of  an  inch  in  a  globe  two 
feet  in  diameter.  It  may  seem  at  first  sight  an  insufficient  basis  for  a 


Fio.  133. 


science  of  the  earth.  We  must  recollect,  however,  that  only  this  crust 
has  been  inhabited  by  animals  and  plants — on  this  crust  only  have  oper- 
ated atmospheric,  aqueous,  and  organic  agencies — and  therefore  on  this 
insignificant  crust  have  been  recorded  all  the  most  important  events  in 
the  history  of  the  earth. 

4. —  General  Surface  Configuration  of  the  Earth. 

The  earth-surface  is  very  irregular.  The  hollows  are  occupied  by  the 
ocean,  and  the  protuberances  constitute  the  continents  and  islands. 
Nearly  three-quarters  of  the  whole  surface  is  covered  by  the  ocean. 
The  mean  heights  of  the  continents  are  given  by  Humboldt  *  as  follows  : 
Europe,  671  feet ;  North  America,  748  feet  ;  South  America,  1,151 
feet ;  Asia,  1,132  feet.  The  mean  height  of  all  land  is  probably  not 
far  from  1,000  feet. 

The  mean  depth  of  the  ocean  is  probably  12,000  to  15,000  feet 
(Thomson).  There  is  probably  water  enough  in  the  ocean,  if  the  ine- 
qualities of  the  earth-surface  were  removed,  to  cover  the  earth  to  a 
depth  of  at  least  8,000  to  9,000  feet. 

The  extreme  height  of  the  land  above  the  sea-level  is  five  miles,  and 
the  extreme  depth  of  the  ocean  is  at  least  as  much.  The  extreme  re- 
lief of  the  solid  earth  is  therefore  not  less  than  ten  miles. 

Cause  of  Land-Surfaces  and  Sea-Bottoms. — The  most  usual  idea 
among  geologists  as  to  the  general  constitution  of  the  earth  is  that  the 
earth  is  still  essentially  a  liquid  mass,  covered  by  a  solid  shell  of  twen- 


ty-five to  thirty  miles  in  thickness ;  and  that  the  great  inequalities, 
constituting  land-surfaces  and  ocean-bottoms,  are  produced  by  the  up- 
bending  and  down-bending  of  this  crust  into  convex  and  concave  arches, 
as  shown  in  Fig.  134.  The  clear  statement  of  this  view  is  sufficient  to 
1  "  Cosmos,"  Sabine's  edition,  vol.  i.,  p.  293. 


168  GENERAL  FORM  AND   STRUCTURE  OF  THE  EARTH. 

refute  it ;  for,  when  it  is  remembered  that  the  arches  with  which  we  are 
here  dealing  have  a  span  of  nearly  a  semi-circumference  of  the  earth,  it 
becomes  evident  that  no  such  arch,  either  above  or  below  the  mean 
level,  could  sustain  itself  for  a  moment.  The  only  condition  under 
which  such  inequalities  could  sustain  themselves  on  a  supporting  liquid 


FIG.  135. — Diagram  illustrating  the  Conditions  of  Equilibrium  of  a  Solid  Crust  on  a  Liquid  Interior. 

is  the  existence  of  inequalities  on  the  under  surface  of  the  crust  next 
the  liquid,  similar  to  those  on  the  upper  surface,  but  in  reverse,  as 
shown  in  Fig.  135.  And  these  lower  or  under-surface  inequalities 
would  have  to  be  repeated  not  only  for  the  largest  inequalities,  viz., 
continental  surfaces  and  ocean-bottoms,  but  also  for  great  mountain 
plateaus.  And  thus  the  hypothesis  breaks  down  writh  its  own  weight. 

Besides,  we  have  already  given  good  reasons  (pages  79  and  80)  for 
believing  that  the  earth  is  substantially  solid.  Upon  the  hypothesis  of 
a  substantially  solid  earth,  we  explain  the  great  inequalities  constitut- 
ing continental  surfaces  and  ocean-bottoms  by  unequal  radial  contrac- 
tion of  the  earth  in  its  secular  co.oling. 

It  is  evident  that,  in  such  secular  cooling  and  contraction,  unless  the 
earth  were  perfectly  homogeneous,  some  parts,  being  more  conductive, 
would  cool  and  contract  more  rapidly  in  a  radial  direction  than  others. 
Thus  some  radii  would  become  shorter  than  others.  The  more  conduc- 
tive, rapidly-contracting  portions,  with  the  shorter  radii,  would  become 
sea-bottoms  ;  and  the  less  conductive,  less  rapidly -contracting  portions, 
with  the  longer  radii,  land-surfaces.  In  other  words,  the  solid  earth 
in  contracting  becomes  slightly  deformed,  and  the  water  collects  in  the 
depressions. 

It  is  only  the  greatest  inequalities,  viz.,  land-surfaces  and  sea-bot- 
toms, which  we  account  for  in  this  way.  Mountain-chains  are  certainly 
formed  by  a  different  process,  which  we  will  discuss  under  that  head 
(p.  240) ;  and  it  is  even  possible  that  the  causes  which  operate  to  pro- 
duce mountain-chains  may  also  produce  these  greater  inequalities. 

The  continuance  of  these  causes  would  tend  constantly  to  increase 
the  extent  and  height  of  the  land,  and  to  increase  the  depth,  but  dimin- 
ish the  extent  of  the  sea.  This,  on  the  whole,  seems  to  have  been  the 
fact  during  the  history  of  the  earth,  as  will  be  shown  in  Part  III. 
Nevertheless,  local  causes,  both  aqueous  and  igneous,  as  already  shown 
in  Part  I.,  have  greatly  modified  the  general  contour,  both  map  and 
profile,  given  by  secular  contraction. 

Laws  of   Continental   Form. — That  the  general  contour  of  conti- 


ROCKS.  169 

nents  and  sea-bottoms  has  been  determined  by  some  general  cause, 
such  as  secular  contraction,  affecting  the  whole  earth,  is  further  shown 
by  the  laws  of  continental  form.  The  most  important  of  these  are  as 
follows : 

1.  Continents  consist  of  a  great  interior  basin,  bordered  by  elevated 
coast-chain  rims.  This  typical  form  is  most  conspicuously  seen  in  North 
and  South  America,  Africa,  and  Australia.  Europe- Asia  is  more  irregu- 
lar, and  therefore  the  typical  form  is  less  distinct.  We  give  in  Fig.  136, 


W 


FIG.  136.— A,  Section  across  North  America  (after  Guyot) ;  S,  Section  across  Australia  (after  Guyot). 

A.  and  B,  an  east-and-west  section  of  North  America  and  of  Australia, 
as  typical  examples  of  continental  structure. 

The  great  rivers  of  the  world,  e.  g.,  the  Nile,  Mississippi,  Amazon, 
La  Plata,  etc.,  drain  these  interior  continental  basins. 

2.  In  each  continent  the  greatest  range  of  mountains  faces  the  great- 
est ocean.     Thus  in  America  the  greatest  range  is  on  the  west,  facing 
the  Pacific  ;  while  in  Africa  the  greatest  range  is  on  the  east,  facing  the 
Indian  Ocean.     In  Asia  the  Himalayas  face  the  Indian  Ocean,  while 
the  Altai  face  the  Polar  Sea.     In  Australia  the  greatest  range  is  to  the 
east,  facing  the  Pacific. 

3.  The  greatest  ranges  have  been  subjected  to  the  greatest  and 
most  complex  foldings  of  the  strata,  and  are  the  seats  of  the  greatest 
metamorphism  (p.  213)  and  the  greatest  volcanic  activity. 

4.  The  outlines  of  the  present  continents  have  been  sketched  in  the 
earliest  geological  times,  and  have  been  gradually  developed  and  per- 
fected in  the  course  of  the  history  of  the  earth.     In  the  case  of  the 
North  American  Continent  this  will  be  shown  in  Part  III. 

The  cause  of  some  of  these  laws  will  be  discussed  under  the  head 
of  Mountain-Chains. 

Rocks. 

In  geology  the  term  rock  is  used  to  signify  any  material  consti- 
tuting a  portion  of  the  earth,  whether  hard  or  soft.  Thus,  a  bed  of 
sand  or  clav  is  no  less  a  rock  than  the  hardest  granite.  In  fact,  it  is 
impossible  to  draw  any  scientific  distinction  between  materials  founded 


170  STEATIFIED   OR  SEDIMENTARY  ROCKS. 

upon  hardness  alone.  The  same  mass  of  limestone  may  be  soft  chalk 
in  one  part  and  hard  marble  in  another ;  the  same  bed  of  clay  may  be 
hard  slate  in  one  part  and  good  brick-earth  in  another ;  the  same  bed 
of  sandstone  may  be  hard  gritstone  in  one  part  and  soft  enough  to  be 
spaded  in  another.  The  same  volcanic  material  may  be  stony,  glassy, 
scoriaceous,  or  loose  sand  or  ashes. 

Classes  Of  Rocks. — All  rocks  are  divided  into  two  great  classes,  viz., 
stratified  rocks  and  unstratified  rocks.  Stratified  rocks  are  more  or 
less  consolidated  sediments,  and  are  usually,  therefore,  more  or  less 
earthy  in  structure  and  of  aqueous  origin.  Unstratified  rocks  have 
been  more  or  less  completely  fused,  and  therefore  are  crystalline  in 
structure  and  of  igneous  origin. 


CHAPTER  II. 

STRATIFIED     OR     SEDIMENTARY     ROCKS. 
SECTION  1. — STRUCTURE  AND  POSITION. 

Stratification. — Stratified  rocks  are  characterized  by  the  fact  that 
they  are  separated  by  parallel  division-planes  into  larger  sheet-like 
masses  called  strata,  and  these  into  smaller  layers  or  beds,  and  these 
again  into  still  smaller  lamince.  These  terms  are  purely  relative,  and 
are  therefore  somewhat  loosely  used.  Usually,  however,  the  term 
stratum  refers  to  the  mineralogical  character ;  the  term  layer  to  sub- 
divisions of  a  stratum  distinguish- 
able by  difference  of  color  or  fine- 
ness ;  and  the  term  lamina  to  those 
smallest  subdivisions,  evidently  pro- 
duced by  the  sorting  power  of  water. 
For  instance,  in  the  annexed  figure, 
«,  b,  and  c,  are  three  strata  of  sand- 
stone, clay,  and  limestone,  each  di- 
visible into  two  layers  differing  in 
fineness  or  compactness  of  the  ma- 
terial, and  all  finely  laminated  by  the  sorting  power  of  water.  The 
lamination,  however,  is  not  represented,  except  in  the  clay  stratum,  b. 

Extent  and  Thickness. — Probably  nine-tenths  of  the  surface  of  the 
land,  and,  of  course,  the  whole  of  the  sea-bottom,  are  covered  with  strati- 
fied rocks.  This  proves  that  every  portion  of  the  surface  of  the  earth 
has  been  at  some  time  covered  with  water.  The  extreme  thickness  of 


STRUCTURE   AND  POSITION.  Jfl 

stratified  rocks  is  certainly  not  less  than  ten  miles  ;  the  average  thick- 
ness is  probably  several  miles. 

Kinds  of  Stratified  Rocks,— Stratified  rocks  are  of  three  kinds,  and 
their  mixtures,  viz.,  arenaceous  or  sand  rocks,  argillaceous  or  clay 
rocks,  and  calcareous  or  lime  rocks.  Arenaceous  rocks,  in  their  inco- 
herent state,  are  sand,  gravel,  shingle,  rubble,  etc.,  and  in  their  com- 
pacted state  are  sandstones,  gritstones,  conglomerates,  and  breccias. 
Conglomerates  are  composed  of  rounded  pebbles,  and  breccias  of  angu- 
lar fragments  cemented  together.  Argillaceous  rocks,  in  their  inco- 
herent state,  are  muds  and  clays;  partially  consolidated  and  finely 
laminated  they  form  shales,  and  thoroughly  consolidated  they  form 
slates.  Calcareous  rocks  are  chalk,  limestone,  and  marble.  They  are 
seldom  in  an  incoherent  state,  except  as  chalk. 

These  different  kinds  of  rocks  graduate  into  each  other  through 
intermediate  shades.  Thus  we  may  have  argillaceous  sandstones,  cal- 
careous sandstones,  and  calcareous  shales  or  marls. 

The  most  important  points  connected  with  stratified  rocks  we  will 
now,  for  the  sake  of  greater  clearness,  bring  out  in  the  form  of  distinct 
propositions.  On  these  propositions  is  based  nearly  the  whole  of 
geological  reasoning. 

I.  Stratified  Rocks  are  more  or  less  Consolidated  Sediments. — The 
evidence  of  this  fundamental  proposition  is  abundant  and  conclusive. 
1.  Beds  of  mud,  clay,  or  sand,  as  already  stated,  may  often  be  trace'd 
by  insensible  gradations  into  shales  and  sandstones.  2.  In  many  places 
the  process  of  consolidation  is  now  going  on  before  our  eyes.  This  is 
most  conspicuous  in  sediments  deposited  at  the  mouths  of  large  rivers 
whose  waters  contain  abundance  of  carbonate  of  lime  in  solution,  or  on 
the  coasts  of  seas  containing  much  carbonate  of  lime.  Thus  the  sedi- 
ments of  the  Rhine  are  now  consolidating  into  hard  stone  (p.  76),  and 
on  the  coasts  of  Florida,  Cuba,  and  on  coral  coasts  generally,  com- 
minuted shells  and  corals  are  quickly  cemented  into  solid  rock  (p.  148). 
3.  All  kinds  of  lamination  produced  by  the  sorting  power  of  water 
which  have  been  observed  in  sediments,  have  also  been  observed  in 
stratified  rocks.  4.  Stratified  rocks  contain  the  remains  of  animals  and 
plants,  precisely  as  the  stratified  mud  of  our  present  rivers  contains 
river-shells,  our  present  beaches  sea-shells,  or  the  mud  of  our  swamps 
the  bones  of  our  higher  animals  drifted  from  the  high  lands.  5.  Im- 
pressions of  various  kinds,  such  as  ripple-marks,  rain-prints,  footprints, 
etc.,  evidently  formed  when  the  rock  was  in  the  condition  of  soft  mud, 
complete  the  proof.  It  may  be  considered  as  absolutely  certain  that 
stratified  rocks  are  sediments.  Arenaceous  and  argillaceous  rocks  are 
the  debris  of  eroded  land,  and  are  therefore  called  mechanical  sedi- 
ments or  fragmental  rocks.  Limestones  are  either  chemical  deposits 
in  lakes  and  seas,  or  are  the  comminuted  remains  of  organisms.  They 


172 


STRATIFIED   OR   SEDIMENTARY  ROCKS. 


are  therefore  either  chemical  or  organic  sediments.  Conglomerates, 
grits,  and  sandstones,  indicate  violent  action ;  shales  and  clays  quiet 
action  in  sheltered^spots.  Limestones  are  sometimes  produced  by  vio- 
lent action — e.  g.,  coral  breccia — sometimes  very  quiet  action,  as  in 
deep-sea  deposits. 

We  have  already  seen  (p.  4)  that  rocks  under  atmospheric  agen- 
cies are  disintegrated  into  soils,  and  these  soils  are  carried  by  rivers 
and  deposited  as  sediments  in  lakes  and  seas.  Now  we  see  that  these 
sediments  are  again  in  the  course  of  time  consolidated  into  rocks,  to  be 
again  raised  by  igneous  agencies  into  land,  and  again  disintegrated  into 
soils,  and  redeposited  as  sediments.  Thus  the  same  material  has  been 
in  some  cases  worked  over  many  times  in  an  ever-recurring  cycle. 
This  is  another  illustration  of  the  great  law  of  circulation,  so  universal 
in  Nature. 

Cause  of  Consolidation. — The  consolidation  of  sediments  into  rocks 
in  many  cases  is  due  to  some  cementing  principle,  such  as  carbonate  of 
lime,  silica,  or  oxide  of  iron,  present  in  percolating  waters.  In  such 
cases  the  consolidation  often  takes  place  rapidly.  In  other  cases  it  is 
due  to  long -continued  heavy  pressure,  and  in  still  others  to  long-con- 
tinued, though  not  necessarily  very  great,  elevation  of  temperature  in 
presence  of  water.  In  these  cases  the  process  is  very  slow,  and  there- 
fore it  has  not  progressed  greatly  in  the  more  recent  rocks. 

II.  Stratified  Rocks  have  been  gradually  deposited. — The  following 

facts  show  that  in  many  cases  rocks  have  been  deposited  with  extreme 
slowness  :  1.  Shales  are  often  found  the  lamination  of  which  is  beau- 
tifully distinct,  and  yet  each  lamina  no  thicker  than  cardboard.  Now, 
each  lamina  was  separately  formed  by  alternating  conditions,  such  as 
the  rise  and  fall  of  tide,  or  the  flood  and  fall  of  river.  2.  Again,  on  the 
interior  of  imbedded  shells  of  mollusca,  or  on  the  outer  surface  of  the 
shells  of  sea-urchins  deprived  of  their  spines, 
are  often  found  attached  other  shells,  as 
shown  in  the  following  figures.  Now,  these 
shells  must  have  been  dead,  but  not  yet 
covered  with  deposit  during  the  whole  time 
the  attached  shell  was  growing.  As  a 
general  rule,  in  fragmental  rocks  the  finest 


FIG.  138—  Serpula  on  Shell  of  an  Echinoderm.  FIG.  139.— Serpulse  on  Interior  of  a  Shell 


STRUCTURE  AND   POSITION. 


173 


materials,  such  as  clay  and  mud,  have  been  deposited  very  slowly,  while 
coarse  materials,  such  as  sand,  gravel,  and  pebbles,  have  been  de- 
posited rapidly.  Limestones,  being  generally  formed  by  the  accumula- 
tion of  the  calcareous  remains  of  successive  generations  of  organisms, 
living  and  dying  on  the  same  spot,  must  have  accumulated  with  extreme 
slowness.  The  same  is  true  of  infusorial  earths. 

It  is  necessary,  therefore,  to  bear  in  mind  that  all  stratified  rocks 
were  formed  in  previous  epochs  by  the  regular  operation  of  agents 
similar  to  those  in  operation  at  present,  and  not  by  irregular  or  cataclys- 
mic action,  as  supposed  by  the  older  geologists.  Thus,  cceteris  paribus, 
the  thickness  of  a  rock  may  be  taken  as  a  rude  measure  of  the  time 
consumed  in  its  formation. 

III.  Stratified  Rocks  were  originally  nearly  horizontal.— The  hori- 
zontal position  is  naturally  assumed  by  all  sediments,  in  obedience  to 
the  law  of  gravity.  When,  therefore,  we  find  strata  highly  inclined  or 
folded,  we  conclude  that  their  position  has  been  subsequently  changed. 
It  must  not  be  supposed,  however,  that  the  planes  which  separate  strata 
were  originally  perfectly  horizontal,  or  that  the  strata  themselves  were 
of  unvarying  thickness,  and  laid  atop  of  each  other  like  the  sheets  of  a 
ream  of  paper.  On  the  contrary,  each  stratum,  when  first  deposited, 
must  be  regarded  as  a  widely  expanded  cake,  thickest  in  the  middle  and 
thinning  out  at  the  edges,  and  interlapping  there  with  other  similar 
cakes.  Fig.  140  is  a  diagram  showing  the  mode  of  interlapping. 


FIG.  140. — Diagram  showing  Thinning  out  of  Beds :  a,  sandstones  and  conglomerates ;  6,  limestones. 

The  extent  of  these  cakes  depends  upon  the  nature  of  the  material.  In 
fine  materials  strata  assume  the  form  of  extensive  thin  sheets,  while 
coarse  materials  thin  out  more  rapidly,  and  are  therefore  more  local. 

The  most  important  apparent  exception  to  the  law  of  original  hori- 
zontality  is  the  phenomenon  of  oblique  or  cross  lamination.  This  kind 
of  lamination  is  formed  by  rapid,  shift- 
ing currents,  bearing  abundance  of 
coarse  materials,  or  by  chafing  of 
waves  on  an  exposed  beach.  Many 
examples  of  similar  lamination  are 
found  in  rocks  of  previous  epochs. 
Figs.  141  and  142  represent  such  ex-  FIG.  wi.-obiique  Lamination, 

amples.     In  some  cases  oblique  lami- 
nation may  be  mistaken  for  highly-inclined  strata ;  careful  examination, 


1Y4:  STRATIFIED  OR  SEDIMENTARY  ROCKS. 

however,  will  show  that  the  strata  are  not  parallel  with  the  laminae. 
The  strata  were  originally  (and  in  the  cases  represented  in  the  figures 
are  still)  horizontal,  while  the  laminae  are  oblique. 


FIG.  142.— Section  on  Mississippi  Central  Railroad  at  Oxford  (after  Hilgard) :  Oblique  Lamination. 

Elevated,  Inclined,  and  Folded  Strata. — We  may  assume,  there- 
fore, that  strata  were  originally  horizontal  at  the  bottom  of  seas  and 
lakes  ;  and,  therefore,  when  we  find  them  in  other  places  and  positions, 
they  have  been  subsequently  disturbed.  Now,  \ve  actually  do  find 
strata  in  every  conceivable  position  and  place  ;  sometimes  they  retain 
their  original  horizontality,  but  are  raised  above  their  original  level ; 
sometimes  they  have  been  squeezed  by  lateral  pressure,  and  thrown  into 

the  most  intricate  contortions 
(Figs.  143,  144,  and  145); 
sometimes  whole  groups  of 
strata  many  thousand  feet 
thick  are  thrown  into  huge 
parallel  folds  or  wrinkles, 
forming  parallel  ranges  of 
mountains  (Figs.  146  and 
147)  ;  sometimes  by  these 
movements  the  strata  are 
broken,  and  one  side  of  the 
fissure  slips  up,  while  the  other 

FIG.  148.— Contorted  Strata  (after  Hitchcock).  side   drops   down,  thus    produ- 

cing what   is    called    a  fault 

(see  page  222).      But  whether  simply  elevated,  or  also  contorted,  or 
broken  and  slipped,  in  nearly  all  cases  large  portions  of  the  original 


FIG.  144.— Contorted  Strata  (from  Logan). 


strata  are  carried  away  by  erosion,  and  they  are  left  in  patches  and 
basins,  or  with  their  upturned  edges  exposed  on  the  surface,  as  shown 


STRUCTURE  AND   POSITION. 


175 


1  c 

FIG.  146.-Section  of  Appalachian  Chain. 


FIG.  14T.— Section  of  the  Jura  Mountains. 

' 

FIG.  148. 

in  Figs.  148,  149,  and  150,  in  which  the  dotted  lines  show  the  part 

removed.       We    are    thus 

enabled  to   examine   strata  ---I~x 

which     would     otherwise 

have    remained  forever  hid 

from    us.       The    exposure 

of  the   edges  of  strata  on 

1  /»  1  -T  11*.     -i-1*!?. 

the    surface    by   erosion    is 

called  outcrop.     There  are  certain  terms  in  constant  use  by  geologists 

which  must  be  explained  in  this  connection. 

Dip  and  Strike. — The  inclination  of  strata  to  an  horizontal  plane  is 


176  STRATIFIED   OR  SEDIMENTARY  ROCKS. 

called  the  dip.     Thus,  in  Fig.  152,  the  strata  dip  25°  toward  the  south. 


c  \  d 

\       \ 

FIG.  151.— Upturned  and  Eroded  Strata,  Elk  Mountains,  Colorado  (after  Hayden). 

The  dip  may  vary  from  0°  to  90°,  from  horizontally  to  vertically.    Fig-. 
153  gives  an  example  of  vertical  strata.     When  in  strong  foldings  the 

strata  are  pushed  over  be- 
yond the  perpendicular,  as 
in  Fig.  150,  we  have  what 
is  called  an  overturn  dip. 
When  strata  dipping  regu- 
larly are  exposed  on  their 
edges,  as  in  Fig.  152,  their 
thickness  may  be  easily 
calculated.  If  we  measure 
the  distance  a  b  and  the  angle  of  dip  c  a  b,  then  c  b,  the  thickness  of 
the  stfata,  is  equal  to  the  sine  of  the  angle  of  dip,  multiplied  by  the  dis- 
tance a  b  (R  =  1  :  a  b 
: :  sine  cab  :  c  b  and  c  b 
=  a  b  X  sine  c  a  b). 

The  angle  of  dip  is  ob- 
tained by  means  of  an  in- 
strument called  a  clinome- 
ter (Fig.  154).  The  most 
convenient  form  is  a  pock- 
et compass  containing  a 
pendulum  to  indicate  the 
angle  of  dip. 

T       .  FIG.  153.— Vertical  Strata. 

It  is  rarely  the  case 
that  the  geologist  is  able  to  get  a  complete  natural  section  of  an  exten- 


STRUCTURE   AXD  POSITION.  177 

sive  series  of  strata.  He  is  usually,  therefore,  compelled  to  construct  a 
more  or  less  ideal  section  from  the  examination  of  outcrops  and  partial 
sections  wherever  he  can  find  them. 


FIG.  154.— Clinometer. 

The  strike  is  the  line  of  intersection  of  strata  with  an  horizontal 
plane,  or  the  direction  of  the  outcrop  of  strata  on  a  level  surface.  It  is 
always  at  right  angles  to  the  dip.  If  the  dip  is  toward  the  north  or 
south,  the  strike  is  east  and  west.  If  the  strata  are  plane,  the  strike  is 
a  straight  line,  but  in  folded  strata  the  strike  may  become  very  sinu- 
ous. The  outcrop  of  strata  upon  the  actual  surface  is  often  extremely 
irregular,  since  this  is  affected  not  only  by  the  foldings  of  the  strata, 
but  by  the  inequalities  of  surface 
produced  by  erosion.  The  intricate 
outcrop  of  rocks,  under  these  circum- 
stances, can  only  be  understood  by 
actual  examination  in  the  field  or  by 
the  use  of  models.1  A  comparatively 
simple  case  of  such  outcrop  is  given 
in  Fig.  155,  and  the  manner  in  which 
the  rocks  are  folded  and  eroded  is 
shown  in  the  section  Fig.  156. 

Anticlines  and  Synclines. — Fold- 
ed strata,  of  course,  usually  dip  al- 
ternately in  opposite  directions,  forming  alternate  ridges  and  hollows, 
or  saddles  and  troughs  (Fig.  156).    A  line  from  which  the  strata  dip  in 

opposite  directions  on  the  two 
sides  is  called  an  anticlinal 
axis,  or  simply  an  anticline  / 
a  line  toward  which  the  strata 
dip  in  opposite  directions  on 

Fte.  156.-Section  of  Undulati'stra'ta.  tllG    tW°    SideS    'S  C&lled  *  ^ 

clinal  axis,  or  a  syncline.   The 

strata,  in  the  case  of  an  anticline,  always  form  a  ridge,  and  in  the  case 
of  a  syncline  a  trough  ;  but,  in  the  actual  surface,  this  is  often  entirely 

1  Sopwith's  Geological  Models. 


FIG.  155.— Plan  of  Undulating  Strata. 


178  STRATIFIED   OR  SEDIMENTARY  ROCKS. 

reversed  by  erosion,  so  that  the  synclines  become  the  ridges  and  the 
anticlines  the  hollows  or  valleys.  Fig.  149  represents  a  section  in 
which  the  anticlines  or  original  ridges  have  become  valleys,  while  the 
synclines  or  original  valleys  have  become  mountain-ridges.  Examples 
of  synclinal  mountains  and  anticlinal  valleys  are  by  no  means  uncom- 
mon. In  both  anticlines  and  synclines  the  strata  are  repeated  on  each 
side  of  the  axis. 

Monoclinal  Axes. — Sometimes  strata  over  large  areas  are  lifted 
bodily  upward  with  little  change  of  inclination,  while  over  contiguous 
areas  they  are  dropped  down,  the  two  areas  being  connected  by  a  sharp 
bend  of  the  strata  instead  of  a  fault.  Such  a  bend  is  called  a  mono- 
clinal  fold  or  axis  (Fig.  157).  Monoclinal  folds  pass  by  insensible  gra- 


dations  into  faults,  and  are  evidently  produced  in  a  similar  manner — 
the  degree  of  flexibility  of  the  strata  determining  whether  the  one  or 
the  other  is  formed.  In  the  plateau  of  Colorado,  where  monoclinal 
folds  are  common,  they  may  be  traced  into  faults.  Fig.  157  is  taken 
from  this  region. 

Unconformity. — We  have  seen  (page  175)  that  land-surfaces  are 
always  composed  of  eroded,  and  usually  of  tilted,  strata.  We  have 
also  seen  (pages  127-130)  that  land-surfaces  are  now  in  some  places 
sinking  and  becoming  sea-bottoms,  while  in  others  sea-bottoms  are  ris- 
ing and  becoming  land-surfaces.  The  same  thing  has  happened  in  every 
geological  epoch.  Now,  whenever  an  eroded  land-surface  sinks  below  the 
water  and  receives  sediments,  these  sediments  will  lie  in  horizontal  layers 
upon  the  upturned  edges,  and  filling  up  the  erosion  hollows  of  the  pre- 
vious strata.  If,  now,  the  two  series  of  strata  be  again  elevated  into 
land-surface,  and  exposed  to  the  inspection  of  the  geologist,  the  relation 
of  the  two  series  to  one  another  will  be  represented  by  the  following 
sections  (Figs.  158  and  159).  When  one  series  of  strata  rests  thus  on 


CLEAVAGE   STRUCTURE. 


179 


the  eroded  surface  or  edges  of  another  series,  the  two  series  are  said  to 
be  unconformable.  Of  course,  the  whole  series  may  be  again  elevated, 
tilted,  and  eroded,  making  the  phenomena  far  more  complex  than  here 


onformity. 


represented.  By  far  the  most  common  case  is  that  of  Fig.  158,  in  which 
the  upper  series  rests  on  the  upturned  edges  of  the  lower  series,  and 
there  is  therefore  a  want  of  parallelism  between  the  two  series ;  and 


FIG.  159.-Unconformity. 

the  term  unconformity  is  usually  defined  as  a  want  of  parallelism ;  but 
it  should  be  applied  also  to  cases  like  Fig.  159,  where  there  is  no  want 
of  parallelism. 

Conformable  strata  indicate  a  period  of  comparative  repose,  during 
which  sediments  were  quietly  deposited.  Unconformity  indicates  a  pe- 
riod of  disturbance,  during  which  the  strata  were  elevated  into  a  land- 
surface,  subjected  to  erosion,  and  again  subsided  to  receive  other  sedi- 
ments. A  section  like  Fig.  158  or  159,  one  of  the  commonest  in  struct- 
ural geology,  indicates  two  periods  of  repose  and  one  of  disturbance. 
The  lapse  of  time  in  the  periods  of  repose  is  represented  by  the  strata ; 
the  lapse  of  time  in  the  period  of  disturbance  is  represented  by  the  ero- 
sion. Every  case  of  unconformity,  therefore,  indicates  a  gap  in  the 
history  of  the  earth — a  period  unrecorded  by  strata  at  that  place. 

Formation.— A  group  of  conformable  strata  often  constitutes  what 
geologists  call  a  formation.  Unconformable  strata  usually  belong  to 
different  formations.  These  divisions,  however,  are  founded  also  upon 
the  character  of  the  contained  fossils.  This  subject  will  be  more  fully 
explained  hereafter. 

Cleavage  Structure* 

We  have  thus  far  spoken  only  of  the  original  and  universal  structure 
of  stratified  rocks,  together  with  the  tiltings,  foldings,  and  erosion,  to 

1  This  structure  is  usually  treated  under  metamorphic  rocks,  as  a  kind  of  metamor- 
phism ;  but  it  is  found  in  rocks  which  have  not  undergone  ordinary  metamorphic  changes, 
and  it  is  produced  by  an  entirely  different  cause. 


180  STRATIFIED   OR   SEDIMENTARY  ROCKS. 

which  they  have  been  subjected.  There  is,  however,  often  found  in 
stratified  rocks  a  superinduced  structure  which  simulates,  and  is  often 
mistaken  for,  stratification.  It  is  called  cleavage  structure,  or  (since  it 
is  usually  found  in  slates)  slaty  cleavage.  This  subject  has  recently 
attracted  much  attention,  and  is  an  admirable  example  of  the  successful 
application  of  physics  to  the  solution  of  problems  in  geology. 

Cleavage  may  be  defined  as  the  easy  splitting  of  any  substance  in 
planes  parallel  to  each  other.  Such  definite  splitting  may  result,  in 
different  cases,  from  entirely  different  causes.  For  example  (a),  under 
the  influence  of  the  sorting  power  of  water,  sedimentary  materials  may 
be  so  arranged  as  to  give  rise  to  easy  splitting  along  the  planes  of  lami- 
nation. Many  rocks  may  be  thus  split  into  large  coarse  slabs  called  flag- 
stones, and  are  used  for  paving  streets,  or  even  sometimes  as  roofing- 
slates.  This  may  be  called  flag-stone  cleavage,  or  lamination  cleavage. 
Again  (5),  the  arrangement  of  the  ultimate  molecules  of  a  mineral  un- 
der the  influence  of  molecular  or  crystalline  forces  gives  rise  to  an  ex- 
quisite splitting  along  the  planes  parallel  to  the  fundamental  faces  of 
the  crystal.  This  is  called  crystalline  cleavage.  Again  (c),  the  ar- 
rangement of  the  wood-cells  under  the  influence  of  vital  forces  gives  rise 
to  easy  splitting  of  wood  in  the  direction  of  the  silver-grain.  This  may 
be  called  organic  cleavage. 

Now,  in  certain  slates  and  some  other  rocks  is  found  a  very  perfect 
cleavage  on  a  stupendous  scale.  Whole  mountains  of  strata  may  be 
cleft  from  top  to  bottom  in  thin  slabs,  along  planes  parallel  to  each 
other.  The  planes  of  cleavage  seem  to  have  no  relation  to  the  strata, 
but  cut  through  them,  maintaining  their  parallelism,  however  the  strata 
may  vary  in  dip  (Fig.  160).  Usually  the  cleavage-planes  are  highly  in- 
clined, and  often  nearly  perpendicular.  It  is  from  the  cleaving  of  such 


FIG.  160.-Cleavage-Planes  cutting  through  Strata. 

slates  that  roofing-slates,  ciphering-slates,  and  blackboard-slates  are 
made.  This  remarkable  structure  has  long. excited  the  interest  of 
geologists,  and  many  theories  have  been  proposed  to  explain  it. 

On  cursory  examination  of  such  rocks,  the  first  impression  is,  that  the 
cleavage  is  but  a  very  perfect  example  of  flag-stone  or  lamination  cleav- 
age— that  the  cleavage-planes  are  in  fact  stratification-planes,  and  that 
we  have  here  an  admirable  example  of  finely  laminated  rocks  which 
have  been  highly  tilted  and  then  the  edges  exposed  by  erosion.  Closer 
examination,  however,  will  generally  show  the  falseness  of  this  view. 


CLEAVAGE  STRUCTURE.  181 

Fig.  161  represents  a  mass  of  slate  in  which  three  kinds  of  structure  are 
distinctly  seen,  viz.,  joint  faces,  A,  J3,  C,  J,  J ;   stratification-planes, 


FIG.  161.— Strata,  Cleavage-Planes,  and  Joints. 

S  S S,  gently  dipping  to  the  right;  and  cleavage-planes,  highly  inclined, 
D  J),  cutting  through  both.  Cleavage-planes  are  therefore  not  stratifi- 
cation-planes. 

Again,  it  has  been  compared  to  crystalline  cleavage,  on  a  huge  scale. 
It  has  been  supposed  that  electricity  traversing  the  earth  in  certain  di- 
rections, while  certain  rocks  were  in  a  semi-fluid  or  plastic  state  through 
heat,  arranged  the  particles  of  such  rocks  in  a  definite  way,  giving  rise 
to  easy  splitting  in  definite  directions.  In  support  of  this  view  it  was 
urged  that  cleaved  slates  are  most  common  in  metamorphic  regions;  and 
metamorphism,  as  we  shall  see  hereafter  (p.  215,  et  seq.),  indicates  the 
previous  plastic  state  of  rocks,  which  is  a  necessary  condition  of  the 
rearrangement  of  the  particles  by  electricity.  The  great  objections  to 
this  theory  are — 1.  That  the  cleavage  is  not  like  crystalline  cleavage, 
between  ultimate  molecules,  and  therefore  perfectly  smooth,  but  be- 
tween discrete  and  quite  visible  granules ;  and,  2.  That  although  the 
phenomenon  is  indeed  most  common  in  metamorphic  rocks,  yet  meta- 
morphism is  by  no  means  a  necessary  condition ;  on  the  contrary,  when 
the  real  necessary  conditions  are  present,  the  less  the  metamorphism 
the  more  perfect  the  cleavage. 

It  is  evident,  therefore,  that  slaty  cleavage  is  not  due  to  any  of  the 
causes  spoken  of  above.  It  is  not  flag-stone  cleavage,  nor  crystalline 
cleavage,  and  of  course  cannot  be  organic  cleavage. 

Sharpe's  Mechanical  Theory.— The  first  decided  step  in  the  right 
direction  was  made  by  Sharpe.  According  to  him,  slaty  cleavage  is  al- 
ways due  to  powerful  pressure  at  right  angles  to  the  planes  of  cleavage, 
by  which  the  pressed  mass  has  been  compressed  in  the  direction  of  press- 
ure and  extended  in  the  direction  of  cleavage.  This  theory  may  be 
now  regarded  as  completely  established  by  the  labors  of  Sharpe,  Sorby, 
Haughton,  Tyndall,  and  others.  We  will  give  a  few  of  the  most  impor- 
tant observations  which  establish  its  truth. 

(a.)  Distorted  Shells. — Many  cleaved  slates  are  full  of  fossils.     In 


182 


STRATIFIED   OR  SEDIMENTARY  ROCKS. 


such  cases  the  fossils  are  always  crushed  and  distorted  as  if  by  powerful 
pressure,  their  diameters  being  shortened  at  right  angles  to  the  cleavage, 
and  greatly  increased  in  the  direction  of  the  cleavage-planes.  The  fol- 
lowing figures  (Fig.  162)  are  examples  of  distortion  by  pressure.  In 


FIG.  162. -Distorted  Fossils  (after  Sharpe). 

Fig.  162,  Z  Z  gives  the  direction  of  the  planes  of  cleavage ;  Figs.  1, 2, 3, 
4,  represent  one  species  ;  5,  6,  7,  8,  another.  In  Fig.  163  still  another 
species  is  represented  in  the  natural  and  distorted  forms. 


Fio.  163.— Cardium  Hillanum  :  A,  natural  form ;  B  and  C,  deformed  by  pressure. 

(J.)  Association  with  Foldings. — Cleavage  is  always  associated  with 
strong  foldings  and  contortions  of  the  strata.     The  folding  of  the  strata 


FIG.  164.— Cleavage-Planes  intersecting  Strata. 


is  produced  by  horizontal  pressure ;  the  strike  of  the  strata,  or  the 
direction  of  the  anticlinal  and  synclinal  axes,  being  of  course  at  right 


CLEAVAGE  STRUCTURE. 


183 


angles  to  the  direction  of  pressure.  Now,  if  cleavage  is  produced  by 
the  same  pressure  which  folded  the  strata,  then  in  this  case  we  ought 
to  find  the  cleavage-planes  highly  inclined,  and  their  strike  parallel 
with  the  strike  of  the  strata  ;  and  such  we  find  is  usually  the  fact.  In 
Fig.  164  the  heavy  lines  represent  the  strata  and  the  light  lines  the 
cleavage-planes,  both  outcropping  on  a  nearly  level  surface,  and  parallel 
to  each  other. 

(c.)  Association  with  Contorted  Laminae. — The  last  evidence  was 
taken  from  foldings  on  a  grand  scale  of  the  crust  of  the  earth ;  but  even 

fine  lines  of   lamination   are   often    thrown   into        

intricate  foldings  by  squeezing  together  in  the 
direction  of  the  lamination-planes.  In  such  case, 
of  course  the  cleavage  ought  by  theory  to  be  at 
right  angles  to  the  original  direction  of  the  lami- 
nation, and  in  such  direction  we  actually  find  them. 
Fig.  165  represents  a  block  of  rock  in  which  three 
lamination-lines  are  visible.  The  lower  one,  f  d, 
consists  of  coarse  sand  which  could  not  mash,  and 
therefore  has  been  thrown  into  folds.  As  the 
specimen  stands  in  the  figure,  the  pressure  has 
been  horizontal ;  the  perpendicular  lines  represent 
the  position  of  the  cleavage-planes.  Fig.  166  rep- 
resents a  beautiful  specimen  of  laminated  slate,  in 
which  the  lamination -planes  have  been  thrown  into 
folds  by  pressure.  The  direction  of  the  pressure  is  obvious.  The  planes 

of  cleavage  are  parallel  to  the 
face,  cp,  and  therefore  at  right 
angles  to  the  pressure. 

(d.)  Flattened  Nodules.— 
In  some  finely-cleaved  slates, 
such  as  are  used  for  writing- 
slates,  it  is  common  to  find 
small  light-greenish,  elliptical 
spots  of  finer  material.  In  clay- 
deposits  of  the  present  day 
it  is  also  common  to  find  im- 
bedded little  round  nodules  of 
finer  material.  It  is  probable 
that  the  greenish  nodules  in 
slates  were  also  rounded  nodules 
of  finer  clay  in  the  original  clay- 
deposit  from  which  the  slate 
was  formed  by  consolidation. 

FIG.  166.-A  Block  of  Cleayed  Slate  (after  Jukes).        But  in  cleaved  slates  these  nod- 


FIG.  165.— Cleavage-Planes 
(after  Tyndall). 


184 


STRATIFIED  OR  SEDIMENTARY  ROCKS. 


ules  are  always  very  much  flattened  in  the  direction  at  right  angles  to 
the  cleavage-planes,  and  spread  out  in  the  direction  of  these  planes. 

(e.)  Apparent  Diamagnetism  of  Cleaved  Slates  .under  Certain  Con- 
ditions.— If  a  bar  of  iron  be  placed  between  the  poles  of  a  magnet, 
it  will  immediately  place  itself  in  the  line  connecting  the  poles  (axial 
position)  ;  but  if  a  bar  of  bismuth  be  similarly  placed,  it  will  assume 
a  position  at  right  angles  to  the  axial  line  (equatorial position).  In 
the  former  case  the  ends  of  the  bar  are  attracted  by  the  poles ;  in  the 
other  they  are  repelled.  Bodies  which,  like  iron,  assume  the  axial  posi- 
tion, are  called  paramagnetic  ;  bodies  which,  like  bismuth,  assume  the 
equatorial  position,  are  called  diamagnetic.  But  Tyndall  has  shown  * 
that  by  strong  compression  a  paramagnetic  substance  may  be  made  to  as- 


FIG.  167.— Illustrating  Behavior  of  Cleaved  Slates  in  the  Magnetic  Field. 

sume  an  equatorial  or  diamagnetic  position.  If  a  cube  of  iron  be -placed 
between  the  poles  JVand  S  of  a  magnet  (Fig.  167,  ^4),  the  cube  will  be  in- 
different as  to  position,  since  the  attraction  along  any  two  lines,  a  #,  c  d, 
at  right  angles  to  one  another,  will  be  equal.  But  if  iron-filings  be  made 
into  a  mass  with  gum,  and  then  subjected  to  strong  compression  in  one 
direction,  and  from  the  pressed  mass  a  cube  be  cut,  this  cube,  placed  in 
the  magnetic  field,  is  no  longer  indifferent,  but  sets  with  its  line  of  great- 
est compression,  a  b  (Fig.  167,  -B),  axial;  the  attraction  along  tliis  line 
being  greater  than  along  any  other  line,  because  the  number  and  prox- 
imity of  the  particles  are  greater  along  this  line.  And  so  much  greater 
is  the  magnetic  attraction  along  this  line  than  along  any  other,  that  this 
diameter  may  be  cut  away  to  a  considerable  extent,  so  as  to  make  a 
short  bar,  and  still  the  line  a  b  will  maintain  its  axial  position  (Fig.  167, 
(7),  and  the  bar  will  seem  to  be  diamagnetic,  i.  e.,  its  long  diameter  will 
be  equatorial ;  not,  however,  because  its  ends  are  repelled,  but  because 
the  attraction  along  the  shorter  diameter  a  b  is  greater  than  along  the 
long  diameter.  If,  therefore,  the  cutting-down  of  the  diameter  a  b  be 
continued,  finally  the  influence  of  length  will  prevail  over  that  of  com- 

1  Philosophical  Magazine,  third  scries,  vol.  xxxvii.,  p.  1,  and  fourth  series,  vol.  ii., 
p.  165. 


CLEAVAGE  STRUCTURE. 


185 


pression,  and  the  bar  will  assume  its  true  axial  position  (Fig.  167,  D). 
Now,  Tyndall,  while  experimenting  upon  the  magnetic  properties  of 
various  bodies,1  found  that  a  short  bar  of  cleaved  slate,  with  its  longer 
diameter  in  the  plane  of  cleavage,  when  placed  in  the  magnetic  field, 
takes  the  equatorial  position ;  although,  if  the  bar  be  slender,  it  at  once 
shows  its  paramagnetism  by  assuming  the  axial  position.  In  other 
words,  cleaved  slate  behaves  exactly  as  if  it  was  a  paramagnetic  pow- 
der pressed  in  the  direction  at  right  angles  to  the  cleavage-planes. 

(/.)  Experimental  Proof. —  Finally,  experiments  by  Sorby  and  by 
Tyndall  show  that  clay  (the  basis  of  slates),  when  subjected  to  power- 
ful pressure,  exhibits  always  a  cleavage,  often  a  very  perfect  cleavage, 
at  right  angles  to  the  line  of  pressure. 

Physical  Theory. — Cleavage  is  certainly  produced  by  pressure,  but 
the  question  still  remains :  How  does  pressure  produce  planes  of  easy 
splitting  at  right  angles  to  its  own  direction  ?  What  is  the  physical 
explanation  of  cleavage  ? 

Sorby's  Theory.2 — Mr.  Sorby's  view  is  that  all  cleaved  rocks  con- 
sisted, at  the  time  when  this  structure  was  impressed  upon  it,  of  a  plastic 
mass,  with  unequiaxed  foreign  particles  disseminated  through  it;  and 
that  by  pressure  the  unequiaxed  particles  were  turned  so  as  to  bring 
their  long  diameters  in  a  direction  more  or  less  nearly  at  right  angles 
to  the  line  of  pressure,  and  thus  determined  planes  of  easy  fracture  in 
that  direction.  Usually,  as  in  slates,  the  plastic  material  is  clay,  and  the 
unequiaxed  particles  are  mica-scales.  Let  A,  Fig.  168,  repre- 
sent a  cube  of  clay  with  mica  disseminated.  If  such  a  cube 
be  dried  and  broken,  the  fracture  will  take  place  principally 
along  the  surfaces  of  the  mica,  which  may  therefore  be  seen 
glistening  on  the  uneven  surface  of  the  fracture ;  but  if  the 
cube,  while  still  plastic,  be  pressed  into  a  flattened  disk, 
then  the  scales  are  turned  with  their  long  diameters  in  the 
direction  of  extension  and  at  right  angles 
to  the  line  of  pressure,  as  in  B,  Fig.  168, 
and  the  planes  of  easy  fracture,  being 
still  determined  by  these  surfaces,  will 
be  in  that  direction. 

In  proof  of  this  view,  Mr.  Sorby  mixed 
clay  with  mica-scales  or  with  oxide-of-iron 
scales,  and,  upon  subjecting  the  mass  to 
powerful  compression  and  drying,  he  al- 
ways found  a  perfect  cleavage  at  right  angles  to  the  line  of  pressure. 
Furthermore,  by  microscopic  examination  he  found  that  both  in  the 
pressed  clay  and  in  the  cleaved  slates  the  mica-scales  lay  in  the  direc- 
tion of  the  cleavage-planes. 

1  Philosophical  Magazine,  4th  series,  vol.  v.,  p.  303.        2  Ib.,  2d  series,  vol.  xi.,  p.  20. 


FIG.  168.— Illustrating  Sorby's  Theory 
of  Slaty  Cleavage  (after  Sorby). 


186 


STRATIFIED   OR  SEDIMENTARY  ROCKS. 


FIG.  169.— Illustrating  Sorby's  Theory 
of  Slaty  Cleavage  (after  Sorby). 


Although  cleavage  is  most  perfect  in  slates,  yet  other  rocks  are 
sometimes  affected  with  this  structure.  In  a  specimen  of  cleaved  lime- 
stone, Sorby  found  under  the  microscope  unequiaxed  fragments  of 
broken  shells,  corals,  crinoid  stems,  etc.  (organic  particles),  ^ 

in  a  homogeneous  limestone-paste,  lying  with  their  long 
diameters   in  the  direction  of  cleavage.      Originally  the 
limestone  was  a  lime-mud  with  (he  supposes)  unequiaxed 
organic  particles  disseminated.     In  some  cases,  however, 
Sorby   recognized  the  very  important 
fact  that  the  organic  fragments,  which 
were  encrinal  joints,  had  been  flattened 
by  pressure — had  changed  their  form 
instead  of  their  position.     A.,  Fig.  169, 
gives  a  section  of  the  mass  in  the  sup- 
posed  original   condition,  and   B  the 
condition  after  pressure.     This  obser- 
vation contained  the  germ  of  the  theory 
proposed  by  Tyndall. 

TyndalTs  Theory.1 — Tyndall  was  led  to  reject  Sorby's  theory  by  the 
observation  that  cleavage  structure  was  not  confined  to  masses  contain- 
ing unequiaxed  particles  of  any  kind,  but,  on  the  contrary,  the  cleavage  is 
more  perfect  in  proportion  as  the  mass  is  free  from  all  such  particles. 
Clay,  deprived  of  the  last  trace  of  foreign  particles  by  the  sorting  power 
of  water,  when  pressed,  cleaved  in  the  most  perfect  manner.  Common 
beeswax,  flattened  by  powerful  pressure  between  two  plates  of  glass 
and  then  hardened  by  cold,  exhibits  a  most  beautiful  cleavage  structure. 
Almost  any  substance — curds,  white-lead  powder,  plumbago — subjected 
to  powerful  pressure,  exhibits  to  some  extent  a  similar  structure.  Tyn- 
dall explains  these  facts  thus :  Nearly  all  substances,  except  vitreous, 
have  a  granular  or  a  crystalline  structure,  i.  e.,  consist  entirely  of  dis- 
crete granules  or  crystals,  with  surfaces  of  easy  fracture  between  them. 
When  such  substances  are  broken,  the  fracture  takes  place  between  the 
crystals  or  granules,  producing  a  rough  crystalline  or  granular  surface, 
entirely  different  from  the  smooth  surface  of  vitreous  fracture.  Marble, 
cast-iron,  earthenware,  and  clay,  are  good  examples  of  crystalline  and 
granular  structure.  Now,  if  a  mass  thus  composed  yield  to  pressure, 
every  constituent  granule  is  flattened  into  a  scale,  and  the  structure  be- 
comes scaly /  and  as  the  surfaces  of  easy  fracture  will  still  be  between 
the  constituent  scales,  we  have  cleavage  at  right  angles  to  the  line  of 
pressure.  A  mass  of  iron,  just  taken  from  the  puddling-furnace  and 
cooled,  exhibits  a  granular  structure  ;  but  if  drawn  out  into  a  bar,  each 
granule  is  extended  into  a  thread,  and  the  structure  becomes  fibrous  ; 

1  Philosophical  Magazine,  2d  series,  vol.  xii.,  p.  35. 


CLEAVAGE  STRUCTURE.  187 

or  if  rolled  into  a  sheet,  each  granule  is  flattened  into  a  scale,  and  we 
have  a  cleavage  structure. 

There  can  be  little  doubt  that  this  is  the  true  explanation  of  slaty 
cleavage.  The  change  of  form  which,  as  we  have  seen,  has  taken  place 
in  the  fossil-shells,  encrinal  joints,  and  rounded  nodules,  has  affected 
every  constituent  granule  of  the  original  earthy  mass,  so  that  the  struct- 
ure becomes  essentially  scaly  instead  of  granular ;  the  cleavage  being 
between  the  constituent  scales.  Sorby,  it  is  true,  in  his  observations 
on  cleaved  limestones,  recognized  the  true  cause  of  cleavage,  viz.,  the 
change  of  form  of  discrete  particles  ;  but  he  regarded  this  as  subordi- 
nate to  change  of  position.  Besides,  the  particles  of  Sorby  were  for- 
eign, which  Tyndall  has  shown  to  be  unnecessary ;  while  the  particles 
of  Tyndall  are  constituent. 

Geological  Application. — It  may  be  considered,  therefore,  as  certain 
that  cleaved  slates  have  assumed  their  peculiar  structure  under  the  in- 
fluence of  powerful  pressure  at  right  angles  to  the  cleavage-planes,  by 
which  the  whole  squeezed  mass  is  mashed  together  in  one  direction  and 
extended  in  another.  Taking  any  ideal  sphere  in  the  original  unsqueezed 
mass  :  after  mashing  the  diameter  in  the  line  of  pressure  has  been  short- 
ened, the  diameter  in  the  line  of  cleavage-^//?  has  been  correspondingly 
extended,  and  the  diameter  in  the  line  of  cleavage-strike  unaffected,  since 
extension  of  this  diameter  in  any  place  must  be  compensated  by  short- 
ening in  a  contiguous  place  right  or  left ;  so  that  the  original  sphere  has 
been  converted  into  a  greatly-flattened  ellipsoid  of  three  unequal  diame- 
ters. The  amount  of  compression  and  extension  may  be  estimated  in  the 
case  a  by  the  amount  of  distortion  of  shells  of  known  form  (Figs.  162 
and  163) ;  in  the  case  c  by  a  comparison  of  the  transverse  diameter  with 
the  length  of  the  folded  line/V?  (Fig.  165) ;  in  the  case  d  by  the  relation 
between  the  diameters  of  the  elliptic  spots.  By  these  means,  but  prin- 
cipally by  the  first,  Haughton  *  has  estimated  that  the  original  sphere  has 
been  changed  into  an  ellipsoid,  whose  greatest  and  shortest  diameters 
are  to  each  other,  in  some  cases,  as  2  :  1,  in  others  as  3  :  1,4:  1,  6  or 
7  :  1,9:1,  and  in  some  even  11 :  1.  The  average  in  well-cleaved  slates, 
according  to  Sorby,  is  about  6  : 1.  Now,  since  this  ratio  is  the  result 
partly  of  compression  and  partly  of  extension,  it  is  evident  that  either 
the  compression  alone  or  the  extension  alone  would  be  the  square  roots 
of  these  ratios.  Therefore,  we  may  assume  the  average  compression 
as  2| :  1,  and  the  average  extension  as  1  :  2|. 

It  is  impossible  to  over-estimate  the  geological  importance  of  these 
facts.  Whole  mountains  of  strata,  whole  regions  of  the  earth's  crust, 
are  cleaved  to  great  and  unknown  depths,  showing  that  the  crust  has 
been  subjected  to  an  almost  inconceivable  force,  squeezing  it  together 
in  an  horizontal  direction  and  swelling  it  upward.  This  upward  swell- 
1  Philosophical  Magazine,  fourth  series,  vol.  xii.,  p.  409. 


188 


STRATIFIED   OR  SEDIMENTARY  ROCKS. 


ing,  or  thickening  of  the  strata  by  lateral  squeezing,  is  a  probable  cause 
of  gradual  elevation  of  the  earth's  crust,  which  has  not  been  noticed  by 
geologists.  We  will  speak  again  of  this  important  subject  in  our  dis- 
cussion of  mountain-formation. 

There  are  reasons  for  believing  that  the  squeezing  did  not  take 
place,  and  the  structure  was  not  formed,  while  the  strata  were  in  their 
original  condition  of  plastic  sediment,  but  after  they  had  been  consoli- 
dated into  rock  and  the  contained  fossils  had  been  completely  petrified, 
otherwise  the  shells  must  have  been  broken  by  the  pressure.  Yet,  on 
the  other  hand,  some  degree  of  plasticity  seems  absolutely  necessary  to 
account  for  so  great  a  compression  in  one  direction  and  extension  in 
another  without  disintegration  of  the  mass.  It  seems  most  probable 
that  at  the  time  the  structure  was  produced  these  rocks  were  deeply 
buried  beneath  other  rocks  and  in  a  somewhat  plastic  state,  through 
the  influence  of  heat  in  the  presence  of  water.  Afterward,  they  were 
exposed  by  erosion. 

Nodular  or  Concretionary  Structure. 

In  many  stratified  rocks  are  found  nodules  of  various  forms  scattered 
through  the  mass  or  in  layers  parallel  to  the  planes  of  stratification. 
Like  slaty  cleavage,  this  structure  is  the  result  of  internal  changes  sub- 
sequent to  the  sedimentation ;  for  the  planes  of  stratification  often  pass 
directly  through  the  nodules  (Figs.  170  and  171).  The  flint  nodules  of 


FIG.  170.  FIG.  171. 

the  chalk,  and  the  clay  iron-stone  nodules  of  the  coal  strata  and  hy- 
draulic lime-balls,  common  in  many  clays,  are  familiar  illustrations  of 
this  structure. 

Cause. — Nodular  concretions  seem  to  occur  whenever  any  substance 
is  diffused  in  small  quantities  through  a  mass  of  entirely  different  mate- 
rial. Thus,  if  strata  of  sandstone  or  clay  have  small  quantities  of  car- 
bonate of  lime  or  carbonate  of  iron  diffused  through  them,  the  diffused 
particles  of  lime  or  iron  will  gradually,  by  a  process  little  understood, 
segregate  themselves  into  more  or  less  spherical  or  nodular  masses,  in 
some  cases  almost  pure,  but  generally  inclosing  a  considerable  quantity 
of  the  material  of  the  strata.  In  this  manner  lime-balls  and  iron-ore 
balls  and  nodules,  so  common  in  sandstones  and  clays,  are  formed.  In 
like  manner,  the  flint  nodules  of  the  chalk  were  formed  by  the  segre- 
gation of  silica,  originally  diffused  in  small  quantities  through  the  chalk- 


NODULAR  OR   CONCRETIONARY  STRUCTURE. 


189 


sediment.  Very  often  some  foreign  substance  forms  the  nucleus  about 
which  the  segregation  commences.  On  breaking  a  nodule  open,  a  shell 
or  some  other  organism  is  often  found 
beautifully  preserved.  These  nodules, 
therefore,  are  a  fruitful  source  of  beau- 
tiful fossils.  In  most  cases,  probably 
in  all  cases,  the  segregating  substance 
must  have  been  to  some  extent  soluble 
in  water  pervading,  or  suspensible  in 
water  percolating,  the  stratum.  Some- 
times the  nodules  run  together,  form- 
ing a  more  or  less  continuous  stratum. 
In  such  cases,  the  segregating  material 
is  more  impure. 

Forms  of  Nodules. — The  typical  and  most  common  form  is  globular. 
This  is  well  seen  in  lime-balls  and  iron-balls.  Sometimes  these  balls 
are  solid,  sometimes  they  have  irregular  cracks  in  the  centre  (Fig.  172), 
sometimes  they  have  a  radiated  structure  (Fig.  173),  sometimes  they 
are  hollow  like  a  shell  (this  is  common  in  iron-balls).  They  vary  in  size 
from  that  of  a  pea  to  six  and  eight  feet  in  diameter.  Often,  however, 
instead  of  the  spherical  form,  they  take  on  various  and  strange  and 


FIG.  173.— Dolomite  containing  Concretions,  Sunderland  (after  Jukes). 

fantastic  shapes  (Fig.  174),  sometimes  like  a  dumb-bell,  sometimes  a 
flattened  disk,  sometimes  a  ring,  sometimes  a  flattened  ellipsoid,  regu- 
larly seamed  on  the  surface  like  the  shell  of  a  turtle  (turtle-stones). 
They  are  often  mistaken  by  unscientific  observers  for  fossils. 

Kinds  of  Nodules  found  in  Different  Strata. — In  sandstone  strata  the 

nodules  are  commonly  carbonate  of  lime  or  oxide  of  iron  (lime  or  iron 
balls).  In  clay  strata  they  are  carbonate  of  lime  or  carbonate  of  iron 
(clay  iron-stone  of  coal  strata),  or  a  mixture  of  these  (Roman  cement 
nodules  of  the  London  clay). 


190 


STRATIFIED   OR  SEDIMENTARY  ROCKS. 


In  limestone  the  nodules  are  always  silica,  and  conversely  silica 
nodules  are  peculiar  to  limestone.  The  flint  nodules  of  the  chalk  are 
remarkable  for  being  arranged  in  planes  parallel  to  the  planes  of  strati- 


FIG.  174.— Limestone  Strata  containing  Concretions. 

fioation  (Fig.  175).     Sometimes  the  siliceous  matter  segregates  in  con- 
tinuous strata  of  siliceous  limestone  (Fig.  176). 

In  the  cases  thus  far  spoken  of,  the  nodules  are  scattered  through 
the  mass  of  the  strata  or  arranged  in  planes  parallel  to  planes  of  stra- 


FIG.  175.— Chalk-Cliffs  with  Flint  Nodules. 

tification.  But  in  some  cases  the  whole  mass  of  the  rock  assumes  a  con- 
cretionary or  concentric  structure  (Fig.  177).  The  cause  of  this  is  still 
more  difficult  to  explain. 

FOSSILS  :  THEIR  ORIGIN  AND  DISTRIBUTION. 

Stratified  rocks,  as  we  have  already  seen,  are  sediments  accumulated 
in  ancient  seas,  lakes,  deltas,  etc.,  and  consolidated  by  time.  As  now, 
so  then,  dead  shells  were  imbedded  in  shore-deposits ;  leaves  and  logs  of 


FOSSILS:   THEIR  ORIGIN  AND   DISTRIBUTION. 


191 


high  land-plants,  and  bones  of  land-animals,  were  drifted  into  swamps 
and  deltas  and  buried  in  mud ;  and  tracks  were  formed  on  flat,  muddy 
shores  by  animals  walking 
on  them.  These  have  been 
preserved  with  more  or  less 
change,  and  are  even  now 
found  in  great  numbers  in- 
closed in  stratified  rocks. 
They  are  called  fossils.  A 
fossil,  therefore,  is  any  evi- 
dence of  the  former  existence 
of  a  living  being.  Fossils 
are  the  remains  of  the  fauna 
and  flora  of  previous  geolog- 
ical epochs.  Their  presence 
is  the  most  constant  charac- 
teristic of  stratified  rocks. 

The  Degrees  of  Preservation  are  very  various. — Sometimes  only  the 

tracks  of  animals,  or  impressions  of  leaves  of  plants,  are  preserved. 
More  commonly  the  bones  or  shells,  or  other  hard  parts  of  animals,  are 


FIG.  176.-Chalk-Cliffs. 


FIG.  177.— Coal-measure  Shale,  weathering  into  Spheroids. 

preserved  with  various  degrees  of  change.  Sometimes  even  the  soft 
and  more  perishable  tissues  are  preserved.  We  will  treat  of  these 
degrees  under  three  principal  heads  : 

1.  Decomposition  prevented  and  the  Organic  Matter  more  or  less 
completely  preserved. — Cases  of  this  kind  are  usually  found  in  compar- 
atively recent  strata,  and  imbedded  either  in  frozen  soils,  or  in  peat,  or 
in  stiff  clays  ;  although  some  cases  of  partial  preservation  of  the  or- 


192  STRATIFIED   OR  SEDIMENTARY  ROCKS. 

ganic  matter  are  found  even  in  old  rocks.  Extinct  elephants  have  been 
found  frozen  in  the  river-bluffs  of  Siberia  so  perfectly  preserved  that 
dogs  and  wolves  ate  their  flesh.  Skeletons  of  men  and  animals  are 
found  in  peat-bogs  and  stiff  clays  of  a  comparatively  recent  formation, 
the  organic  matter  of  which  is  still  preserved.  In  clays  of  the  Tertiary 
period  the  imbedded  shells  still  retain  the  epidermis,  and  even  in  the 
Lias  (mesozoic)  shells  are  found  retaining  the  nacreous  lustre.  Coal  is 
vegetable  matter  changed  but  not  destroyed.  It  is  found  in  almost 
every  formation,  even  down  to  the  oldest.  Every  degree  of  change  may 
be  traced  in  different  specimens  of  fossil  wood,  between  perfect  wood 
and  perfect  coaL 

2.  Petrifaction:  Organic  Form  and  Structure  preserved. — In  t::e 
last  case  the  organic  matter  is  more  or  less  preserved.  In  the  case 
now  to  be  described  the  organic  matter  is  entirely  gone ;  but  the  or- 
ganic form  and  the  organic  structure  are  preserved  in  mineral  matter. 
This  is  what  is  usually  called  petrifaction  or  mineralization.  The  best 
example  of  this  is  petrified  wood.  In  a  good  specimen  of  petrified 
wood,  not  only  the  external  form  of  the  trunk,  not  only  the  general 
structure  of  the  stem — viz.,  pith,  wood,  and  bark — not  only  the  radiating 
silver-grain  and  the  concentric  rings  of  growth,  are  discernible,  but 
even  the  microscopic  cellular  structure  of  the  wood,  and  the  exquisite 
sculpturings  of  the  cell-walls  themselves,  are  perfectly  preserved,  so 
that  the  kind  of  wood  may  often  be  determined  by  the  microscope  with 
the  utmost  certainty.  Yet  not  one  particle  of  the  organic  matter  of 
the  wood  remains.  It  has  been  entirely  replaced  by  mineral  matter ; 
usually  by  some  form  of  silica.  The  same  is  true  of  shells  and  bones 
of  animals ;  but  as  shells  and  bones  consist  naturally  partly  of  organic 
and  partly  of  mineral  matter,  very  often  it  is  only  the  organic  matter 
which  is  replaced,  although  sometimes  the  original  mineral  matter  is 
also  replaced  by  silica  or  other  mineral  substance.  The  radiating 
structure  of  corals  or  the  microscopic  structure  of  teeth,  bones,  and 
shells,  is  often  beautifully  preserved.  This  kind  of  preservation  for 
shells  and  corals  is  most  common  in  limestones  and  clays ;  for  wood,  in 
gravels. 

Theory  of  Petrifaction. — If  wood  be  soaked  in  a  strong  solution  of 
sulphate  of  iron  (copperas)  and  dried,  and  the  same  process  be  re- 
peated until  the  wood  is  highly  charged  with  this  salt,  and  then 
burned,  the  structure  of  the  wood  will  be  preserved  in  the  peroxide  of 
iron  left.  Also,  it  is  well  known  that  the  smallest  fissures  and  cavities 
in  rocks  are  speedily  filled  by  infiltrating  waters  with  mineral  matters. 
Now,  wood  buried  in  soil  soaked  with  some  petrifying  material  becomes 
highly  charged  with  the  same,  and  the  cells  filled  with  infiltrated  mat- 
ter, and  when  the  wood  decays  the  petrifying  material  is  left,  retaining 
the  structure  of  the  wood.  But  this  is  not  all,  for  in  Nature  there  is 


FOSSILS:    THEIR   ORIGIN  AND   DISTRIBUTION.  ^93 

an  additional  process,  not  illustrated  either  by  the  experiment  or  by 
the  example  of  infiltrated  fillings.  As  each  particle  of  organic  matter 
passes  away  by  decay,  a  particle  of  mineral  matter  takes  its  place,  until 
finally  the  whole  of  the  organic  matter  is  replaced.  Petrifaction,  there- 
fore, is  a  process  of  substitution,  as  well  as  interstitial  filling.  Now, 
it  so  happens,  probably  from  the  different  nature  of  the  process  in  the 
two  cases,  that  the  interstitial  filling  always  differs,  either  in  chemical 
composition  or  in  color,  from  the  substituting  mate- 
rial. Thus  the  structure  is  still  visible,  though  the 
mass  is  solid.  If  Fig.  178  represent  a  cross-section 
of  three  petrified  wood-cells,  the  matter  filling  the  cells 
(b)  is  always  different  from  the  matter  forming  the 
cell-wall  (a). 

The  most  common  petrifying  materials  are  silica, 

carbonate  of  lime,  and  sulphide  of  iron  (pyrites).  In  the  case  of  petri- 
faction by  pyrites  the  process  is  quite  intelligible,  but  the  structure  is 
usually  very  imperfectly  preserved.  If  water  containing  sulphate  of 
iron  (FeSOJ  come  in  contact  with  decaying  organic  matter,  the  salt  is 
deoxidized  by  the  organic  matter,  the  latter  passing  off  as  carbonic 
acid  and  water,  and  the  former  becomes  insoluble  sulphide  (FeS),  and 
is  deposited.  Now,  as  each  particle  of  organic  matter  passes  away  as 
CO,  and  H2O,  the  molecule  of  iron  sulphate  which  effected  the  change 
is  itself  changed  into  insoluble  sulphide,  and  takes  its  place. 

The  process  of  replacement  by  silica  (silicification)  is  less  clear,  but 
it  is  probably  as  follows :  Silica  is  found  in  solution  in  many  waters, 
being  held  in  this  condition  by  small  quantities  of  alkali  present  in  the 
waters.  In  contact  with  decomposing  wood  the  alkali  is  neutralized 
by  the  humic,  ulmic,  and  other  acids  of  decomposition,  and  the  silica 
therefore  deposited. 

3.  Organic  Form  only  preserved. — In  the  third  case  organic  mat- 
ter and  organic  structure  are  both  lost,  and  only  organic  form  is  pre- 


served. This  kind  of  fossilization  is  most  commonly  seen  in  shells.  It 
may  be  subdivided  into  four  subordinate  cases,  represented  in  section 
by  «,  b,  c,  and  d  of  Fig.  179.  In  this  figure  the  horizontal  lines  represent 
the  original  sediment  which  may  or  may  not  have  consolidated  into 
rock ;  the  vertical  lines  represent  a  subsequent  filling  of  different  and 
usually  finer  material.  In  a  we  have  a  mould  of  the  external  form  of 
13 


194 


STRATIFIED   OR  SEDIMENTARY  ROCKS. 


the  shell  preserved  in  sediment.  The  shell  with  the  undecayed  animal 
was  imbedded,  and  afterward  entirely  dissolved  away,  leaving  only  the 
hollow  mould.  In  b  the  same  process  has  taken  place,  only  the  mould 
has  been  subsequently  filled  by  infiltration  of  slightly  soluble  matters. 
In  this  case  we  have  both  the  mould  and  the  cast  of  the  external  form  ; 
the  mould  being  formed  of  sediment,  and  the  cast  of  infiltrated  matter. 
These  are  always  of  different  materials,  i.  e.,  different  either  in  chemical 
composition  or  in  state  of  aggregation.  In  c  we  have  a  mould  of  the 
external  form  in  sediment,  and  a  cast  of  the  internal  form  in  the  same 
material,  with  an  empty  space  between,  having  the  exact  form  and 
thickness  of  the  shell.  In  this  case,  the  already  dead  and  empty  shell 


FIG.  180.— o,  Cast  of  interior;  6,  natural  form. 


Fro.  181. — a,  Natural  form  ;  &,  cast 
of  interior  and  mould  of  exterior. 


FIG.  182. — Trigonia  Longa,  showing  cast  (a)  of  the 
exterior  and  (6)  of  the  interior  of  the  shell. 


was  imbedded  in  sediment,  which  also  filled  its  interior  ;  afterward  the 
shell  was  removed,  leaving  an  empty  space.  In  d  this  empty  space 
was  subsequently  filled  by  infiltration.  In  shore  and  river  deposits  of 
the  present  day  it  is  very  common. to  find  shells  imbedded  in,  and  filled 
with,  sand  or  mud.  In  the  more  recent  tertiary  rocks  shells  are  com- 
monly found  in  the  same  condition  precisely ;  but  in  the  older  rocks 
more  commonly  the  original  shell  is  removed,  and  the  space  either  left 
empty  or  filled  by  infiltration.  Cases  c  and  d  are  well  represented  by 
Figs.  180,  181,  and  182.  Cases  like  a  and  c  are  most  commonly  found 
in  porous  rocks  like  sandstone ;  b  and  c?,  especially  the  latter,  are  found 


DISTRIBUTION  OF  FOSSILS  IN  THE  STRATA.  195 

in  all  kinds  of  rocks.  By  far  the  most  common  infiltration  fillings  are 
carbonate  of  lime  and  silica. 

Often  we  find  impressions  of  the  forms  of  small  portions  only  of  the 
original  organism,  as  of  the  leaves  of  trees,  or  the  feet  of  animals  walk- 
ing on  the  soft  mud  of  the  flat  shores  of  ancient  bays.  Such  tracks 
were  afterward  covered  up  with  river  or  tidal  deposit,  and  thus  pre- 
served. On  cleaving  the  rock  along  the  lamination-planes  we  have  on 
one  side  a  mould  and  on  the  other  the  cast  of  the  foot. 

Between  cases  1  and  2  every  stage  of  gradation  may  be  traced. 
The  amount  of  change,  as  a  general  fact,  varies  with  the  age  of  the 
rock ;  but  is  still  more  dependent  on  the  kind  of  rock  and  the  degree 
of  metamorphism  (p.  213).  In  an  impermeable  rock,  like  clay,  the 
changes  are  much  more  slow  than  in  a  porous  rock,  like  sandstone. 

• 
Distribution  of  Fossils  in  the  Strata. 

The  nature  of  the  fossil  species  found  in  rocks  is  determined  partly 
by  the  kind  of  rock,  partly  by  the  country  where  the  rock  is  found,  and 
partly  by  the  age  of  the  rock. 

1.  Kind   of  Rock. — It  has  been  already  stated  (p.  162)  that  the 
species  of  lower  marine  animals  vary  with  the  depth.     They  also  vary 
with  the  kind  of  bottom.     Thus,  along  shore-lines  and  on  sand-bottom 
the  species  differ  from  those  in  deep  water  and  on  mud-bottom.    Shells 
are  found  mostly  along  shore-lines,  corals  in  opener  seas,  and  foramini- 
fera  in  deep  seas.     The  same  was  true  in  every  previous  epoch.     We 
might  expect,  therefore,  and  do  find,  that  the  lower  marine  fossils  of 
sandstones,  shales,  and  limestones,  differ  even  when  these  strata  belong 
to  the  same  country  and  geological  epoch.    The  higher  marine  animals, 
such  as  fishes,  cuttle-fish,  etc.,  swimming  freely  in  the  sea,  are  more 
independent  of  bottoms,  and  we  find  their  skeletons  and  shells  equally 
in  all  kinds  of  strata.     Land  animals  perish  on  land,  and  their  skeletons 
are  drifted  into  bays,  river-deltas,  and  lakes,  and  buried  there  mostly  in 
fresh-water  or  brackish-w.ater  deposits  of  sand  and  clay.     It  is,  there- 
fore, in  such  strata  that  their  remains  are  commonly  found. 

2.  The  Country  where  found.— We  have  already  seen  (p.  155)  that 
the  fauna  and  flora  of  different  countries  at  the  present  time  differ  as 
to  species,  and  often  as  to  genera  and  families ;  the  difference  being 
generally  in  proportion  to  the  difference  in  .climate  and  the  physical 
barriers  intervening.     The  same  was  true  of  the  faunas  and  floras  of 
previous  epochs,  and  therefore  of  the  fossils  of  the  same  age  in  differ- 
ent countries.    The  fossil  species  of  the  same  epoch  in  America,  and  in 
Europe  and  in  Asia,  are  not  usually  identical,  although  there  may  be  a 
general  resemblance.     The  geographical  diversity,  however,  is  small  in 
the  lowest  and  oldest  rocks,  and  becomes  greater  and  greater  as  we 


196  STRATIFIED  OR  SEDIMENTARY  ROCKS. 

pass  upward  into  newer  and  newer  rocks,  and  is  greatest  in  the  fauna 
and  flora  of  the  present  day. 

3.  The  Age.— This  introduces  the  subject  of  the  laws  of  distri- 
bution of  organisms  in  time,  or  of  fossils  vertically  in  the  series  of 
stratified  rocks.  The  subject  will  be  more  fully  treated  in  Part  III., 
of  which  it  constitutes  the  principal  portion.  We  now  bring  out  only 
so  much  as  is  necessary  as  a  basis  of  classification  of  stratified  rocks. 

(a.)  Geological  Fauna  and  Flora. — As  we  pass  from  the  oldest 
and  lowest  rocks  upward  to  the  newest  and  highest,  we  find  that  all  the 
species,  most  of  the  genera,  and  many  of  the  families,  change  many  times. 
Now,  all  the  species  of  animals  and  plants  inhabiting  the  earth  at  one 
time  constitute  the  fauna  and  flora  of  that  geological  time.  Geological 
faunae,  therefore,  have  changed  many  times.  In  a  conformable  series 
of  rocks  the  change  from  one  fossil  fauna  or  flora  to  another  succeeding 
is  always  gradual,  the  species  of  the  later  fauna  or  flora  gradually 
replacing  those  of  the  earlier.  But  between  two  series  of  unconform- 
able  strata  the  change  is  sudden  and  complete — as  if  one  fauna  and 
flora  had  been  suddenly  destroyed  and  another  introduced.  It  must  be 
remembered,  however,  that  unconformity  always  indicates  a  great 
lapse  of  time  unrepresented  at  the  place  of  observation  by  strata  or 
fossils.  It  is  therefore  probable  that  the  apparent  suddenness  of  the 
change  is  only  the  result  of  our  ignorance  of  the  fauna  and  flora  of  the 
period  unrepresented.  Nevertheless,  as  unconformity  always  indi- 
cates changes  of  physical  geography,  and  therefore  of  climate,  it  is 
probable  that  in  the  history  of  the  earth  there  were  periods  of  great 
changes,  marked  by  unconformity  of  strata,  during  which  changes  of 
species  were  more  rapid,  separated  by  periods  of  comparative  quiet, 
marked  by  conformity,  during  which  the  species  were  either  un- 
changed, or  changed  slowly.  Such  a  period  is  called  a  geological 
period  or  geological  epoch,  and  the  rocks  formed  during  a  geological 
period,  or  epoch,  is  called  &  formation. 

There  are,  therefore,  two  tests  of  a  formation  and  a  corresponding 
geological  period,  viz.,  1.  Conformity  of  the  strata,  or  rock-system, 
and,  2.  General  similarity  of  fossils,  or  life-system  /  and  two  modes 
of  separating  formations  and  corresponding  periods,  viz.,  unconformity 
of  the  rock-system,  and  great  and  sudden  change  of  the  life-system. 
A  geological  formation,  therefore,  may  be  defined  as  a  group  of  con- 
formable rocks  containing  similar  fossils,  usually  separated  from  other 
similar  groups  containing  different  fossils  by  unconformity.  A  geo- 
logical period  may  be  defined  as  a  period  of  comparative  quiet,  during 
which  the  physical  geography,  climate,  and  fauna  and  flora,  were  sub- 
stantially the  same,  usually  separated  from  other  similar  periods  by 
changes  of  physical  geography  and  climate,  which  resulted  in  changes 
of  fauna  and  flora.  Of  these  two  tests,  however,  the  life-sj^stem  is 


CLASSIFICATION  OF  STRATIFIED   ROCKS.  19^ 

usually  considered  the  most  important,  and  in  case  of  disagreement 
must  control  classification. 

(b.)  Geological  Faunce  and  Florae  differ  more  than  Geographical 
Faunae  and  Florae. — If  there  were  no  geographical  diversity,  species 
of  the  same  age  would  be  identical  all  over  the  earth,  and  therefore  it 
would  be  easy  to  determine  strata  of  the  same  age  (geological  horizon). 
On  the  other  hand,  if  geographical  diversity  in  any  age  were  as  great 
as  the  diversity  between  two  successive  ages,  then  it  would  seem  im- 
possible to  establish  a  geological  horizon.  But  this  law  states  that  the 
difference  between  two  successive  faunas  is  greater  than  between  two 
contiguous  faunae.  In  other  words,  the  species  of  successive  periods, 
or  fossils  of  successive  formations,  differ  from  each  other  more  than 
species  of  the  same  period  or  fossils  of  the  same  formation  in  different 
parts  of  the  earth.  There  is  a  general  similarity  in  the  species  of  the 
same  period  all  over  the  surface  of  the  earth.  Kence  by  comparison  of 
fossils  it  is  possible  to  determine  what  strata,  in  different  portions  of 
the  earth,  belong  to  the  same  period  (to  synchronize  strata).  The  strata 
all  over  the  earth,  which  were  formed  at  the  same  time,  are  said  to  be- 
long to  the  same  geological  horizon.  Strata  of  the  same  horizon  are 
determinate  by  similarity  of  fossils  with  considerable  certainty,  until 
we  come  up  to  the  tertiary  rocks.  In  all  the  newer  rocks,  however,  the 
geographical  diversity  is  so  great  as  to  interfere  seriously  with  the 
ability  to  synchronize  by  means  of  comparison  of  fossils.  Another 
method,  therefore,  is  used  for  these  higher  rocks. 

(c.)  By  examining  and  comparing  fossils  from  the  lowest  to  the 
highest  rocks,  it  has  been  observed  that  there  is  a  steady  approach  of 
the  fossil  faunas  and  florae  to  the  present  faunae  and  florae,  first  in  the 
families,  then  in  the  genera,  and  finally  in  the  species.  The  species  of 
fossil  molluscous  shells  begin  to  be  identical  with  molluscous  species  of 
the  present  day  only  in  the  tertiary  rocks,  and  the  proportion  of  iden- 
tical species  steadily  increases  as  we  pass  upward.  Thus  in  the  newer 
rocks,  just  where  the  other  method  (comparison  of  fossil  faunas  with  one 
another)  begins  to  fail,  we  mav  svnchronize  strata  of  different  localities, 
by  comparing  their  shell  fauna  with  the  shell  fauna  of  the  present  day, 
in  the  same  localities.  Those  are  said  to  be  of  the  same  age  which 
contain  the  same  percentage  of  shells  identical  with  those  of  the  present 
day. 

SECTION  2. — CLASSIFICATION  OF  STRATIFIED  ROCKS. 

Geology  is  essentially  a  history.  Stratified  rocks  are  the  leaves  on 
which  this  history  is  recorded.  The  fundamental  idea  of  every  clas- 
sification is  therefore  relative  age.  The  object  to  be  attained  in  clas- 
sification is,  first,  to  arrange  all  rocks  in  chronological  order,  so  that 
the  history  may  be  read  as  it  was  written  ;  and  then,  second,  to  collect 


198  STRATIFIED   OR  SEDIMENTARY  ROCKS. 

them  into  larger  and  smaller  groups,  called  systems,  series,  formations, 
corresponding  to  the  great  eras,  periods,  epochs,  of  the  earth's  history. 
There  are  several  different  methods  of  determining  the  relative  age  of 
rocks : 

1.  Order  of  Superposition. — It  is  evident,  from  the  manner  in  which 
stratified  rocks  are  formed — viz.,  by  sedimentation — that  their  original 
position  indicates,  with  absolute  certainty,  their  relative  age,  the  lower 
being  older  than  the  higher.     If,  therefore,  the  original  position  of  any 
series  of  strata  be  retained  or  not  very  greatly  disturbed,  and  we  have 
a  good  section,  the  relative  age  of  the  strata  which  compose  the  series 
may  be  easily  determined.     But  the  strata,  as  we  have  already  seen, 
have  in  many  cases  been  crushed  and  contorted  and  folded  in  the  most 
intricate  manner,  sometimes  even  turned  over ;  have  been  broken  and 
slipped,  and  large  masses  carried  away  by  erosion,  and  often  so  changed 
by  heat  and  other  agents,  that  their  stratification  is  nearly  or  quite 
obliterated.     For  these  reasons  it  is  often  very  difficult  to  determine 
the  relative  position,  and  thus  to  construct  an  ideal  section  of  the 
strata  of  a  series  of  rocks,  even  in  a  single  locality.     Nevertheless,  in 
spite  of  all  these  difficulties,  the  method  of  superposition  is  conclusive, 
and  takes  precedence  of  all  others  whenever  it  can  be  applied.     In 
spite  of  all  these  difficulties,  if  the  whole  geological  series  were  present 
in  any  one  locality,  it  would  be  comparatively  easy  to  construct  the 
geological  chronology. 

But  a  series  of  rocks  in  any  one  locality  cannot  give  us  the  whole 
history  of  the  earth.  Since  sedimentation  only  takes  place  at  the  bot- 
tom of  water,  those  places  which  were  land-surfaces  during  any  geo- 
logical epoch  received  no  deposit,  and  therefore  the  strata  representing 
that  epoch  must  be  wanting  there.  Now,  as  there  have  been  frequent 
oscillations  of  land-surfaces  and  sea-bottoms  in  past  times,  similar  to 
those  taking  place  at  the  present  time,  we  find  that  in  every  known 
local  series  of  strata  there  exist  many  and  great  gaps,  so  many  and  so 
great  that  the  record  may  be  regarded  as  only  fragmentary.  Such  gaps 
are  usually  indicated  by  unconformity.  It  is  the  task  of  the  geolo- 
gist, by  extensive  comparison  of  rocks  in  all  countries,  to  fill  up  these 
gaps,  and  make  a  continuous  series.  The  leaves  of  the  book  of  Time 
are  scattered  hither  and  thither  over  the  surface  of  the  earth,  and  it  is 
the  duty  of  the  geologist  to  gather  and  arrange  them  according  to  their 
paging.  This  is  done  by  comparison  of  rocks  of  different  localities, 
partly  by  their  lithological  character,  but  principally  by  the  fossils  which 
they  contain. 

2.  Lithological  Character. — At  the  present  time,  in  our  seas  and 
lakes,  deposits  are  forming  composed  of  sand,  clav,  mud,  and  lime,  of  ev- 
ery kind,  in  different  localities.     The  same  has  taken  place  in  previous 
epochs.     Sandstones,  limestones,  and  slates,  not  differing  greatly  from 


CLASSIFICATION  OF  STRATIFIED  ROCKS.  199 

those  forming  at  the  present  time,  except  in  degree  of  consolidation, 
have  been  formed  in  every  geological  period.  Lithological  charac- 
ter, therefore,  is  no  test  of  age.  In  comparing  rocks  of  widely-sepa- 
rated localities,  as,  for  example,  the  rocks  of  different  continents,  dif- 
ference of  lithological  character  is  no  evidence  of  difference  of  age,  nor 
similarity  of  lithological  character  of  any  value  in  determining  a  geo- 
logical horizon.  But,  as  deposits  are  now  being  formed  of  a  similar  char- 
acter over  considerable  areas,  so  also  we  find  strata  (the  deposits  of  pre- 
vious epochs),  continuous  and  unchanged  in  lithological  character,  over 
large  tracts  of  country.  Therefore,  in  contiguous  localities,  similarity 
of  lithological  character  becomes  a  very  valuable  means  of  identifying 
strata.  If,  in  two  localities  not  too  widely  separated,  we  find  a  similar 
rock,  e.  g.,  a  sandstone  of  similar  grain  and  color,  we  conclude  that 
they  probably  belong  to  the  same  age,  or  are,  in  fact,  the  same  stratum. 

3.  Comparison  of  Fossils. — This  is  by  far  the  best,  and  in  widely- 
separated  localities  the  only,  method  of  determining  the  age  of  rocks. 
The  principle  of  this  method  is  that  every  geological  epoch  has  its  own 
fauna  and  flora  by  which  it  may  be  identified  everywhere  in  spite  of 
those  slight  differences  which  result  from  geographical  diversity ;  and, 
therefore,  similarity  of  fossils  shows  similarity  of  age.  There  are,  how- 
ever, certain  limitations  to  the  application  of  this  method  which  must 
be  borne  in  mind  : 

(a.)  The  lower  marine  species  are  much  affected  by  depths  and  bot- 
toms, and  therefore  we  should  expect  that  sandstone  fossils,  limestone 
fossils,  and  slate  fossils,  would  differ  in  species  even  in  the  same  epoch. 
Again,  in  lake  and  delta  deposits,  the  entombed  species  would  prob- 
ably be  entirely  different  from  those  of  marine  deposits.  We  must  be 
careful,  therefore,  to  compare  fossils  of  rocks  formed  under  similar  con- 
ditions. 

(b.)  We  must  also  make  due  allowance  for  geographical  diversity. 
This,  as  we  have  already  stated,  becomes  greater  and  greater  as  we 
pass  up  the  series  of  rocks.  In  the  lower  or  older  rocks  the  geographi- 
cal diversity  is  small ;  in  strata  of  the  same  age  in  different  countries 
the  fossils  are  quite  similar,  most  of  the  genera  and  many  of  the  species 
being  undistinguishable.  It  is  therefore  comparatively  easy,  by  com- 
parison of  fossils,  to  synchronize  the  strata  and  determine  the  geological 
horizon.  In  the  middle  rocks  the  geographical  diversity  is  greater,  but 
the  general  similarity  is  still  considerable — the  difference  between  or- 
ganisms of  consecutive  epochs  (geological  faunae  and  florae)  is  still  much 
greater  than  the  difference  between  organisms  of  the  same  epoch  in  dif- 
ferent countries  (geographical  faunas  and  floras) ;  and,  therefore,  it  is 
still  quite  possible,  by  comparison  of  fossils,  to  synchronize  the  strata. 
In  the  higher  or  newer  rocks  the  geographical  diversity  has  become  so 
great  that  we  are  compelled  to  determine  age  and  synchronize  strata, 


200 


STRATIFIED   OR   SEDIMENTARY  ROCKS. 


no  longer  entirely  by  comparison  of  fossils  of  the  different  localities 
with  each  other,  but  also  by  the  comparison  of  the  fossils  of  each  local- 
ity with  the  living  species  in  the  same  locality.  In  these  rocks  we  de- 
termine relative  age  by  relative  percentage  of  living  species,  and  simi- 
larity of  age  (geological  horizon)  by  similarity  of  this  percentage. 

Manner  of  constructing  a  Geological  Chronology. — The  manner  in 
which  a  geological  chronology  has  actually  grown  up,  under  the  com- 
bined labors  of  the  geologists  of  all  countries,  may  be  briefly  stated  as 
follows :  First,  the  order  of  superposition,  and  therefore  the  relative 
ages  of  the  strata  composing  the  rock-series  of  many  different  countries, 
were  determined  independently ;  next,  by  comparison  of  these,  partly 
by  lithological  character,  if  the  localities  are  contiguous,  and  partly  by 
fossils,  the  geologist  determines  those  which  are  synchronous  and  those 
which  are  wanting  in  each  locality.  Thus,  out  of  several  local  series, 
by  intercalation,  he  constructs  a  more  complete  ideal  series.  In  case  of 
doubt,  he  strives  to  find  places  where  the  doubtful  strata  come  together, 
and  observes  their  relative  position.  In  Fig.  183,  A  and  B  represent 


FIG.  183— Diagram  illustrating  the  Mode  of  determining  the  Chronological  Order  of  Strata. 

two  contiguous  localities  in  which  by  independent  study  the  relative  po- 
sitions and  ages  of  6  and  7  strata  respectively  have  been  determined. 
By  comparison,  the  rocks  of  the  two  series  are  found  to  consist  of 
eleven  strata  of  different  ages,  some  being  wanting  in  the  one  and 
some  in  the  other  locality.  The  figure  represents  the  strata  as  con- 
nected and  traceable  from  one  locality  to  the'  other,  but  the  intervening 
portions  between  A  and  B  may  be  removed  by  erosion,  as  shown  by  the 
dotted  line,  or  covered  with  water.  In  such  case,  the  actual  overlapping 
cannot  be  observed,  if  it  ever  existed,  but  the  comparison  in  other  re- 
spects is  the  same.  In  widely-separated  localities  of  course  the  compar- 
ison can  only  be  made  by  means  of  fossils.  Thus  as  the  examination  of 
the  earth's  surface  progresses,  with  every  new  country  examined  some 
gaps  are  filled  up,  and  the  series  becomes  more  perfect.  Many  gaps  still 
remain  unfilled.  The  series  will  continue  to  be  made  more  perfect,  and 
the  chronology  more  complete,  until  the  geological  examination  of  the 
earth-surface  is  complete. 


CLASSIFICATION  OF  STRATIFIED   ROCKS. 


201 


The  second  object  to  be  attained  by  classification  is  the  division  and 
subdivision  of  the  whole  series  into  larger  and  smaller  groups,  corre- 
sponding to  the  eras,  periods,  and  epochs  of  time. 

The  following  is  an  outline  of  the  classification  of  Dana,  slightly 
modified.  Except  in  the  uppermost  part  it  is  carried  only  as  far  as 
periods  : 


EBAS. 

AGES. 

PERIODS. 

EPOCHS. 

5.  Psychozoic. 

7.  Age  of  Man. 

Human, 

22     Recent. 

4.  Cenozoic. 

6   The  Age    of  Mam- 
mals. 

f  Quaternary, 
[  Tertiary, 

!  Terrace. 
Champlain. 
Glacial. 
(  Pliocene. 
20    -J  Miocene. 
(  Eocene. 

3.  Mesozoic. 

5.   The    Age    of   Rep- 
tiles. 

(  Cretaceous, 
•}  Jurassic, 
(  Triassic, 

19 
18 
17 

2.  Palaeozoic. 

Carboniferous  Age.  "| 
4.  The  Age  of  Aero-  1 
gens  and    Am-  f 
phibians. 

{Permian, 
Carboniferous, 
Sub-carboniferous, 

16 
15 
14 

Devonian. 
3.  The  Age  of  Fishes. 

fCatskill, 
I  Chemung, 
"I  Hamilton, 
[Corniferous, 

13 
12 
11 
10 

Sttunan.        (>A 
2.  The  Age  of  Inverte- 
brates.           /, 

-?.       -'-^ 

'Oriskany,                     9 
i  "Helderberg,                8 
Salina,                          7 
>  Niagara,-^  ^_U-    6 
^Trenton,                ^    5 
•<  .Canadian,                    4 
Primordial,  /,,...      3 

1.  Archaean,  or 
Eozoic. 

1.  Archaean. 

(  Huronian, 
|  Laurentian, 

2 
1 

As  we  have  already  stated,  the  gaps  in  the  series  are  usually  indi- 
cated by  unconformity.  Now,  since  unconformity  always  indicates  move- 
ments of  the  crust,  changes  of  the  outlines  of  sea  and  land,  changes  of 
climate,  and  consequent  changes  in  the  fauna  and  flora,  these  gaps  mark 
the  times  of  great  revolutions  in  the  earth's  history,  and  are  therefore 
the  natural  boundaries  of  the  eras,  periods,  etc.  The  whole  rock-series, 


202  UNSTRATIFIED   OR   IGNEOUS   ROCKS. 

therefore,  is  divided,  by  means  of  unconformity  and  the  character  of  the 
fossils,  into  larger  groups  called  systems,  and  these  again  into  smaller 
groups  called  series  and  formations.  The  largest  groups  are  founded 
upon  universal,  or  almost  universal,  unconformity,  and  a  consequent  very 
great  difference  in  character  of  organisms;  the  smaller  groups  are 
founded  upon  a  less  general  unconformity  and  less  difference  in  char- 
acter of  the  organisms.  Corresponding  with  the  great  divisions  and 
subdivisions  of  the  rock-system  are  the  eras,  ages,  periods,  and  epochs 
of  the  history.  The  several  terms  expressing  the  divisions  and  sub- 
divisions, both  of  the  rocks  and  of  the  history,  are  unfortunately  used  in 
a  loose  manner.  We  will  try  to  use  them  in  the  manner  indicated.  It 
will  be  observed  that  the  divisions  are  founded  upon  (a)  unconformity, 
and  (b)  change  in  fossils.  These  generally  accompany  each  other,  since 
they  are  produced  by  the  same  cause,  viz.,  change  of  physical  geography. 
In  some  localities,  however,  they  may  be  in  discordance.  In  this  case, 
the  change  of  fossils  is  considered  the  more  important,  and  controls 
classification. 


CHAPTER  III. 

UNSTRATIFIED   OR  IGNEOUS  ROCKS. 

Characteristics. — The  unstratified  are  distinguished  from  the  strati- 
fied rocks — a.  By  the  absence  of  true  stratification  or  lamination  of 
sorted  materials ;  b.  By  the  absence  of  fossils ;  c.  Usually  by  a  crystal- 
line or  else  a  glassy  structure ;  and,  d.  By  their  mode  of  occurrence, 
explained  below. 

Origin. — They  have  evidently  been  consolidated  from  a  fused  or 
semi-fused  condition,  and  are  therefore  called  igneous  rocks.  This  ori- 
gin is  clearly  shown  by  their  structure,  by  their  occurrence  in  dikes 
and  tortuous  veins,  by  the  effect  they  often  produce  upon  the  stratified 
rocks  with  which  they  come  in  contact,  and  by  their  resemblance  to 
ordinary  lavas. 

Mode  of  Occurrence. — They  occur — a.  Underlying  the  strata,  and 
forming  the  great  mass  of  the  interior  of  the  earth ;  b.  Forming  the 
peaks  and  axes  of  many  mountain-chains ;  c.  Filling  fissures,  often  of 
great  extent,  in  the  stratified  rocks,  or  other  igneous  rocks  ;  d.  Over- 
lying strata,  as  if  erupted  through  fissures  and  outpoured  on  the  surface ; 
and,  e.  Lying  conformably  between  the  strata,  as  if  forced  between 
them  in  a  melted  condition,  or  else  outpoured  upon  the  bed  of  the  sea, 
and  afterward  covered  with  sediments.  All  these  positions  are  illus- 
trated in  Fig.  184.  In  all  these  modes  of  occurrence,  the  observed  rock 
is  connected  with  an  underlying  mass,  of  which  it  is-but  the  extension. 


GRANITIC  ROCKS. 


FIG.  184.— Diagram  showing  Mode  of  Occurrence  of  Igneous  Rocks. 

Extent  on  the  Surface. — The  appearance  of  these  rocks  on  the  sur- 
face is  far  less  extensive  than  that  of  the  stratified  rocks.  Certainly 
not  more  than  one-tenth  of  the  surface  of  the  earth  is  composed  of 
them.  But  beneath  they  are  supposed  to  constitute  the  great  mass  of 
the  earth. 

Classification  of  Igneous  Rocks. — Igneous  rocks  are  best  classified, 
not  by  means  of  their  relative  age,. but  partly  by  their  lithological 
character,  and  partly  by  their  mode  of  occurrence.  They  are  thus 
divisible  into  three  groups,  viz.,  the  granitic  rocks,  the  trappean  or 
fissure-eruption  rocks,  and  the  volcanic  or  crater-eruption  rocks.  After 
describing  these,  we  will  notice  briefly  attempts  at  classification  on 
other  bases. 

1. — GRANITIC  ROCKS. 

The  rocks  of  this  group  are  characterized  by  a  coarse-grained, 
speckled  or  mottled  appearance,  arising  from  the  fact  that  they  are 
formed  by  the  aggregation  of  distinct  crystals  or  masses  of  different 


FIG.  185. -Graphic  Granite:  A,  cross-section ;  J3,  longitudinal  section. 

colors.  Granite,  which  is  the  type  of  this  group,  consists  of  quartz, 
feldspar,  and  mica.  Sometimes  the  mica  is  wanting,  and  the  quartz  is 
in  the  form  of  curiously-bent  laminae,  which  on  cross-section  resem- 
ble Hebrew  or  Arabic  characters,  disseminated  in  a  mass  of  feld- 
spar. The  rock  is  then  called  graphic  granite  (Fig.  185) ;  when  the 


204 


UNSTRATIFIED   OR  IGNEOUS  ROCKS. 


mica  is  replaced  by  hornblende,  the  rock  is  called  syenite  y1  when,  in 
addition  to  the  quartz  and  feldspar,  both  mica  and  hornblende  occur,  it 
is  called  syenitic  granite.  Most  of  the  granite  in  this  country  is  syenitic. 
The  dark  specks  in  granite  are  due  to  mica  or  to  hornblende ;  the  opaque 
white,  or  reddish,  or  greenish,  with  distinct  cleavage,  is  feldspar,  and 
the  grayish  glassy  is  quartz. 

Chemical  Composition  and  Kinds. — Quartz  is  pure  silica  or  silicic 
acid  (SiOa).  Feldspar  of  this  group  is  an  acid  silicate  of  alumina  and 
alkali,  potash  or  soda  (orthoclase).  Hornblende  is  a  basic  silicate  of 
magnesia  and  lime,  with  also  oxide  of  iron  and  alumina.  Remembering 
also  that  hornblende  is  a  black  mineral,  while  quartz  and  feldspar  are 
either  colorless  or  very  light-colored,  it  is  evident  that  this  group  may 
be  divided  into  two  sub-groups,  the  one  more  acid,  and  the  other  more 
basic ;  and  in  proportion  as  quartz  and  feldspar  predominate,  the  rock  is 
lighter  colored,  less  dense,  and  more  acid ;  in  proportion  as  hornblende 
predominates,  it  is  darker,  heavier,  and  more  basic.  Granite  may  be 
taken  as  the  type  of  the  more  acid  sub-group,  and  the  darker  varieties 
of  syenite  as  the  type  of  the  more  basic  sub-group.  These  sub-groups 
graduate  insensibly  into  each  other.  Since  a  general  characteristic  of 
the  granitic  group  is  the  existence  of  free  quartz  in  notable  quantity, 
this  group,  taken  as  a  whole,  is  usually  regarded  as  more  acid  than  the 
other  two  groups. 

Mode  Of  Occurrence. — Granitic  rocks  commonly  occur  forming  the 
axes  and  peaks  of  mountain-chains  (Fig.  186,  A),  or  as  rounded  masses, 
of  greater  or  less  extent,  coming  up  through  stratified  rocks  of  the  older 

series  (Fig.  186,  J3).  They 
also  sometimes  occur  as  tor- 
tuous veins,  running  from 
an  underlying  mass  into  the 
stratified  rocks  above,  as  if 
forced  by  heavy  pressure, 
while  in  a  fused  condition, 
into  small,  irregular  fissures 
of  the  overlying  strata  (Fig. 
187,  A  and  _B),  and  some- 
times, though  rarely,  as  dikes  filling  great  fissures,  as  in  the  elvans  of 
Cornwall;  but  it  is  doubtful  whether  these  should  be  considered  as 
true  granites.  They  are  probably  a  quartz-porphyry. 

We  have  no  distinct  evidence  that  granite  is  ever  an  eruptive  rock, 
i.  e.,  that  it  has  ever  been  forced  upward  through  great  fissures  of  the 
earth's  crust,  and  outpoured  on  the  surface  in  the  manner  of  lavas  and 

1  Some  writers  use  the  term  syenite  to  designate  a  rock  consisting  of  feldspar  and 
hornblende  only.  In  this  case  syenite  would  differ  from  diorite  only  in  the  form  of  the 
feldspar,  which  in  the  former  is  orthic  (orthoclase),  and  in  the  latter  clinic  (plagioclase). 


FIG.  1S6.— Diagram  illustrating  Mode  of  Occurrence  of 
Granite. 


TRAPPEAN   OR  FISSURE-ERUPTION  ROCKS. 


205 


traps.  Hence  also  ashes,  cinders,  tufas,  or  other  evidences  of  contact 
of  a  fused  mass  with  the  atmosphere,  have  never  been  found  in  connec- 
tion with  granites.  Most  geologists,  therefore,  believe  that  granite,  al- 
though it  may  be  irruptive  or  intrusive  by  pressure,  as  explained  above, 


FIG.  187.— Granite  Veins. 

is  never  an  eruptive  rock.  Granitic  rocks  are  most  probably  formed  at 
great  depths,  and  remain  where  they  are  formed.  Whenever,  there- 
fore, they  appear  on  the  surface,  it  is  probable  they  have  been  exposed 
by  extensive  denudation. 

2. — TRAPPEAN  OR  FISSURE-ERUPTION  ROCKS. 

Some  geologists  identify  these  with  volcanic  rocks,  regarding  their 
lithological  differences,  especially  their  more  crystalline  structure,  as  the 
result  only  of  the  fact  that  they  have  been  subjected  to  erosion,  which 
has  exposed  their  deeper  parts,  while  of  modern  lavas  we  see  only  the 
surface.  But  the  distinctive  character  of  these  rocks  consists  not  in 
their  age,  but  in  their  mode  of  occurrence,  having  come  up  through 
fissures  and  spread  out  on  the  surface  as  extensive  sheets,  rather  than 
through  craters,  and  run  off  in  streams. 

General  Characteristics.  —  Trappean  rocks,  differ  from  granitic  in 
being  usually  finer  grained  ;  in  usually,  though  not  always,  wanting 
quartz  and  mica ;  and  in  their  mode  of  occurrence,  explained  below.  In 
general  appearance  they  vary  greatly.  They  are  sometimes  minutely 
speckled  with  distinct  crystals,  sometimes  compact  or  crypto-crystalline, 
sometimes  even  glassy  or  scoriaceous  or  tufaceous,  like  modern  lavas. 
They  consist  usually  of  only  two  minerals,  viz.,  feldspar  (or  other  allied 
mineral  replacing)  and  hornblende  or  augite.  These  may  be  inti- 
mately mixed  or  in  distinct  crystals. 

Varieties. — The  varieties  are  so  numerous  and  run  by  such  insen- 
sible gradations  into  each  other,  that  it  is  useless  to  do  more  than 
mention  the  principal  types.  Like  granitic  rocks,  they  are  divided  into 


ACID  8EK1ES. 


Porphyry, 

Felstone, 

Phonolite, 

Trachyte, 

Pumice, 

Obsidian. 


BASIC   SERIES. 


Diorite, 

Dolerite, 

Melaphyr, 

Basalt, 

Black  obsidian. 


206  UNSTRATIFIED  OR  IGXEOUS  ROCKS. 

two  sub-groups — a  more  acid  and  a  more  basic.  In  proportion  as  feld- 
spar predominates,  the  rock  is  lighter  colored  and  more  acid  ;  in  pro- 
portion as  hornblende  or  augite  predominates,  it  is  darker  colored  and 
more  basic.  Felstone,  phonolite,  porphyry,  trachyte,  and  the  light- 
colored  obsidians,  are  types  of  the  acid  series ;  diorite,  dolerite,  basalt, 
and  black  obsidians,  of  the  basic,  as  shown  in  the  following  table. 

Phonolite  is  a  grayish,  crypto-crystalline  rock,  composed  chiefly  of 
feldspar,  which  breaks  or  joints  into  thin  slabs  like  slate,  and  rings  under 

the  hammer.  Felstone  and  petrosi- 
lix  have  a  composition  similar  to 
phonolite.  The  term  porphyry  is 
very  loosely  applied  to  a  great  va- 
riety of  rocks  in  which  large  crystals 
are  imbedded  in  a  more  compact 
matrix.  Felsite  porphyry,  which  may 
be  considered  the  type,  consists  of  a 
grayish  or  reddish  feldspathic  mass,  containing  large  crystals  of  lighter- 
colored  and  purer  feldspar.  Quartz-porphyry  contains  distinct  crystals 
of  quartz,  etc.  But  any  rock  is  called  porphyritic  which  contains  large 
crystals,  giving  the  mass  a  spotted  appearance.  Thus  there  may  be  a 
porphyritic  diorite,  porphyritic  diabase,  or  even  porphyritic  granite. 
Trachyte  is  a  light-colored,  feldspathic  rock,  having  a  rough  feel,  from 
the  presence  of  small  crystals  of  feldspar. 

Diorite  is  a  distinctly  crystalline,  and  therefore  a  distinctly  speckled, 
dark-colored  rock,  consisting  of  feldspar  and  hornblende.  Dolerite  has 
a  similar  general  appearance,  but  finer  grained,  and  a  similar  composi- 
tion, except  that  augite  replaces  hornblende  as  the  basic  ingredient. 
Melaphyr  and  diabase  may  be  regarded  as  varieties  of  dolerite.  Basalt 
is  a  very  dark,  crypto-crystalline  variety  of  dolerite.  Many  of  these 
more  basic  varieties  of  rock,  when  somewhat  changed  by  weathering, 
are  called  green-stones. 

The  glassy  and  scoriaceous  varieties  are  found,  but  are  not  so  com- 
mon in  the  trappean  as  in  the  recent  lavas. 

These  extreme  varieties  pass  insensibly  into  each  other  by  change 
of  proportion  of  their  mineral  ingredients,  and  into  the  granitic  series 
by  the  addition  of  quartz  and  mica.  Some  varieties  of  quartz-porphyry 
thus  pass  into  granite,  and  some  varieties  of  diorite  into  syenite. 

Mode  of  Occurrence. — Trap-rocks  usually  occur  in  vertical  sheets, 
filling  great  fissures  intersecting  the  strata,  or  in  extensive  horizontal 
sheets  outpoured  on  the  surface.  Sometimes  similar  sheets  are  found 
between  the  strata  as  if  outpoured  on  the  sea-bottom,  and  afterward 
covered  with  sediments.  In  some  cases,  however,  such  bedded  traps 
are  metamorphic,  and  not  truly  eruptive.  Trap-dikes  (as  the  fillings 
of  fissures  are  called)  vary  in  thickness  from  a  few  inches  to  fifty  or 


TRAPPEAN   OR  FISSURE-ERUPTION  ROCKS. 


207 


Fio.  188,-Dikes. 


even  one  hundred  feet ;  they  are  often  fifty  to  one  hundred  miles  long, 
and  extend  downward  to  unknown  depth.  When  there  is  no  overflow- 
ing portion  observable,  but  the  dike  simply  outcrops  along  the  surface, 
then  it  is  probable  either  that  the  overflow  has  been  subsequently  removed 
by  erosion,  or  else  that  the  liquid  matter  filled  a  fissure  which  originally 
did  not  reach  the  surface  (as  at/,  Fig.  184),  and  has  subsequently  been 
exposed  by  erosion.  In  either  case,  such  an  outcropping  dike  is  an 
evidence  of  extensive  denudation. 
Sometimes  the  outcropping  dike 
has  resisted  the  erosion  more  than 
the  country  rock,  and  the  dike  is 
left  standing,  like  a  low  wall,  run- 
ning over  the  face  of  the  country 
(Fig.  188,  a) ;  at  other  times  the 
country  rock  has  resisted  more 
than  the  dike,  and  the  place  of  the  dike  is  marked  by  a  slight  depression 
like  a  ditch  (Fig.  188,  b).  These  appearances  have  given  rise  to  the 
term  dike.  They  are  represented  in  section  in  the  figure. 

Outpoured  masses  of  fissure-eruption  rocks  are  often  of  immense 
thickness  and  extent,  forming,  in  some  cases,  the  chief  bulk  of  whole 
mountain-chains.  There  is  also, 

.mr.nmm          _, 


frequently,   abundant   evidence 

of  repeated   outflows  over  the 

same    region,    forming    sheets, 

piled  one  atop  of  another,  as 

shown  in  the  figure.     In  such 

cases,  on  account  of  the  peculiar 

columnar  structure  described  on 

page  209,  the  crumbling  of  the 

rocks  gives  rise  to  somewhat  regular  terraces  or  benches.     From  this 

circumstance  has  arisen  the  term  trap,  from  the  Swedish  word  trappa, 

a  stair. 

The  great  lava-flood  of  the  Northwest  covers  an  area  of  150,000  to 
200,000  square  miles,  and  is  3,000  to  4,000  feet  thick  in  its  thickest 
part,  where  cut  through  by  the  Columbia  River.  In  another  place,  at 
least  seventy  miles  distant,  where  cut  into  2,500  feet  deep  by  the  Des- 
chutes  River,  at  least  thirty  successive  sheets  may  be  counted.  Another 
great  lava-flood,  according  to  Gilbert,  covers  25,000  square  miles  of 
Arizona,  and  is  3,000  feet  thick  in  its  thickest  portions.1 

The  scoriaceous  and  tufaceous  conditions,  though  not  so  common  as 
in  crater-eruption  rocks,  are  also  often  found  in  connection  with  trap, 
clearly  indicating  the  contact  of  melted  rock  with  the  atmosphere. 

Effect  of  Dikes  on  the  Intersected  Strata,— The  strata  forming  the 

1  Wheeler's  "  Report— Geology,"  1874. 


f---^^*-'""-'-  — — ^        ^sU-ETT^    -7-J  M^IJT     _  -          I~~ 


208  UNSTRATIFIED  OR  IGNEOUS  ROCKS. 

bounding  walls  of  a  dike  are  almost  always  greatly  changed  by  the 
intense  heat  of  the  fused  matter.  Limestones  and  chalk  are  changed 
into  crystalline  marble  ;  clay  is  baked  into  porcelain  jasper  ;  impure 
sandstones  are  changed  into  a  speckled  rock  resembling  gneiss  ;  seams 
of  bituminous  coal  are  changed  into  anthracite,  or  sometimes  into  coke. 
In  all  cases  the  original  stratification  and  the  contained  fossils  are  more 
or  less  completely  destroyed.  These  effects  extend  sometimes  only  a 
few  feet,  sometimes  many  yards,  from  the  dike  itself. 

Age — how  determined.— "When  two  dikes  intersect  each  other,  then, 
of  course,  the  intersecting  must  be  younger  than  the  intersected  dike. 
In  this  manner  the  relative  age  of  dikes  intersecting  the  same  region 
may  often  be  determined.  The  absolute  age  of  igneous  rocks  can  only 
be  determined  by  means  of  the  strata  with  which  they  are  associated. 
If  a  dike  is  found  either  intersecting  (c)  or  outpoured  upon  the  sur- 
face of  strata  of  known  age  (t?,  Fig.  184),  the  dike  must  be  younger 
than  the  strata.  If  a  dike  (c'),  intersecting  strata  and  outcropping  on 
the  surface,  is  found  overlaid  by  other  strata  through  which  it  does  not 
break,  then  the  igneous  injection  is  younger  than  the  former  and  older 
than  the  latter.  The  series  of  events  indicated  is  briefly  as  follows  : 
first,  the  older  series  of  sediments  has  been  formed  ;  then  fissures 
formed  and  filled  by  igneous  injection  ;  then  erosion  has  carried  away 
the  upper  portion  of  the  strata  and  its  included  dike,  so  that  the  dike 
outcrops  along  the  eroded  surface  ;  and,  lastly,  the  whole  has  been 
submerged  and  again  covered  with  sediment. 

3. — VOLCANIC  ROCKS. 

Characteristics. — Volcanic  differ  from  trappean  rocks,  in  being  still 
finer  grained  ;  in  being  more  frequently  glassy,  scoriaceous,  and  tufa- 
ceous  ;  but  especially  in  their  mode  of  occurrence,  having  come  up 
through  craters  instead  of  fissures,  running  off  as  streams  instead  of 
spreading  as  extensive  sheets,  and  accumulating  in  the  form  of  isolated 
cones  instead  of  covering  great  areas  of  country.  Some  believe,  also, 
that  as  a  group  they  are  more  basic  than  either  of  the  others. 

Varieties. — These  have  already  been  described  under  Volcanoes. 
It  is  almost  impossible  to  draw  any  clear  lithologic  distinction  be- 
.tween  this  and  the  last  group.  Many  of  the  varieties  described  under 
the  last — as,  for  example,  trachyte  and  basalt,  pumice  and  obsidian — 
belong  equally  here.  The  rocks  of  this  group  also  are  divisible  into 
two  sub-groups  :  a  feldspathic — lighter-colored,  more  acid  group  ;  and 
an  augitic — darker-colored  and  more  basic  group.  As  already  explained 
under  Volcanoes,  both  of  these  may  exist  under  all  the  different  physi- 
cal conditions  of  stony,  glassy,  and  scoriaceous  lava,  and  also  as  sand 
and  ashes.  Of  the  acid  series,  trachyte,  the  lighter-colored  obsidians, 


STRUCTURES  FOUND   IN  ERUPTIVE   ROCKS. 


209 


and  pumice,  are  good  types  ;  and  of  the  basic  series  are  basalt  and  the 
black  obsidian,  and  black  scoriae. 

Of  Certain  Structures  found  in  many  Eruptive  Hocks. 

Columnar  Structure. — Many  kinds  of  eruptive  rock,  both  trappean 
and  volcanic,  exhibit  sometimes  a  remarkable  columnar  structure. 
This  is  most  conspicuous  in  basalt,  and  is  therefore  often  called  basaltic 
structure.  Sheets  and  dikes  of  this  rock  are  often  found  composed 
wholly  of  regular  prismatic  jointed  columns,  closely  fitting  together, 


FIG.  190.— Columnar  Basalt,  New  South  Wales  (Da 


varying  in  size  from  a  few  inches  to  a  foot  or  more,  and  in  length  from 
several  feet  to  fifty  or  one  hundred  feet.  When  these  columns  have 
been  well  exposed  on  cliffs  by  the  action  of  waves,  or  on  river-banks  by 
the  erosive  action  of  currents,  or  even  by  atmospheric  disintegration, 


FIG.  191.— Basaltic  Columns  on  Se 


2ntary  Eock,  Lake  Superior  (after  Owen). 


they  produce  a  very  striking  scenic  effect  (Figs.  190,  191).  In  Europe 
the  Giant's  Causeway,  on  the  coast  of  Ireland,  and  Fingal's  Cave,  in 
the  island  of  Staffa,  on  the  west  coast  of  Scotland,  are  conspicuous  ex- 
amples. In  the  United  States  we  have  examples  in  Mount  Holyoke, 
14 


210  UNSTRATIFIED  OR  IGNEOUS  ROCKS. 

on  the  Connecticut  River ;  in  the  Palisades  of  the  Hudson  River  ;  in 
the  traps  on  the  shores  of  Lake  Superior ;  and  especially  in  splendid 
cliffs  of  the  Columbia  and  Deschutes  Rivers,  in  Oregon. 

Direction  Of  the  Columns. — The  direction  of  the  columns  is  usually 
at  right  angles  to  the  cooling  surface.  In  horizontal  sheets,  therefore, 
the  columns  are  vertical,  but  in  dikes  they  are  horizontal  (Fig.  192). 
A  dike  left  standing  above  the  general  surface  of  country  sometimes 


FIG.  192.— Columnar  Dike,  Lake  Superior  (after  Owen). 

presents  the  appearance  of  a  long  pile  of  cord-wood.  In  some  cases 
the  columns  are  curved  and  twisted  in  a  manner  not  easy  to  explain ; 
sometimes,  instead  of  columnar,  a  ball  structure  is  observed. 

Cause  of  Columnar  Structure. — There  is  little  doubt  that  this 
structure  is  produced  by  contraction  in  the  act  of  cooling.  Many  sub- 
stances break  in  a  prismatic  way  in  contracting.  Masses  of  wet  starch, 
or  very  fine  mud  exposed  to  the  sun,  crack  in  this  way.  In  basalt  the 
structure  is  more  regular  than  in  any  other  known  substance.  The 
subject  of  the  cause  of  jointed  columnar  structure  has  been  very  ably 
discussed  by  Mr.  Mallet.1 

Volcanic  Conglomerate  and  Breccia.— If  a  stream  of  fused  rock, 

whether  from  a  crater  or  a  fissure,  run  down  a  stream-bed,  it  gathers 
up  the  pebbles  in  its  course,  and  after  solidification  forms  a  conglom- 
erate which  differs  from  a  true  conglomerate  (p.  171)  in  the  fact  that 
the  uniting  paste  is  igneous  instead  of  sedimentary.  In  a  similar 
manner  volcanic  breccias  are  formed  by  the  flowing  of  a  lava-stream  over 
a  surface  covered  with  rubble. 

1  Philosophical  Magazine,  August  and  September,  1875. 


OTHER  MODES  OF   CLASSIFICATION   OF   IGNEOUS   ROCKS.         211 

Amygdaloid. — Still  another  structure,  very  common  in  lavas  and 
traps,  is  the  amygdaloidal.  The  rock  called  amygdaloid  greatly  resembles 
volcanic  conglomerate,  being  appar- 
ently composed  of  almond-shaped  peb- 
bles in  an  igneous  paste,  but  is  formed 
in  a  wholly  different  way.  Outpoured 
traps,  and  especially  lava-streams,  are 
very  often  vesicular,  i.  e,  filled  with 
vapor-blebs,  usually  of  a  flattened  el- 
lipsoidal form.  In  the  course  of  time 
these  cavities  are  filled  with  silica, 
carbonate  of  lime,  or  some  other  mate- 
rial, by  infiltrated  water  holding  these 
matters  in  solution.  Sometimes  the 
filling  has  taken  place  very  slowly  by  FlG"  193- 

successive  additions  of  different-colored  material.  Thus  are  formed  the 
beautiful  agate  pebbles,  or  more  properly  amygdules,  so  common  in  trap. 
The  most  common  filling  is  silica,  because  water  percolating  through 
igneous  rocks  is  always  alkaline,  and  holds  silica  in  solution. 

Other  Modes  of  Classification  of  Igneous  Hocks. 

There  is  no  subject  connected  with  geology  which  is  in  a  state  of 
greater  confusion  than  the  classification  and  nomenclature  of  igneous 
rocks.  It  seems  proper  to  mention  some  of  the  different  views  enter- 
tained. 

Many  geologists  think  that  the  three  groups  mentioned  above  are 
characteristic  of  different  periods  of  the  earth's  history,  and  therefore 
associated  with  strata  of  different  ages.  They  think  that  granites  are 
associated  only  with  the  Archaean  and  Palaeozoic  strata,  traps  with 
Mesozoic,  and  volcanic  rocks  with  Tertiary  and  modern  strata,  and  that 
therefore  the  earliest  eruptions  were  granitic,  then  trappean,  and  last 
volcanic. 

Again,  many  think  that  erupted  matters  of  different  times  have 
become  progressively  more  basic.  They  think  that,  although  each  group 
may  be  divided  into  a  more  acid  and  a  more  basic  sub-group,  yet  as  a 
whole  the  granitic  group  is  the  most  acid,  the  volcanic  most  basic,  and 
the  trappean  intermediate,  as  shown  in  the  diagram  on  page  212. 

Again,  these  two  views,  usually  held  by  the  same  persons,  are  by 
them  connected  with  a  third  view  in  regard  to  the  original  constitu- 
tion of  the  earth's  crust.  On  first  cooling  the  outer  layer  is  supposed 
to  have  been  highly  oxidized  and  highly  siliceous  or  acid,  in  other  words 
granitic;  beneath  was  a  less  oxidized  and  less  acid  layer,  and  so  on, 
becoming  less  and  less  acid  or  more  and  more  basic.  The  first  erup- 
tions were  from  the  outer  layer,  and  therefore  granitic.  Afterward, 


212  UNSTRATIFIED   OR  IGNEOUS  ROCKS. 

as  tlie  solid  crust  grew  thicker  and  thicker,  the  eruptions  were  from 
deeper  and  deeper  layers,  and  therefore  more  and  more  basic. 

But  as  to  age  there  can  be  no  doubt  that 
.  granite,  though  most  commonly  found  in  the 

older  rocks,  is  associated  with  strata  of  all 
ages  up  to  the  Middle  Tertiary  ;  and  trappean 
eruptions  (if  we  use  this  term  to  mean  fissure- 


Acid.  fcasic. 


Granitic. 
Granite.  Syenite. 

Trappean. 
Porphyry.      I       Diorite. 


,        : 


Volcanic. 


Trachyte. 


Basalt. 


eruptions)  have  occurred  until  the  later  ter- 
tiary. The  granite  of  Mont  Blanc  was  pushed 
up  about  the  end  of  the  Eocene  period  (Lyell), 
and  the  lava-flood  of  the  Northwest  was  out- 
poured from  fissures  in  the  Cascade  Range  at 
the  end  of  the  Miocene.  Also,  as  to  compo- 
sition^ trachyte  has  much  the  same  compo- 
sition as  granite,  except  that  more  of  the 
silica  is  in  combination  and  less  of  it  free  in 
the  former  than  in  the  latter.  Similarly,  the  dark,  very  hornblendic 
varieties  of  syenite  have  much  the  same  chemical  (though  not  miner- 
alogical)  composition  as  basalt. 

Again,  others,  with  great  show  of  reason,  think  that  much  if  not  all 
the  difference  between  the  three  groups,  in  mineralogical  character  and 
crystalline  structure,  is  due  to  the  different  depths  at  which,  and  slow- 
ness with  which,  the  solidification  took  place  ;  for  slowness  of  cooling 
tends  to  produce  a  more  complete  separation  and  crystallization  of 
different  minerals  from  the  fused  glassy  magma  ;  they  think,  therefore, 
that  if  trachyte  could  be  traced  down  deep  enough,  it  would  be  found 
to  pass  into  porphyry,  and  finally  into  granite,  and  similarly  basalt 
would  become  diorite,  and  finally  dark  syenites.1  On  this  view,  what 
we  cannot  do,  erosion  has  done  for  us  ;  and  granite  is  most  commonly 
associated  with  the  older  rocks  only  because  these  have  been  most 
eroded,  and  therefore  their  deeper  parts  exposed.  Similarly,  a  less 
erosion  of  the  Mesozoic  or  Secondary  rocks  has  exposed  porphyries  and 
diorites.  Of  the  modern  lavas,  only  the  upper  parts  are  exposed.  If 
we  take  this  view,  then  the  only  true  distinction  among  igneous  rocks 
is  founded  on  the  mode  of  eruption,  viz.,  fissure-eruption  rocks  and 
crater-eruption  rocks  ;  and  each  of  these  may  assume  the  form  of  lava, 
or  trap,  or  granite,  according  to  the  depth  of  formation. 

The  confusion  in  the  classification  and  nomenclature  of  igneous 
rocks  is  still  further  increased  by  the  undoubted  fact  that  nearly  all  the 
varieties  of  igneous  rocks  mentioned  above  are  found  also  among  meta- 
morphic  rocks,  which  have  not  been  erupted  at  all.  This  subject  is 
further  treated  under  the  head  of  Metamorphism. 

1  This  gradual  change  has  very  recently  been  distinctly  observed  in  Southeastern  Eu- 
rope by  Judd.—  Geological  Magazine,  1876,  vol.  xxxii.,  p.  292. 


METAMORPHIC   ROCKS.  213 


CHAPTER  IV. 
METAMORPHIC    ROCKS. 

THERE  is  a  third  class  of  rocks,  intermediate  in  character  between 
the  ordinary  sedimentary  and  the  igneous  rocks,  and  therefore  put  off 
until  these  had  been  described.  The  rocks  of  this  class  are  stratified, 
like  the  sedimentary,  but  crystalline,  and  usually  non-fossiliferous,  like 
the  igneous  rocks.  They  graduate  insensibly  on  the  one  hand  into  the 
true  unchanged  sediment,  and  on  the  other  into  true  igneous  rocks. 

Origin. — Their  origin  is  evidently  sedimentary,  like  other  stratified 
rocks,  but  they  have  been  subsequently  subjected  to  heat  and  other 
agents  which  have  changed  their  structure,  sometimes  entirely  destroy- 
ing their  fossils  and  even  their  lamination  structure,  and  inducing  in- 
stead a  crystalline  structure.  The  evidence  of  their  sedimentary  origin 
is  found  in  their  gradation  into  unchanged  fossiliferous  strata ;  the 
evidence  of  their  subsequent  change  by  heat,  in  their  gradation 
into  true  igneous  rocks.  For  this  reason  they  are  called  metamorphic 
rocks. 

Position. — All  the  lowest  and  oldest  rocks  are  metamorphic.  The 
converse,  however,  viz.,  that  metamorphic  rocks  are  always  among  the 
oldest,  is  by  no  means  true.  Metamorphism  is  not,  therefore,  a  test  of 
age.  Metamorphic  rocks  are  found  of  all  ages  up  to  the  Tertiary.  The 
Coast  Range  of  California  is  much  of  it  metamorphic,  although  the  strata 
belong  to  the  Tertiary  and  Cretaceous  periods.  Metamorphism  seems  to 
be  universal  in  the  Laurentian,  is  general  in  the  Palaeozoic,  frequent  in 
the  Mesozoic,  exceptional  in  the  Tertiary,  and  entirely  wanting  in  recent 
sediments.  It  is  therefore  less  and  less  common  as  we  pass  up  the  series 
of  rocks.  The  date  of  metamorphism  is  also  different  from  that  of 
the  origin  of  the  strata.  Metamorphism  has  taken  place  in  all  geologi- 
cal periods,  and  is  doubtless  now  progressing  in  deeply-buried  strata. 

Metamorphism  is  also  generally  associated  with  foldings,  tiltings,  in- 
tersecting dikes,  and  other  evidences  of  igneous  agency,  and  is  there- 
fore chiefly  found  in  mountainous  regions.  It  is  also  usually  found  only 
in  very  thick  strata. 

Extent  on  the  Earth-Surface. — These  rocks  exist,  outcropping  on 
the  surface,  over  wide  regions.  Nearly  the  whole  of  Canada  and  Labra- 
dor, a  large  strip  on  the  eastern  slope  of  the  Appalachians,  and  a  large 
portion  of  the  mountainous  regions  of  the  western  border  of  this  con- 
tinent, are  composed  of  them.  Beneath  the  surface  they  probably  un- 
derlie all  other  stratified  rocks.  Their  thickness  is  also  often  immense. 


214:  METAMORPHIC   ROCKS. 

The  Laurentian  series  of  Canada  is  probably  40,000  feet  thick,  and 
metamorphic  throughout. 

Principal  Kinds. — The  principal  kinds  of  metamorphic  rocks  are: 
Gneiss,  mica-schist,  chlorite-schist,  talcose-schist,  hornblende-schist, 
clay-slate,  quartzite,  marble,  and  serpentine. 

Gneiss,  the  most  universal  and  characteristic  of  these  rocks,  has  the 
general  appearance  and  mineral  composition  of  granite,  except  that  it  is 
more  or  less  distinctly  stratified.  Often,  however,  the  stratification  can 
only  be  observed  in  large  masses.  Gneiss  runs  by  insensible  grada- 
tions, on  the  one  hand,  into  granite,  and  on  the  other,  through  the  more 
perfectly  stratified  schists,  into  sandy  clays  or  clayey  sands. 


FIG.  194— Gneiss. 

The  schists  are  usually  grayish  fissile  rocks,  made  up  largely  of  scales 
of  mica,  or  chlorite,  or  talc.  Hornblende-schist  is  similarly  made  up  of 
scales  of  hornblende,  and  is  therefore  a  very  dark  rock.  The  fissile 
structure  of  schists  is  due  to  the  presence  of  these  scales,  and  is  therefore 
wholly  different  from  that  of  slates.  It  is  called  foliation-structure. 

/Serpentine  is  a  compact,  greenish  magnesian  rock.  The  other  varie- 
ties need  no  description.  Hornblende-schists  run  by  insensible  grada- 
tions into  clay-slates  on  the  one  hand,  and  into  diorites  and  syenites  on 
the  other. 

All  these  kinds  may  be  regarded  as  changed  sands,  limestones,  and 
clays,  the  infinite  varieties  being  the  result  of  the  difference  in  the  original 
sediments  and  the  degrees  of  metamorphism.  Sands  and  limestones  are 
often  found  very  pure  ;  such  when  metamorphosed  produce  quartzite  and 
marble.  Clays,  on  the  contrary,  are  almost  always  impure,  containing 
sand,  lime,  iron,  magnesia,  etc.  Such  impure  clays,  if  sand  is  in  excess, 
produce  by  metamorphosis  gneiss,  mica-schist,  and  the  like  ;  but  if 
lime  and  iron  are  in  considerable  quantities  they  produce  hornblende- 
schist  or  clay-slate ;  if  magnesia,  talcose-schist.  The  origin  of  serpen- 
tine is  not  well  understood ;  but  it  is  evidently  a  changed  magnesian 
clay.  All  gradations  between  such  clays  and  serpentine  may  be  found 
in  the  Tertiary  and  Cretaceous  strata  of  the  Coast  Range  of  California. 


THEORY   OF   METAMORPHISM.  215 

Theory  of  Metamorphism. 

There  are  few  subjects  more  obscure  than  the  cause  of  metamor- 
phism,  and  the  conditions  under  which  it  occurs.  Some  important  light 
has  been  thrown  on  it,  however,  recently.  For  the  sake  of  clearness,  it 
will  be  better  to  divide  metamorphism  into  two  kinds,  somewhat  dif- 
ferent in  their  causes,  viz.,  local  and  general. 

Local  Metamorphism  is  that  produced  by  direct  contact  with  evident 
sources  of  intense  heat,  as  when  dikes  break  through  stratified  rocks. 
As  already  seen  (p.  207),  under  these  circumstances,  impure  sandstones 
are  changed  into  schists,  or  into  gneiss ;  clays,  into  slates,  or  into  porce- 
lain jasper;  limestones,  into  marbles;  and  bituminous  coal,  into  coke, 
or  into  anthracite.  In  these  cases  it  is  evident  that  the  cause  of  the 
change  is  the  intense  heat  of  the  incandescent,  fused  contents  of  the 
dike  at  the  moment  of  filling.  In  such  cases  of  local  metamorphism,  the 
effects  usually  extend  but  a  few  yards  from  the  wall  of  the  dike. 

General  Metamorphism. — But  in  many  cases  we  cannot  trace  the 
change  to  any  evident  source  of  intense  heat.  Rocks,  thousands  of  feet 
in  thickness,  and  covering  hundreds  of  thousands  of  square  miles,  are 
universally  changed.  The  principal  agents  of  this  general  metamorphism 
seem  to  be  heat,  water,  alkali,  pressure. 

That  heat  is  a  necessary  agent  is  sufficiently  evident  from  the  gen- 
eral similarity  of  the  results  to  local  metamorphism.  But  that  the  heat 
was  not  intense,  and  therefore  not  sufficient  of  itself  to  produce  the  ef- 
fects, is  also  quite  certain.  For  (a.)  metamorphic  rocks  are  often  found 
interstratified  with  unchanged  rocks.  Intense  heat  would  have  affected 
them  all  alike,  or  nearly  alike.  (#.)  Many  minerals  are  found  in  meta- 
morphic rocks  which  will  not  stand  intense  heat.  As  an  example,  carbon 
has  been  found  in  contact  with  magnetic  iron-ore,  although  it  is  known 
that  this  contact  cannot  exist,  even  at  the  temperature  of  red-heat,  with- 
out reduction  of  the  iron-ore,  (V.)  The  effect  of  simple  dry  heat,  as 
shown  in  cases  of  local  metamorphism,  does  not  extend  many  yards. 
(d.}  Water- cavities  are  found  abundantly  in  metamorphic  rocks.  This 
will  be  more  fully  explained  farther  on. 

Water. — Heat  combined  with  water  seems  to  be  the  true  agent. 
Recent  experiments  of  Daubree,  Senarmont,  and  others,  prove  that 
water  at  400°  C.  (=  752°  Fahr.)  reduces  to  a  pasty  condition  nearly  all 
ordinary  rocks  ;  moreover,  that  at  this  temperature  crystals  of  quartz, 
feldspar,  mica,  augite,  etc.,  are  formed.  Such  a  pasty  or  aqueo-fused 
mass  slowly  cooled  would  form  a  crystalline  rock  containing  crystals  of 
quartz,  feldspar,  mica,  etc.  ;  in  other  words,  would  be  metamorphic.  The 
quantity  of  water  necessary  for  these  effects  is  shown  by  experiment 
to  be  very  small — only  five  to  ten  per  cent.  In  other  words,  the  in- 
cluded water  of  sediments  is  amply  sufficient. 


216  METAMORPHIC   ROCKS. 

Alkali. — Alkaline  carbonates,  or  alkaline  silicates,  so  common  in 
natural  waters,  greatly  promote  the  process,  causing  the  aqueo-igneous 
pastiness  or  aqueo-igneous  fusion  to  take  place  at  a  much  lower  tem- 
perature. 

Pressure. — Simple  pressure  is  not  itself  a  direct  agent,  but  the 
necessary  condition  for  the  action  of  the  others,  since  it  is  impossible  to 
have  high  temperature  in  the  presence  of  water  without  corresponding 
pressure. 

It  is  evident,  therefore,  that  while  metamorphism  by  dry  heat  would 
require  a  temperature  of  2,000°  to  3,000°,  in  the  presence  of  water  the 
same  result  is  produced  at  300°  or  400°  C.  ;  or  in  the  presence  of  alkali, 
even  in  small  amount,  probably  at  300°  or  400°  Fahr. 

Application. — All  these  agents  are  found  associated  in  deeply-buried 
sediments.  Series  of  outcropping  strata  are  often  found  20,000  or  even 
40,000  feet  thick.  The  lower  strata  of  such  a  series,  by  the  regular 
increase  of  interior  heat  alone,  must  have  been,  before  uptilting,  at  a 
temperature  of  between  700°  and  800°  Fahr.,  a  temperature  sufficient, 
with  their  included  water,  to  produce  complete  aqueo-igneous  pasti- 
ness, and  therefore,  by  cooling  and  crystallization,  complete  meta- 
morphism. 

Suppose,  then,  a  b  5,  Fig.  195,  represent  the  contour  of  land  and 
sea-bottom  at  the  beginning  of  any  period,  and  the  dotted  line  i  i  the 


FIG.  195.— Diagram  illustrating  the  Invasion  of  Sediments  by  the  Interior  Heat. 

isogeotherm  of  800°.  If,  now,  sediments  40,000  to  50,000  feet  thick 
be  deposited  so  that  the  sea-bottom  is  raised  to  V  b',  then  the  isotherm 
of  800°  will  rise  to  i'  i'  and  invade  the  lower  portions  of  the  sediments 
with  their  included  water.  Such  sediments  would  be  completely 
changed  in  their  lower  portions,  and  to  a  less  extent  higher  up.  It  is 
probable  that  even  300°  to  400°  Fahr.  is  sufficient  to  produce  a  consid- 
erable degree  of  change ;  or  even  200°,  if  alkali  be  present. 

Crushing. — Although  simple  gravitative  pressure  is  only  a  condi- 
tion, and  not  a  cause,  of  heat,  horizontal  pressure  with  crushing  of  the 
crust,  by  the  conversion  of  mechanical  energy  into  heat,  becomes,  as 
Mallet  has  shown,1  an  active  source  of  this  agent.  Now,  in  all  cases 

1  "Philosophical  Transactions,"  1873,  p.  147. 


ORIGIN   OF   GRANITE.  217 

of  metamorphism  we  find  ample 'evidences  of  such  horizontal  crushing 
in  the  associated  foldings  and  cleavage  of  the  strata. 

Explanation  Of  Associated  Phenomena.  —  This  theory  readily  ex- 
plains— 1.  Why  metamorphism  is  always  associated  with  great  thick- 
ness of  strata;  2.  Why  the  oldest  rocks  are  most  commonly  meta- 
morphic,  since  these  have  usually  had  the  newer  rocks  piled  upon 
them,  and  have  been  subsequently  exposed  by  erosion.  The  newer 
rocks  are  sometimes  also  metamorphic,  but  in  these  cases  they  are  very 
thick.  3.  It  also  explains  the  interstratification  of  metamorphic  with 
unchanged  rocks  ;  since  some  rocks  are  more  easily  affected  by  heated 
water  than  others,  and  the  composition  of  the  included  water  may  be 
also  different,  some  containing  alkali  and  some  not.  4.  It  also  explains 
its  association  with  foldings  of  strata  and  with  mountain-chains,  as  will 
be  more  fully  explained  hereafter. 

If  metamorphism  is  only  produced  in  deeply-buried  sediments,  then 
the  exposure  of  such  rocks  on  the  surface  can  only  result  from  exten- 
sive erosion. 

Origin  of  Granite. 

There  is  much  reason  to  believe  that  most  granites  are  npt  the  re- 
sult of  simple  dry  fusion,  as  is  usually  supposed ;  but,  on  the  contrary, 
only  the  last  term  of  metamorphism  of  highly-siliceous  sediments.  Ac- 
cording to  this  view,  incipient  pastiness  by  heat  and  water  makes 
gneiss ;  complete  pastiness,  completely  destroying  stratification,  makes 
granite.  The  principal  arguments  for  this  view  may  be  briefly  stated 
as  follows : 1 

1.  In  many  localities  in  mountain-regions,  and  nowhere  better  than 
in  the  Sierras  of  California,  every  stage  of  gradation  may  be  observed 
between  clayey  sandstones  and  gneiss,  and  between  gneiss  and  granite. 
So  perfect  is  this  gradation,  that  it  is  impossible  to  draw  sharply  the 
distinction.     Even  geologists  who  believe  that  granite  is  the  primitive 
rock  have  been  compelled  to  admit  that  there  is  also  a  metamorphic 
granite,  scarcely  distinguishable  from  primitive  granite. 

2.  Not  only  gneiss,  but  even  granite,  is  sometimes  interstratified 
with  unchanged  fossiliferous  rock. 

3.  Chemists  recognize  two  kinds  of  silica,  viz.,  an  amorphous  va- 
riety of  specific  gravity  2.2,  and  a  crystallized  variety,  specific  gravity 
2.6.     These  two  varieties  differ  from  each  other  not  only  in  density, 
but  also  in  chemical  properties,  the  former  being  much  more  easily 
attacked  by  alkalies  than  the  latter.     By  solidification  from  fusion  (dry 
way)  only  the  variety  of  specific  gravity  2.2  can  be  formed,  while  the 
variety  2.6  is  formed  only  by  slow  deposit  from  solution  (humid  way). 

1  Rose,  Philosophical  Magazine,  xix.,  p.  32 ;  Delesse,  "  Archives  des  Sciences,"  vol. 
vii.,  p.  190;  Hunt,  American  Journal  of  Science  and  Arts,  new  series,  vol.  i.,  pp.  82,  182. 


218  METAMORPHIC   ROCKS. 

Now,  the  quartz  of  granite  is  always  of  the  variety  2.6,  and  therefore 
must  have  been  formed  in  presence  of  water. 

4.  The  several  minerals  of  which  granite  is  composed  will  not  sep- 
arate from  a  fused  granitic  magma  on  cooling;  but,  on  the  contrary, 
fused  granite  solidifies  into  a  highly-siliceous  glass.     The  only  answer 
to  this,  as  well  as  to  the  preceding,  is  that  the  behavior  of  the  granitic 
magma,  when  fused  on  a  large  scale  and  cooled  slowly  in  the  laboratory 
of  Nature,  is  possibly  different  from  its  behavior  when  melted  in  small 
masses  and  cooled  less  slowly  in  our  laboratories. 

5.  Crystals  of  quartz,  feldspar,  and  mica,  are  frequently  formed  in 
Nature  by  the  humid  process,  as,  for  example,  in,  metamorphic  rocks ; 
and  have  also  been  artificially  formed  by  the  same  process  by  Daubree, 
Senarmont,  and  others,  as  already  stated  (p.  215) ;  but  they  have  never 
been  formed  artificially  by  the  dry  way. 

6.  In  nearly  all  rocks  and  minerals  microscopic  cavities  are  found 
indicating  the  conditions  under  which  crystallization  or  solidification 
took  place.     If  crystals  are  formed  by  sublimation,  they  contain  vacu- 
ous cavities.     If  they  are  formed   by  solidification  from  fusion  (dry 
way),  and  if  gases  are  present,  they  may  contain  air-blebs  ;  but,  if  they 
crystallize  slowly  from  a  glassy  magma,  they  contain  spots  of  glassy 
matter  or  glass  cavities,  as  in  slags  and  lavas.     If  they  are  formed  by 
crystallization  from  solution,  then  they  have  fluid  cavities.     Now,  not 
only  are  these  fluid  cavities  found  in  metamorphic  rocks,  but  also  in  the 
quartz  and  feldspar  of  granite.     "  A  thousand  millions  of  these  micro- 
scopic cavities  in  a  cubic  inch  is  not  at  all  unusual ;  and  the  inclosed 
water  often  constitutes  one  to  two  per  cent,  of  the  volume  of  the 
quartz."  *     Besides  these  fluid  cavities,  however,  glass  cavities  are  also 
found  in  the  quartz  and  feldspar  of  granite.     These  facts  point  plainly 
to  the  agency  of  both  heat  and  water  in  the  formation  of  granite. 

Even  the  temperature  at  which  metamorphic  rocks  and  granite 
solidified  has  been  approximately  determined  by  Mr.  Sorby.  The  prin-- 
ciple  on  which  this  is  done  is  as  follows :  If  crystallization  from  solution, 
or  solidification  in  the  presence  of  water,  take  place  at  ordinary  temper- 
atures, then  the  fluid  cavities  will  be  full ;  but  if  at  high  temperatures, 
and  the  mass  subsequently  cools,  then  by  the  contraction  of  the  con- 
tained liquid  a  vacuous  space  will  be  formed  which  will  be  larger,  in 
proportion  to  the  amount  of  contraction,  and  therefore  to  the  tempera- 
ture of  solidification.  Knowing,  therefore,  the  relative  sizes  of  the 
vacuole  and  the  contained  water,  and  the  coefficient  of  expansion  of  the 
water  and  the  rock,  the  temperature  at  which  the  cavity  would  fill 
(which  is  the  temperature  of  solidification)  may  be  calculated.  Some- 
times this  temperature  may  be  gotten  by  actual  experiment,  i.  e.,  by 
heating  until  the  cavity  fills.  By  this  method  Mr.  Sorby  has  calculated 
1  Sorby,  Quarterly  Journal  of  the  Geological  Society,  vol.  xiv.,  pp.  3*29,  453. 


ORIGIN  OF  GRANITE.  219 

the  temperature  of  solidification  of  certain  metamorphic  rocks  of  Corn- 
wall as  392°  Fahr.,  and  of  some  granites  as  482°,  and  others  only  212°. 

It  seems  almost  certain,  therefore,  that  most  granites  have  not  been 
formed  by  dry,  igneous  fusion.  Yet  that  this  rock  has  been  in  a  liquid  or 
pasty  condition  is  perfectly  certain  from  its  occurrence  in  tortuous  veins. 
Therefore  it  has  been  rendered  pasty  by  heat  in  the  presence  of  water 
under  great  pressures,  such  as  always  exist  in  deeply -buried  strata.  The 
weight  of  the  superincumbent  strata,  or  else  pressure  by  folding  and 
crushing  of  the  strata,  has  forced  it  into  cracks  and  great  fissures. 

What  we  have  said  of  granite  applies  of  course  to  the  whole  gra- 
nitic group.  Granitic  rocks  are  often  only  the  last  term  of  the  metamor- 
phism  cf  sediments ;  granite  being  produced  from  the  more  siliceous 
sediments,  and  dark  syenites  from  the  mere  basic  impure  clays.  But 
we  cannot  stop  with  this  group.  It  is  certain  that  many  if  not  all  the 
rocks  of  the  Trappean  group  also  may  be  made  by  metamorphism  of 
sediments.  Many  bedded  diorites,  dolerites,  and  felsites,  are  undoubt- 
edly formed  in  this  way,  for  the  gradations  can  be  distinctly  traced  into 
slates.  Prof.  Dana  *  has  recently  recognized  this  as  so  certain  that  he 
proposes  the  addition  of  the  prefix  metct  to  these  to  indicate  their 
origin.  Thus  he  recognizes  a  syenite  and  a  metasyenite,  a  diorite  and 
a  metadiorite,  dolerite  and  metadolerite,  felsite  and  metafelsite,  etc., 
and  we  might  add  granite  and  metagranite. 

Many  geologists  push  these  views  so  as  to  include  also  even  the 
true  lavas.  Deeply-buried  sediments  under  gentle  heat  in  the  presence 
of  water  and  pressure  undergo  incipient  change  and  form  metamorphic 
rocks ;  under  greater  heat  become  pasty  and  form  granite,  metasyenites, 
metadiorites,  metafelsites,  etc.  ;  under  still  greater  heat,  increased 
probably,  as  Mallet  suggests,  by  mechanical  energy  in  crushed  strata 
being  converted  into  heat,  become  completely  fused,  and  are  then  out- 
poured upon  the  surface  either  by  the  elastic  force  of  the  steam  gener- 
ated, or  by  the  pressure  and  squeezing  produced  by  the  folding  of  the 
crust  of  the  earth,  so  common  in  mountainous  regions.  According  to 
this  view,  every  portion  of  the  earth's  crust  has  been  worked  over  and 
over  again,  passing  through  the  several  conditions  of  soil,  sediment, 
stratified  rock,  metamorphic  rock,  and  igneous  rock,  perhaps  many 
times  in  the  course  of  the  geological  history  of  the  earth,  and  we  look 
in  vain  for  the  primitive  rock  of  the  earth's  crust. 

1  American  Journal  of  Science  and  Arts,  vol.  xi.,  p.  119,  February,  1876. 


220  STRUCTURE  COMMON  TO  ALL  ROCKS. 


CHAPTER  V. 
STRUCTURE  COMMON  TO  ALL  ROCKS. 

WE  have  thus  far  given  a  brief  description  of  the  three  classes  of 
rocks,  their  structure  and  mode  of  occurrence.  There  are  still,  how- 
ever, several  important  kinds  of  structure  which  are  common  to  all 
these  classes  of  rocks,  and  require  description.  These  are  joints,  fissures, 
and  veins.  Mountain-chains,  as  involving  all  kinds  of  rocks  and  all 
kinds  of  structure,  must  be  taken  up  last. 

SECTION  1.— JOINTS  AND  FISSURES. 
Joints. 

All  rocks,  whether  stratified  or  igneous,  are  divided,  by  cracks  or 
division-planes,  in  three  directions,  into  separable  irregularly  prismatic 
blocks  of  various  sizes  and  shapes.  These  cracks  are  called  joints.  In 
stratified  rocks  the  planes  between  the  bedding  constitute  one  of  these 
division-planes,  while  the  other  two  are  nearly  at  right  angles  to  this 
and  to  each  other,  and  are  true  joints.  In  igneous  rocks  all  the 
division-planes  are  of  the  nature  of  joints.  In  sandstone  these  blocks 
are  large  and  irregularly  prismatic ;  in  slate,  small,  confusedly  rhom- 


Fio.  196. — Regular  Jointing  of  Limestone. 


boidal ;  in  shale,  long,  parallel,  straight ;  in  limestone,  large,  regular, 
cubic;  in  basalt,  regular,  jointed,  columnar  ;  in  granite,  large, irregularly 
cubic  or  irregularly  columnar.  On  this  account  a  perpendicular  rocky 


FISSURES,   OR  FRACTURES..  221 

cliff  usually  presents  the  appearance  of  huge,  irregular  masonry,  with- 
out cement. 

•   The  cause  of  joints  is  probably  the  shrinkage  of  the  rock  in  the  act 
of  consolidation  from  sediments  (lithification),  as  in  stratified  rocks,  or 


FIG.  197.— Granitic  Columns. 

in  cooling  from  a  previous  condition  of  high  temperature,  as  in  the 
igneous  and  metamorphic  rocks. 

Fissures,  or  Fractures. 

These  must  not  be  confounded  with  joints.  Joints  are  cracks  in 
the  individual  strata  or  beds;  fissures  are  fractures  in  the  earth's 
crust,  passing  through  many  strata,  and  even  sometimes  through  many 
formations.  The  former  are  produced  by  shrinkage ;  the  latter  by 
movements  of  the  earth's  crust.  Fissures,  therefore,  are  often  fifty  or 
more  miles  in  length,  thirty  to  fifty  feet  in  width,  and  pass  downward 
to  unknown  but  certainly  very  great  depths. 

Cause. — The  cause  of  great  fissures  is  evidently  always  movements, 
and  usually  foldings  or  wrinkling  of  the  earth's  crust,  produced  probably 
by  contraction  of  the  interior  portions,  as  will  be  explained  under  Moun- 
tain-Chains, page  240.  The  natural  tendency  of  such  foldings  would  be 
to  form  a  parallel  system  of  fissures  in  the  direction  of  the  folds,  and 
therefore  at  right  angles  to  the  direction  of  the  folding  force.  Fissures 
are  usually  thus  found  in  systems  parallel  among  themselves,  and  to 
the  axes  of  mountain-chains.  Through  such  fissures  igneous  rocks  in  a 
fused  condition  are  often  forced,  forming  dikes  and  overflowing  sheets. 
Besides  the  principal  fissures  just  explained,  Hopkins  has  shown  that, 
in  the  case  of  the  formation  of  mountains,  there  would  be  formed  also 
other  smaller  fissures  at  right  angles  to  these. 

Often  the  walls  on  the  two  sides  of  a  fissure  do  not  correspond  with 
each  other,  but  one  side  has  been  pushed  up  higher  or  dropped  down 
lower  than  the  other.  Such  a  displacement  is  called  a  fault,  a  slip,  or 
dislocation.  This  may  occur  in  fissures  in  any  kind  of  rock,  but  is  most 
marked  and  most  easily  distinguished  in  stratified  rocks.  When  the 


222 


STRUCTURE   COMMON  TO   ALL   ROCKS. 


strata  are  sufficiently  flexible  to  admit  it,  they  are  bent  instead  of  brok- 
en, and  a  monocline  is  formed  instead  of  a  fault  (Fig.  198).  When  the 
fissure  is  filled  at  the  moment  of  its  formation  with  fused  matter  from 
beneath,  it  is  called  a  dike.  When  it  is  not  filled  at  the  moment  of  its 
formation  with  igneous  injection,  but  slowly  afterward  with  other  mat- 
ter, and  by  a  different  process,  it  is  called  a  vein. 


FIG.  19S. -Section  of  Nutria-Fold,  New  Mexico  (after  Gilbert). 

Faults. — In  faults  the  extent  of  vertical  displacement  varies  from  a 
few  inches  to  hundreds  or  even  thousands  of  feet.  In  the  Appalachian 
chain  there  occur  faults  in  which  the  vertical  dislocation  is  5,000  to 
20,000  feet.  In  Southwest  Virginia,  according  to  Rogers,  there  is  a 
line  of  fracture  extending  parallel  to  the  Appalachian  chain  for  eighty 
miles,  in  which  there  is  a  vertical  slip  of  8,000  feet,1  the  Lower  Silurian 
being  brought  up  on  one  side  until  it  comes  in  conjunction  with  the 
Lower  Carboniferous  on  the  other  (Fig.  199).  In  Western  Pennsylvania, 


FIG.  199.-Fault  in  Southwest  Virginia :  a,  Silurian ;  d,  Carboniferous  (after  Lesley). 

according  to  Lesley,  there  is  another  fault  extending  for  twenty  miles, 
in  which  the  lowermost  of  the  Lower  Silurian  is  brought  up  on  a  level 
with  the  uppermost  of  the  Upper  Silurian,  the  whole  Silurian  strata  be- 
ing at  this  place  20,000  feet  thick,  so  that  one  may  stand  astride  of  the 
fissure  with  one  foot  on  the  Trenton  limestone  (Lower  Silurian),  and 
the  other  on  the  Hamilton  shales  (Devonian).2  On  the  north  side  of 
the  Uintah  Mountains  there  is  a  slip,  according  to  Powell,  of  nearly 
20,000  feet,3  The  Sevier  Valley  fault,  Utah,  may  be  traced  partly  as  a 
slip,  partly  as  a  monocline,  for  225  miles  (Gilbert). 

1  Dana's  "  Manual,"  p.  399.  5  "  Manual  of  Coal,"  p.  147. 

3  "  Exploration  of  Colorado  River,"  p.  156. 


FISSURES,  OR   FRACTURES. 


223 


If  such  slips  were  suddenly  produced  by  violent  convulsion,  then,  at 
the  time  of  formation,  there  must  have  been  a  steep  (Fig.  200),  or 
sometimes  even  an  overhanging,  escarpment  (Fig.  199),  equal  to  the 
displacement.  In  some  cases  there  is  such  an  escarpment  or  line  of 
steep  mountain-slope  corresponding  to  the  line  of  slip.  In  the  Colo- 
rado Plateau  region  the  north  and  south  cliffs  are  produced  by  faults 
(Powell).  The  Zandia  Mountains,  New  Mexico,  are  produced  by  a 


Fit;.  200. 

drop  of  11,000  feet  on  the  western  side,  leaving  an  escarpment  still 
7,000  feet  high  (Gilbert).  In  the  Basin  Range  region  also  many  of 
the  ridges  are  formed  by  faults.  But  in  most  cases  there  is  no  such 
escarpment,  the  two  sides  of  the  fault  having  been  cut  down  to  one 
level  by  subsequent  erosion,  so  that  the  unpractised  eye  detects  nothing 
unusual  along  the  line  of  fracture  and  slip.  In  Fig.  200  the  strong 

I 


FIG.  201.— Strata  repeated  by  Faults. 

line  a  a  shows  the  present  surface,  while  the  dotted  line  bbb  shows  the 
surface  after  the  displacement  as  it  would  be  if  unaffected  by  erosion. 
In  many  cases,  however,  it  seems  more  probable  that  there  never 
existed  any  such  escarpment  as  represented  in  Fig.  200,  but  that  the 
displacement  was  produced  by  a  slow,  creeping  motion,  or  else  by  a 
succession  of  smaller  sudden  slips  probably  accompanied  with  earth- 


224 


STRUCTURE   COMMON  TO   ALL   ROCKS. 


quakes  (p.  106),  and  thus  that  the  slipping  and  the  denudation  have 
gone  on  together  pari  passu.  In  Fig.  243,  on  page  263,  the  upper 
part  shows  the  great  Uintah  fault  restored,  while  the  lower  part  shows 
the  actual  condition  of  things  produced  by  erosion. 


FIG.  202.— Section  through  Portion  of  Plateau  Region  of  Utah,  showir 
(after  Howell). 


:  a  Succession  of  Faults 


When  faults  occur  in  inclined  outcropping  strata,  the  same  series 
of  strata  may  be  repeated  several  times,  as  in  Fig.  201.  In  such  a 
case,  the  observer  walking  over  the  surface  of  the  cbuntry  from  A  to  B 


FIG.  203.-Fault  with  Change  of  Dip:  d,  dike. 

might  suppose  here  a  series  of  nine  strata,  whereas  there  are  but  three 
strata,  a,  b,  c,  three  times  repeated.     Fig.  202  is  a  natural  section  show- 

ing  this.      Sometimes  the  dip  of  the 

strata  on  the  two  sides  of  a  fault  are 
not  parallel,  the  change  of  inclination 
being  effected  at  the  time  of  the  dis- 
placement, as  shown  in  Fig.  203.  Upon 
the  eroded  surface  of  such  dislocated 
strata,  by  subsequent  subsidence,  other 
strata  may  be  unconformably  depos- 
ited (Fig.  204). 


FIG.  204.— Unconformity  on  Faulted  Strata. 


Law  of  Slip. — In  faults  the  plane  of  fracture  is  sometimes  vertical, 
but  much  more  generally  it  is  more  or  less  inclined.  In  such  cases,  in 
by  far  the  larger  number  of  great  faults,  the  strata  on  the  upper  side 
(hanging  wall)  of  the  fracture  have  dropped  down,  while  the  strata  on 
the  lower  side  (foot-wall)  have  gone  up,  as  in  Figs.  205  and  206.  This 
would  probably  be  the  case  if,  after  the  fracture,  the  relation  of  parts 
was  adjusted  by  gravity  alone.  In  some  cases  of  strongly-folded  strata, 
however,  the  hanging  wall  seems  to  have  been  pushed  and  made  to 
slide  upward  over  the  foot-wall  as  if  by  powerful  horizontal  squeezing. 
This  is  the  case  with  the  great  slip  in  Southwestern  Virginia,  repre- 
sented in  Fig.  199.  Examples  of  this  kind,  however,  are  exceptional. 
In  several  hundred  cases  of  great  fissures,  examined  by  Phillips,  in 


MINERAL  VEINS. 


225 


England,  nearly  all  followed  the  law  given  above.1     Fig.  205  is  a  sec- 
tion across  Yarrow  Colliery,  in  which  all  the  slips  follow  this  law.     Of 


FIG.  205.— Section  across  Yarrow  Colliery,  showing  the  Law  of  Faults  (after  De  la  Beche). 

the  numerous  slips  figured  by  Powell,  Gilbert,  and  Howell,  as  occurring 
in  the  Plateau  and  Basin  Range  region,  nearly  all  follow  this  law.  Fig. 
206  is  a  section  illustrating  this  fact. 


East  *— 

FIG.  206.— Section  of  Pahranagat  Range,  Nevada,  showing  the  Law  of  Faults  (after  Gilbert). 

SECTION  2. — MINERAL  VEINS. 

All  rocks,  but  especially  metamorphic  rocks  in  mountain-regions, 
are  seamed  and  scarred  in  every  direction,  as  if  broken  and  again 
mended,  as  if  wounded  and  again  healed.  All  such  seams  and  scars,  of 
whatever  nature  and  by  whatever  process  formed,  are  often  called  by 
the  general  name  of  veins.  It  is  better,  however,  that  dikes  and  so- 
called  granite-veins,  or  all  cases  of  fissures  filled  at  the  moment  of 
formation  by  igneous  injection,  should  be  separated  from  the  category 
of  veins.  True  veins,  then,  are  accumulations,  mostly  in  fissures,  of 
certain  mineral  matters  usually  in  a  purer  and  more  sparry  form  than 
they  exist  in  the  rocks.  The  accumulation  has  in  all  cases  taken  place 


Kinds. — Thus  limited,  veins  are  of  three  kinds  :  Veins  of  segrega- 
tion, veins  of  infiltration,  and  great  fissure-veins.  These  three,  how- 
ever, graduate  into  each  other  in  such  wise  that  it  is  often  difficult  to 
determine  to  which  we  must  refer  any  particular  case.  Some  writers 
make  many  other  kinds. 

1.  Veins  of  Segregation. — In  these  the  vein-matter  does  not  differ 
greatly  from  the  inclosing  rock.  Such  are  the  irregular  lines  of  granite 
in  granite,  the  lines  differing  from  the  inclosing  rock  only  in  color  or 
texture ;  also  irregular  veins  of  feldspar  in  granite  or  in  gneiss.  Under 
the  same  head  belong  also  the  irregular  streaks,  clouds,  and  blotches,  so 

1  Phillips's  "  Geology,"  p.  35. 
,  15 


226  STRUCTURE  COMMON  TO  ALL  ROCKS. 

common  in  marble.  In  these  cases  there  seems  to  be  no  distinct  line  of 
separation  between  the  vein  and  the  inclosing  rock — no  distinct  wall  to 
the  vein.  The  reason  is,  these  veins  are  not  formed  by  the  filling  of  a 
previously-existing  fissure,  but  by  the  segregation  of  certain  materials, 
in  certain  spots  and  along  certain  lines,  from  the  general  mass  of  the 
rock,  either  when  the  latter  was  in  plastic  condition  from  heat  and 
water,  or  else  by  means  of  percolating  water,  somewhat  as  concretions 
of  lime,  clay,  iron-ore,  and  flint,  are  formed  in  the  strata  (p.  188). 

2.  Veins  of  Infiltration. — Metamorphic  rocks  have,  probably  in  all 
cases,  been  subjected  to  powerful  horizontal  pressure.    Besides  the  wide 
folds  into  which  such  rocks  are  thus  thrown  and  the  great  fissures  thus 
produced,  the  strata  are  often  broken  into  small  pieces  by  means  of  the 
squeezing  and  crushing.     The  small  fissures  thus  produced  are  often 
filled  by  lateral  secretion  from  the  walls,  or  else  by  slowly-percolating 
waters  holding  in  solution  the  more  soluble  matters  contained  in  the 
rocks.    The  process  is  similar  to  the  filling  of  cavities  left  by  imbedded 
organisms  (p.  193),  and  still  more  to  the  filling  of  air-blebs  in  traps  and 
lavas,  and  the  formation  of  agates  and  carnelian  amygdules  (p.  211). 
In  veins  of  this  kind,  therefore,  a  beautiful  ribbon-structure  is  often 
produced  by  the  successive  deposition  of  different-colored  materials  on 
the  walls  of  the  fissure.     Veins  of  this  kind  also,  since  they  are  the 
filling  of  a  previously-existing  fissure,  have  distinct  walls.     The  filling 
consists  most  commonly  of  silica  or  of  carbonate  of  lime. 

3.  Fissure  -  Veins. — These  are  fillings  of  the  great  fissures  produced 
by  movements  of  the  earth's  crust.     When  these  fissures  are  filled  at 
the  time  of  formation  by  igneous  injection,  they  are  called  dikes ;  but 
if  subsequently  with  mineral  matter,  by  a  different  process,  to  be  dis- 
cussed hereafter,  they  are  fissure-veins.      These  veins,  therefore,  like 
dikes,  outcrop  over  the  surface  of  the  country  often  for  many  miles, 
fifty  or  more.     Like  dikes,  also,  they  are  often  many  yards  in  width, 
and  extend  to  unknown,  but  certainly  very  great,  depths.     Like  dikes 
and  fissures,  also,  they  occur  in  parallel  systems. 

Characteristics. — The  most  obvious  characteristics  of  the  veins  of 
this  class  are  their  size,  their  continuity  for  great  distances  and  to 
great  depths,  and  their  occurrence  in  parallel  systems.  As  the  vein 
is  a  filling  of  a  previously-existing  fissure,  the  distinction  between  the 
vein  and  the  wall-rock  is  usually  quite  marked.  In  many  cases,  in  fact, 
the  vein-filling  is  separated  from  the  wall-rock  by  a  layer  of  earthy  or 
clayey  matter  called  a  selvage,  as  if  the  sides  of  the  open  fissure  had 
become  foul  by  percolating  waters  carrying  clay,  before  the  fissure  was 
filled  with  mineral  matter.  The  contents  of  fissure-veins  are  also  far 
more  varied  than  those  of  other  classes. 

Irregularities. — Although  more  regular  than  other  kinds,  yet  fis- 
sure-veins are  also  often  quite  irregular — sometimes  branching,  some- 


MINERAL   VEINS. 


227 


times  narrowing  or  pinching  out  in  some  parts  and  widening  in  others 
(Fig.  207),  sometimes  dividing  and  again  coming  together,  and  thus 
inclosing  a  portion  of  the  wall-rock  (Fig.  208).  Such  an  inclosed  mass 
of  country  rock  in  the  midst  of  a  vein  is  called  a  "  horse"  Many  of 
these  irregularities  are  probably  the  result  of  movements  after  the  fis- 
sure was  formed,  or  even  after  it  was  filled.  Thus,  it  abed  (Fig.  207) 
be  one  wall  of  an  irregular  vein,  then  it  is  probable  that  a'  b'  c  d'  was 
the  original  position  of  this  wall ;  but,  before  it  was  filled,  it  slipped  up 


FIG.  20T. -Irregularities  in  Veins. 


Fto.  208. — Irregularities  of  Veins. 


to  its  present  position.  Again,  movements  may  reopen  a  fissure  after 
it  is  filled.  In  such  cases,  if  the  adhesion  of  the  filling  to  the  wall  is 
strong,  portions  of  the  wall- rock  are  torn  away;  and  if  a  second  filling 
takes  place,  a  " horse "  is  formed.  Thus  aaa  and  bbb  (Fig.  208) 
represent  the  two  original  walls  of  an  irregular  vein ;  but  subsequent 
movement  reopened  the  fissure  to  b'  b'  b'  and  tore  away  the  horse  H, 
after  which  the  vein  was  again  filled. 

Veins,  of  course,  usually  intersect  the  strata;  but  in  some  cases 
where  strata-planes,  or  else  cleavage-planes,  are  highly  inclined,  the 
opening  is  between  these  planes,  and  the  veins  are,  therefore,  conform- 
able with  them. 

Metalliferous  Veins. — Some  metals,  particularly  iron,  occur  prin- 
cipally in  great  beds,  being  accumulated  by  a  process  already  described 
(p.  136).  Others,  especially  lead,  often  accumulate  in  flat  cavities  be- 
tween the  strata,  especially  of  limestone.  But  most  metals  occur  in 
veins.  All  the  kinds  of  veins  mentioned  above  may  contain  metals,  but 
the  segregative  veins  are  usually  too  irregular  and  uncertain,  and  the 
infiltrative  veins  too  small,  to  be  profitable.  True,  profitable  metal- 
liferous veins  are  almost  always  great  fissure-veins.  We  will  speak, 
therefore,  only  of  these,  and  the  further  description  of  fissure-veins  is 
best  undertaken  under  this  head. 

Contents. — The  contents  of  metalliferous  veins  are  of  two  general 


228  STRUCTURE  COMMON  TO  ALL  ROCKS. 

kinds,  viz.,  vein-stuffs  and  ores.  The  principal  vein-stuffs  are  quartz, 
carbonate  of  lime  (calc-spar),  carbonate  of  baryta,  carbonate  of  iron, 
sulphate  of  baryta  (heavy  spar),  and  fluoride  of  calcium  (fluor-spar). 
By  far  the  most  common  of  these  is  quartz,  and  next  is  calc-spar. 
Often,  however,  the  vein-stuff  is  an  aggregate  of  minerals  forming  a  true 
rock.  Nearly  the  whole  of  a  vein  consists  usually  of  vein-stuff.  The 
ore  exists  in  comparatively  small  quantities,  sometimes  forming  a  cen- 
tral rib  or  sheet,  as  if  deposited  last  (Fig.  209) ;  sometimes  in  irregular 
isolated  masses  called  bunches  or  pockets,  or  in  small  strings,  or  grains, 
irregularly  scattered  through  the  vein-stuff  and  extending  often  a  little 
way  into  the  wall-rock. 

The  chemical  forms  in  which  metals  occur  are  very  various ;  some- 
times they  occur  as  pure  metal  (as  always  in  the  case  of  gold  and  plat- 
inum, and  sometimes  in  the  case  of  silver  and  copper),  but  more  com- 
monly in  the  form  of  metallic  sulphides,  metallic  oxides,  and  metallic 
carbonates.  Of  these  the  metallic  sulphides  are  by  far  the  most  com- 
mon. It  is  worthy  of  remark  that  all  these  forms  are  comparatively 
very  insoluble.  The  same  is  true  of  the  vein-stuffs. 

Ribboned  Structure. — The  ribboned  or  banded  structure,  already 
spoken  of  under  Veins  of  Infiltration,  is  very  commonly  found  in  great 
fissure-veins.  This  structure  is  as  characteristic  of  veins  as  the  colum- 
nar structure  is  of  dikes.  The  layers  on  the  two  sides  usually  corre- 
spond to  each  other  (Fig.  209)  ;  sometimes  the  successive  layers  are  of 
different  color,  giving  rise  to  a  beautiful,  striped  appearance.  Some- 


times the  successive  layers  on  both  sides  are  of  different  materials,  as 
in  Fig.  210,  in  which  the  central  rib,  <7,  is  galena,  and  a  or,  b  b,  c  c,  are 
successive  layers  of  quartz,  fluor,  and  baryta.  Sometimes,  in  cases  of 
quartz-filling,  the  layers  are  agate,  except  the  centre,  which  is  filled  up 
with  a  comb  of  interlocking  crystals,  as  in  Fig.  211.  The  same  occurs 
often  in  amygdules,  the  last  filling  being  crystalline.  Sometimes  there 
is  evidence  of  successive  openings  and  fillings,  as  in  Fig.  212,  where  a 
represents  quartz  crystals,  interlocking  in  the  centre  and  based  on  agate 
layers,  b  b,  while  c  represents  quartz  with  disseminated  copper  pyrites. 


MINERAL  VEINS. 


229 


In  this  case  it  seems  probable  that  1  and  2  were  the  walls  when  the 
agate  and  quartz  filling  took  place,  and  that  afterward  the  fissure  was 


I      CL     I 


FIG.  211. 

reopened  along  2,  so  that  the  walls  became  2  and  3,  and  the  new  fis- 
sure thus  formed  was  filled  with  cuprif- 
erous quartz.  The  same  is  well  shown 
in  Fig.  213,  where  a,  #,  c,  <?,  e,/",  are 
successive  quartz-combs,  separated  by 
2,  3,  4,  5,  6,  which  are  clay  selvages,  and 
therefore  old  walls. 

Age. — The  relative  age  of  veins  in 
the  same  region  is  determined  in  the 
same  way  as  that  of  dikes,  viz.,  by  the 
manner  in  which  they  intersect  each  other;  the  intersecting  vein  being, 
of  course,  younger  than  the  intersected  vein.  Thus  in  Fig.  214,  which 
is  a  section  of  a  hill-side  in  Cornwall,  it  is  evident  that  the  tin-vein,  «, 
is  the  oldest,  since  it  is  intersected  and  slipped  by  all  the  others.  The 
copper-vein,  #,  is  older  than  the 
clay-filled  fissure,  c.  There  is 
a  fourth  fissure,  d,  younger  than 
«,  but  its  relation  to  b  and  c  is 
not  shown  in  the  section. 

The  absolute  age  of  fissure- 
veins,  or  the  geological  period 
in  which  the  fissure  was  formed, 
can  only  be  determined  by  the 
stratified  rocks  through  which 
it  breaks.  The  lead-veins  of 
Cornwall  (b  b,  Fig.  216)  break 

through  the  Cretaceous.  Their  fissures  were  probably  formed  by  the 
changes  or  oscillations  which  closed  the  Cretaceous  and  inaugurated  the 
Tertiary  period.  The  auriferous  veins  of  California  break  through  the 
Jurassic ;  and,  as  there  are  good  reasons  for  believing  that  the  Sierras 
were  formed  at  the  end  of  the  Jurassic,  it  is  probable  that  these  fissures 


230  STRUCTURE  COMMON  TO  ALL  ROCKS. 

were  formed  at  that  time,  by  the  foldings  of  the  strata  consequent  upon 
the  pushing  up  of  this  range.  The  filling,  of  course,  was  a  slow,  sub- 
sequent operation,  but  commenced  then. 

Surface-Changes. — Mineral  veins  seldom  or  never  outcrop  on  the 
surface  in  the  condition  we  have  described  them.  On  the  contrary, 
there  are  certain  changes  which  they  undergo  through  the  influence  of 
atmospheric  agencies,  which  render  their  appearance  along  their  out- 
crop quite  different  from  that  of  the  same  vein  at  some  depth  below. 
A  knowledge  of  these  changes  is,  of  course,  of  the  greatest  practical 
importance.  They  are,  however,  extremely  various,  differing  not  only 
according  to  the  metallic  contents,  but  also  according  to  the  nature  of 
the  vein-stuffs,  and  therefore  must  be  learned  by  observation  in  each 
country.  We  will  give  three  of  the  most  constant  as  illustrations. 

Cupriferous  Veins. — The  original  form  in  which  copper  seems  to 
exist  in  veins  is  copper  pyrites,  a  double  sulphide  of  copper  and  iron 
(CuFeaSa).  Now,  along  the  back  or  outcrop  of  copper-veins,  to  a  depth 
of  thirty  to  sixty  feet,  the  vein  usually  contains  no  copper  at  all,  but 
consists  of  vein-stuff  (more  or  less  changed,  according  to  its  nature), 
among  which  are  scattered  masses  of  a  dark  reddish  or  brownish 
hydrated  peroxide  of  iron,  in  a  light,  spongy  condition.  This  peculiar 
form  of  peroxide  of  iron,  so  characteristic  of  the  outcrop  of  copper- 
veins,  is  called  by  the  Cornish  miners  gossan,  and  by  the  German  and 
French  miners  iron  hat  (eiserner  hut ;  chapeau  defer).  Below  the  influ- 
ence of  atmospheric  agencies  the  vein  is  in  its  original  condition,  i.  e., 
consists  of  vein-stone  containing  disseminated  masses  of  copper  pyrites. 
Just  at  the  junction  of  the  changed  with  the  unchanged  vein — i.  e.,  run- 
ning along  the  back  of  the  vein 
at  a  depth  varying  from  thirty 
to  sixty  feet — occur  rich  accu- 
mulations of  copper,  as  native 
copper,  red  and  black  oxides  of 

coPP"'Sreen  and  """  carbon- 
ates  of  copper,  etc.  These  facts 
are  illustrated  by  Fig.  215, 

which  *  a  section  Of  the  DUCk- 

town  mines  of  Tennessee.  The 
irregular  line,  s  s,  is  the  outline 
of  a  hill,  along  the  crest  of 

FIG.  215.-Ducktown  (Tennessee)  Copper- Vein,  showing    which    the    Vein    Outcrops  ;    the 
Surface-Changes  (after  Safford).  \     ' 

part  b  consists  almost  wholly  of 

gossan,  with  only  small  masses  of  quartz-vein  stuff ;  a  is  the  rich  accu- 
mulation of  copper-ore,  here  about  two  or  three  feet  thick ;  and  c  is 
the  unchanged  vein,  consisting  of  vein-stuff,  inclosing  arsenical  pyrites, 
and  copper  pyrites  in  very  large  quantities. 


MINERAL  VEINS.  231 

These  phenomena  may  be  explained  as  follows  :  There  can  be  no 
doubt  that  the  gossan  represents  copper  pyrites,  from  which  the  copper 
has  been  entirely  washed  out,  leaving  the  iron  in  an  oxidized  condition. 
Thus  the  whole  of  the  copper  from  b  (and  probably  from  much  more 
than  5,  for  the  process  of  denudation  has  gone  on  pari  passu  with  the 
process  of  leaching)  has  been  leached  out  and  accumulated  at  a.  Fur- 
ther, it  is  probable  that  the  process  was  as  follows :  When  copper 
pyrites  is  exposed  to  moist  air,  the  copper,  iron,  and  sulphur,  are 
all  oxidized,  forming  sulphate  of  copper  and  oxide  of  iron,  thus  : 
2(CuFeSa)  +  HO=2CuSO4  +  Fe2O3.  The  soluble  sulphate  is  then  washed 
out  and  carried  downward,  while  the  insoluble  iron  peroxide  is  left 
in  a  spongy  condition.  This  much  seems  certain,  but  by  what  sub- 
sequent process  the  copper  takes  all  the  forms  actually  found  at  a,  is 
little  understood,  although  it  is  probable  that  the  carbonate  is  produced 
by  the  reaction  on  the  sulphate  of  waters  containing  alkaline  carbonate 
or  bicarbonate  of  lime.1 

PlumMferous  Veins. — The  natural  or  original  form  in  which  lead 
occurs  in  veins  is  sulphide  of  lead,  or  galena.  But  along  the  backs  or 
outcrops  of  lead-veins  it  is  found  more  commonly  as  carbonate.  The 
explanation  seems  to  be  as  follows :  Lead  occurs  mostly  in  veins  inter- 
secting, or  in  sheets  between,  strata  of  limestones.  It  is  probable  that 
the  galena  (PbS)  is  oxidized  by  meteoric  agencies  and  becomes  sulphate 
(PbSOJ,  and  then  the  sulphate,  by  reaction  with  the  carbonate  of  lime 
derived  from  the  wall-rock  or  from  the  calc-spar  of  the  vein-stuff,  be- 
comes carbonate,  thus:  PbSO4+CaCO3=PbCO3  +  CaS04.  In  proof 
of  this  process  it  is  stated 2  that  galena,  thrown  out  of  the  old  mines  of 
Derbyshire  among  rubbish  of  limestone,  has  all,  in  the  course  of  ages, 
been  changed  into  carbonate. 

Auriferous  Quartz- Veins. — Gold  is  found  either  in  quartz-veins  in- 
tersecting metamorphic  slates  (quartz-mines)  or  in  gravel-drifts  in  the 
vicinity  of  these  (placer-mines).  Originally  it  existed  in  the  quartz- 
veins  usually  associated  with  metallic  sulphides,  particularly  the  sul- 
phide of  iron  (pyrites).  If  the  pyrites  be  dissolved  in  nitric  acid,  the 
gold  is  left  as  minute  threads  and  crystals.  Evidently,  therefore,  it  exists 
in  minute  threads  and  crystals  scattered  through  the  pyrites.  Now, 
when  such  a  vein  is  exposed  to  meteoric  agencies,  the  pyrites  is  oxi- 
dized, partly  as  soluble  sulphate,  and  carried  away,  and  partly  as  insol- 
uble reddish  peroxide,  which  remains.3  The  quartz-vein  stone  is,  there- 
fore, left  in  a  honey-comb  condition  by  the  removal  of  the  pyrites,  and 

1  Bischof,  "  Chemical  and  Physical  Geology."  vol.  iii.,  p.  509. 

2  De  la  Beche,  "  Geological  Observer,"  p.  794. 

3  Probably  the  iron  sulphide  is  oxidized  to  the  condition  of  sulphate,  then  reduced  to 
carbonate  by  water  containing  alkaline  carbonate  or  bicarbonate  of  lime,  and  lastly  per- 
oxidized  by  exchanging  carbonic  acid  for  oxygen  (Bischof). 


232  STRUCTURE  COMMON  TO  ALL  ROCKS. 

more  commonly  stained  of  a  rusty  color  by  the  peroxide.  Among  the 
cells  of  this  rusty  j  cellular  quartz  the  gold  is  found  in  minute,  sharp 
grains,  evidently  left  by  the  removal  of  the  pyrites.  Hence,  in  an 
auriferous  quartz-Vein,  along  the  outcrop  to  a  depth  of  thirty  to  sixty 
feet  (i.  e.,  as  far  as  meteoric  agencies  extend),  gold  is  found  free  in 
small  grains  among  the  cellular  quartz ;  but  below  the  reach  of  these 
agencies  it  is  inclosed  in  the  undecomposed  pyrites. 

Placer-Mines. — If  a  mountain-slope,  along  which  outcrop  auriferous 
quartz-veins,  be  subjected  to  powerful  erosion  by  water-currents,  then 
in  the  stream-beds  will  be  found  gravel-drifts,  composed  partly  of  the 
country  rock  and  partly  of  the  quartz  vein-stone.  Among  the  gravel 
will  be  found  particles  of  gold,  washed  out  from  the  upper  parts  of  the 
veins.  By  the  sorting  power  of  water  the  heavy  gold  particles  are  apt 
to  accumulate  mostly  near  the  bed  of  the  gravel-deposit  (bed-rock). 
These  gravel-deposits  are  the  placers.  In  these,  the  gold-particles,  like 
the  stone-fragments,  are  always  rounded  and  worn  by  attrition. 

Some  Important  Laws  affecting  the  Occurrence  and  the  Richness  of 
Metalliferous  Veins. 

1.  Metalliferous  veins  occur  mostly  in  disturbed  and  highly-meta- 
morphic  regions,  where  the  strata  are  tilted,  and  folded,  and  metamor- 
phosed.    The  tilting  and  folding  are  necessary  to  the  formation  ot  fis- 
sures; and  the  conditions  under  which  metamorphism  takes  place  seem 
necessary  for  the  subsequent  filling  with  mineral  matter.     Mineral  veins, 
therefore,  occur  mostly  in  mountain-regions,  and  in  the  vicinity  of  more 
or  less  obvious  evidences  of  igneous  agency.     Lead-veins  seem  to  be  an 
exception  to  this  rule.     They  are  often  found  in  undisturbed  regions, 
-where  the  rocks  are  entirely  unchanged.     The  rich  lead-mines  of  Illinois, 
Iowa,  and  Missouri,  are  notable  examples,  the  country  rock  being  hori- 
zontal, fossiliferous  limestones  of  the  Palaeozoic  era. 

2.  Metalliferous  veins  occur  mostly  in  the  older  rocks.     In  Great 
Britain,  for  example,  no  profitable  veins  occur  above  the  Trias.     This 
rule,  which  was  regarded  as  of  great  importance  by  the  older  geologists, 
is  not  so  regarded  now.     There  seems  to  be  no  close  connection  between 
the  occurrence  of  metalliferous  veins  and  simple  age  alone ;  the  con- 
nection is  rather  with  metamorphism.     Metamorphism,  as  we  have  seen, 
(p.  213),  is  most  common  in  the  older  rocks,  and  becomes  more  and 
more  exceptional  as  we  pass  upward.     The  occurrence  of  metalliferous 
veins  follows  the  same  law.     But  when  the  newer  rocks  are  metamor- 
phic,  they  are  as  likely  to  contain  veins  as  are  rocks  of  the  older  series. 
The  metalliferous  veins  of  California  occur  in  Jurassic,  Cretaceous,  and 
even  Tertiary  strata ;  but  these  strata  are  there  highly  metamorphic,  and 


LAWS  AFFECTING  METALLIFEROUS  VEINS.  233 

strongly  folded.     In  Bohemia,  also,  and  elsewhere,  metalliferous  veins 
occur  in  the  higher  series  (Phillips,  p.  549). 

3.  Parallel  veins  are  apt  to  have  similar  metallic  contents,  while 
veins  running  in  different  directions  (unless  at  right  angles)  are  apt  to 
contain  different  metallic  contents.  Thus,  the  nearly  east-and-west 
lodes  of  Cornwall,  #,  #,  a  (Fig.  216),  contain  tin  and  copper,  while  the 


FIG.  216. — Map  of  Cornwall :  a,  tin  and  copper ;  6,  lead  and  iron. 

north-and-south  courses,  b  b,  contain  lead  and  iron.  The  auriferous 
veins  of  California  are  parallel  to  each  other  and  to  the  Sierras,  except 
a  few  smaller  ones,  which  are  at  right  angles  to  these.  The  reason  of 
this  rule  is,  that  parallel  fissures  belong  to  the  same  system,  and  were 
therefore  formed  at  the  same  time,  broke  through  the  same  strata,  and 
were  filled  under  similar  conditions,  and  therefore  with  the  same  mate- 
rials ;  while  fissures  running  in  different  directions  (unless  in  some  cases 
at  right  angles,  p.  221)  were  probably  formed  at  different  times,  broke 
through  different  strata,  and  were  filled  under  different  conditions. 
Thus,  the  east-and-west  veins  of  Cornwall  break  only  through  the 
Trias,  while  the  north-and-south  veins  break  through  the  Cretaceous. 
The  auriferous  veins  of  California  all  break  through  the  Jurassic  ;  they, 
or  their  fissures,  were  all  produced  at  the  same  time,  viz.,  at  the  time  of 
pushing  up  of  the  Sierras. 

4.  A  change  of  country  rock  of  an  outcropping  vein  is  apt  to  deter- 
mine some  change,  either  in  the  contents  or  in  the  richness  of  the  vein. 
Nevertheless,  there  is  not  that  close  connection  between  the  nature  of 
the  country  rock  and  the  vein-contents  which  obtains  in  infiltrative 
veins.  The  reason  is,  that  infiltrative  veins  derive  their  contents  en- 
tirely from  the  wall-rock  on  either  side,  while  fissure-veins  derive  their 
contents  from  all  the  strata  through  which  they  break,  even  to  great 
depths,  and  especially  from  the  deeper  strata.  The  nature  of  the  sur- 


234  STRUCTURE   COMMON  TO  ALL   ROCKS. 

face  or  country  rock  is,  therefore,  only  one  factor,  determining  the  vein- 
contents. 

5.  Metallic  veins  are  usually  richer  near  their  point  of  intersection 
with   granite  or  with  an  igneous  dike,  especially  if   the   strata  have 
suffered  metamorphism.     This  shows  the  influence  of  such  heat  as  is 
present  in  metaraorphism,  in  determining  the  metallic  contents. 

6.  If  two  veins  cross  each  other,  especially  if  at  small  angle,  one  or 
both  are  apt  to  be  richer  at  the  point  of  crossing.     No  sufficient  reason 
has  been  given  for  this  law.    It  is  probably  due  to  the  reaction  of  waters 
bearing  different  materials  circulating  in  the  two  fissures. 

7.  Since  veins  are  the  fillings  of  fissures,  they  are  often  slipped  by 
each  other  or  by  dikes  or  by  simple  unfilled  fissures.     If  a  metalliferous 
vein  is  thus  slipped,  according  to  the  law  of  slips  already  given  (p.  224) 
the  foot-wall  of  the  vein  has  usually  gone  upward,  and  the  hanging  wall 
dropped  downward.      The  great  importance  of  this  law  in  practical 
mining  is  sufficiently  obvious.     All  the  slips  of  Fig.  214,  except  that 
made  by  the  fissure  c,  follow  this  law. 

8.  The  sitrf ace-indications  are  to  be  learned  by  attentive  observa- 
tion in  each  case.     We  have  already  given  these  in  the  case  of  copper, 
lead,  and  gold. 

Theory  of  Metalliferous  Veins. 

Our  knowledge  of  the  conditions  under  which,  and  the  chemical  pro- 
cess by  which,  fissures  have  been  filled  with  mineral  matter,  is  yet,  un- 
fortunately, very  imperfect.  Many  vague  and  crude  theories  have  been 
proposed.  Some  have  supposed  that  they  have  been  filled  in  the  man- 
ner of  dikes  and  granite  veins,  by  igneous  injection ;  others,  that  these 
fissures,  opening  below  into  the  regions  of  incandescent  heat,  have  been 
filled  by  sublimation,  i.  e.,  by  vaporization  of  certain  materials  and 
their  condensation  in  the  fissures  above.  Some  suppose  that  electric 
currents,  such  as  are  known  by  observation  to  traverse  certain  veins, 
have  been  the  chief  agents  in  the  transference  and  accumulation  of  the 
mineral  matter.  Still  others  have  thought  that  great  fissures  have  filled 
in  the  same  manner  as  the  smaller  fissures,  and  cavities  of  every  kind 
found  in  the  rocks,  viz.,  by  infiltration  of  soluble  matters  from  the  fis- 
sured rocks.  There  is  certainly  considerable  analogy  between  small 
in filtrative  veins  and  great  fissure-veins  in  their  mode  of  formation;  yet 
there  is  a  decided  difference.  The  fillings  of  infiltrative  veins  are  de- 
rived, in  each  part,  entirely  from  the  bounding  rock  on  either  sid.e.  The 
fissure  is  filled  by  a  lateral  secretion  from  its  walls  ;  the  broken  rocks 
heal  themselves  "  by  first  intention  "  by  means  of  a  plasma  oozing  from 
the  sides.  But  great  fissure-veins  derive  their  contents  in  each  part 
from  all  the  strata  to  great  depths,  and  especially  from  the  deeper 
strata.  Hence  the  contents  of  these  veins  are  far  more  varied. 


THEORY  OF  METALLIFEROUS  VEIXS.  235 

Outline  of  the  Most  Probable  Theory. — The  contents  of  mineral 
veins  seem  to  have  been  deposited  from  hot  alkaline  solutions  coming 
up  through  the  fissures ;  in  other  words,  from  hot  alkaline  springs.  We 
will  attempt  to  show  this  first  for  the  vein-stuffs,  especially  quartz,  and 
then  for  the  metallic  ores,  especially  the  metallic  sulphides. 

Vein-Stuffs. — 1.  They  were  deposited  from  solutions,  (a.)  The 
ribbon-structure  and  the  interlocked  crystals  (Fig.  211)  suggest  at  once 
successive  deposition  from  solution,  especially  as  a  similar  structure 
occurs  in  the  fillings  of  cavities  of  all  kinds,  which  could  not  have  been 
filled  in  any  other  way.  (b.)  Quartz  is  by  far  the  most  common  of  all 
vein-stuffs.  Now,  as  already  explained  (p.  217),  there  are  two  varieties 
of  quartz — one  having  a  specific  gravity  of  2.2,  the  other  2.6.  The  dry 
way  produces  only  quartz-glass,  which  has  a  specific  gravity  of  2.2, 
while  the  variety  of  specific  gravity  2.6  cannot  be  formed  except  by  the 
humid  way.  In  fact,  this  variety,  as  far  as  we  know,  is  always  produced 
by  slow  deposition  from  solution.  Now,  the  quartz  of  veins  is  always 
the  variety  2.6,  and  therefore  was  produced  by  slow  deposit  from  solu- 
tion. The  beautiful  crystals  so  often  found  in  veins  could  be  produced 
in  no  other  way.  (c.)  We  have  already  seen  (p.  218)  that  fluid 
cavities  are  a  proof  of  formation  by  humid  process.  Now,  such  fluid 
cavities  are  especially  abundant  in  vein-stuffs  generally.  They  are 
best  seen  in  quartz-vein  stuffs,  because  of  their  transparency.  (<?.)  Not 
only  quartz,  but  many  other  minerals  found  among  vein-stuffs  are  of 
such  nature  that  it  is  difficult  or  impossible  to  understand  how  they 
could  have  been  formed  except  by  the  humid  way,  as  they  will  not 
stand  fusing  temperature. 

2.  The    solutions  were  hot.     (a.)  Fissures   running  deep  into  the 
interior  of  the  earth  could  hardly  remain  empty  of  water.     But  from 
their   great   depth  the  contained  waters  must  be  hot.      The  solvent 
power  of  water,  when  heated  to  high  temperature  under  pressure,  is 
well  known.     Scarcely  any  substance  wholly  resists  it.     (b.)  The  fluid 
cavities  found  in  quartz  and  other  vein-stuffs  are  not  usually  entirely 
filled,  but  contain  a  small  vacuous  space.     Such  a  vacuous  space  in- 
dicates (p.  218)  that  the  inclosed  liquid  was  at  high  temperature  at  the 
time  of  being  inclosed,  and  has  since  contracted  on  cooling.     By  heat- 
ing the  mineral  until  the  cavity  fills  and  the  vacuous  space  disappears, 
we  ascertain  the  temperature  of  deposit.      Now,  by  this  process  the 
temperature  of  deposit  of  vein-minerals  has  been  ascertained  to  vary 
from  ordinary  temperatures  even  up  to  300°  and  350°. J     (c.)  The  in- 
variable association  of  metalliferous  veins  with  metamorphism  demon- 
strates the  agency  of  heat. 

3.  The  solutions  were  alkaline.     Alkaline  carbonates  and  alkaline 

1  Sorby,  Philosophical  Magazine,  vol.  xv.,  p.  152 ;  Quarterly  Journal  of  the  Geological 
Society,  vol.  xiv.,  p.  453,  et  seq. 


236  STRUCTURE  COMMON  TO  ALL  ROCKS. 

sulphides  are  the  only  natural  solvents  of  quartz,  the  commonest  of 
vein-stuffs.  Moreover,  when  these  waters  contain  excess  of  carbonic 
acid,  as  is  almost  always  the  case,  they  dissolve  also  the  carbonates  of 
lime,  baryta,  iron,  etc.,  the  next  most  common  forms  of  vein-stuffs.  In 
California  and  Nevada  such  alkaline  carbonate  and  alkaline  sulphide 
springs  abound,  and  are  daily  depositing  silica  (quartz)  and  carbonates 
of  lime  and  of  iron,  and  even  in  some  cases  filling  fissures. 

Metallic  Ores. — There  seems  no  reason  to  doubt,  then,  that,  in  most 
cases  at  least,  vein-stuffs  have  been  deposited  from  hot  alkaline  solu- 
tions. Now,  it  is  evident,  from  their  intimate  association  with  the  vein- 
stuffs,  that  the  metallic  ores  must  have  been  deposited  from  the  same 
solution.  The  exact  nature  of  the  solvent  and  the  chemical  reaction 
is  still  very  doubtful.  We  may  imagine  many  by  either  of  which  the 
deposit  might  take  place :  1.  Metallic  sulphides  are  by  far  the  most 
common  form  of  ore,  and  even  when  other  forms  exist  we  may  in  many 
cases  trace  them  to  sulphide  as  their  original  form  (p.  230,  et  seq.).  But 
metallic  sulphides  are  soluble  in  alkaline  sulphides,  and  these  latter  are 
often  found  associated  with  alkaline  carbonates  in  hot  springs,  as  in 
California  and  elsewhere.  Such  waters  would  hold  in  solution  silica, 
carbonates  of  lime,  etc.,  and  metallic  sulphides,  and,  coming  up  through 
fissures,  would  deposit  them  by  cooling  in  the  fissures.  Or,  2.  Alka- 
line carbonate  waters  holding  in  solution  silica  and  lime  carbonate  for 
vein-stone,  and  also  containing  alkaline  sulphide,  meeting  and  min- 
gling in  the  same  fissure  with  other  waters  containing  metallic  sul- 
phates, by  reaction  would  precipitate  metallic  sulphides  ( NaS  + 
MSO4=NaS04  +  MS).  This  seems  to  be  the  reaction  by  which  the 
inky  waters  of  some  of  the  hot  springs  of  the  California  geysers  are 
formed.  Or,  3.  The  alkaline  carbonates  still  remaining  for  vein- 
stone, metallic  sulphates,  in  solution  in  the  same  waters  with  organic 
matter  (and  all  meteoric  waters  contain  organic  matter  in  solution), 
would  be  reduced  to  the  form  of  metallic  sulphide,  which,  being  insol- 
uble, would  be  deposited.1  For  greater  clearness  we  annex  a  table  ex- 
pressing these  processes : 

f  Alk.S  +  MS  in  sol0  deposit  MS  by  cooling. 
Alk.CO3  +  HCO3+  J  Alk.S  +  MSO4  meeting  "    MS  "  reaction. 

{ MSO4  +  org°  matr  "    MS   "  reduction. 

There  are  many  difficulties  in  the  way  of  every  attempt  to  place 
these  reactions  in  a  clear  and  distinct  form,  but  in  spite  of  these  diffi- 
culties there  seems  little  reason  to  doubt  that  great  fissures  have  been 

1  It  might  at  first  seem  that  there  is  a  chemical  difficulty  in  this  last  case — that  me- 
tallic  sulphate  cannot  coexist  in  solution  with  alkaline  carbonate,  but  would  be  precipi- 
tated as  metallic  carbonate.  But  it  is  evident  that  this  reaction  would  not  take  place  in 
a  weak  metallic  solution,  in  the  presence  of  excess  of  carbonic  acid,  since  in  this  case  the 
metallic  carbonate  is  soluble. 


THEORY   OF  METALLIFEROUS  VEINS.  237 

filled  by  deposit  from  hot  alkaline  waters  holding  various  mineral  sub- 
stances in  solution.  The  more  insoluble  substances  are  deposited  in 
the  vein,  while  the  more  soluble  substances  reach  the  surface  as  mineral 
springs. 

This  view  is  powerfully  supported  by  the  phenomena  of  hot  alkaline 
springs  in  California  and  Nevada.  The  Steamboat  Springs,  near  Vir- 
ginia City,  Nevada  (so  called  from  the  periodic  eruption  of  hot  water 
and  steam),  come  up  through  fissures  in  comparatively  recent  volcanic 
rock.  The  waters  are  strongly  alkaline,  and  deposit  silica  in  abundance. 
By  this  deposit  the  fissures  are  gradually  filling  up  and  forming  veins. 
Some  fissures  are  now  partially  and  some  entirely  filled.  The  ribbon- 
structure  in  some  cases  is  perfect.  Moreover,  sulphides  of  several  of 
the  metals,  viz.,  iron,  lead,  mercury,  copper,  and  zinc,  have  been  found 
in  the  quartz  vein-stuff.  Here,  then,  we  have  true  metalliferous  veins 
forming  under  our  very  eyes.1 

Thus,  then,  there  seems  no  longer  any  room  for  doubt  that  metallif- 
erous veins  are  actually  deposited  from  hot  alkaline  solutions  coming 
up  through  fissures.  It  is  only  the  exact  chemical  reaction  which  is  yet 
obscure.  The  work  of  the  geologist  is  all  but  complete ;  the  problem 
must  now  be  turned  over  to  the  chemist.  It  may  be  interesting,  how- 
ever, before  leaving  this  subject,  to  consider  separately  the  auriferous 
veins  of  California,  and  apply  to  them  the  principles  set  forth  above. 

Auriferous  Veins  of  California. — Gold  is  one  of  the  most  insoluble 
of  substances,  and  the  occurrence  of  this  metal  in  veins  has  always  been 
regarded  as  a  difficulty  in  the  way  of  the  solution  theory.  The  only 
free  solvent  of  gold  is  a  solution  of  free  chlorine ;  but  this  does  not 
exist  in  Nature.  Nevertheless,  gold  is  known  to  be  slightly  soluble  in 
the  salts,  especially  the  persalts  of  iron.  These  salts,  especially  the 
sulphate  and  persulphate  of  iron,  are  the  probable  solvents  of  gold. 
There  is  also  a  silicate  of  gold,  which,  according  to  Bischof,  is  slightly 
soluble  under  certain  conditions. 

There  is  abundant  evidence  that  the  auriferous  quartz-veins  of  Cali- 
fornia have  been  deposited  from  hot  solutions.  These  veins  exhibit  in 
many  cases  the  characteristic  ribbon-structure.  They  exhibit  also  the 
water-cavities  characteristic  of  deposits  from  solutions,  and  the  vacuous 
spaces,  indicating  that  the  solutions  were  hot.  By  actual  experiment,2 
the  temperatures  at  which  the  vacuous  spaces  disappear,  and  therefore 
at  which  the  deposit  took  place,  has  been  ascertained — being  180°, 
212°,  350°,  and  even  more.3  Again,  there  can  be  no  doubt  that  the 
associated  metallic  sulphides  were  deposited  from  the  same  solutions  as 
the  vein-stuffs,  for  they  are  completely  inclosed  in  the  latter.  But  the 

1  Arthur  Phillips,  American  Journal  of  Science,  vol.  xlvii.,  p.  194 ;  and  Philosophical 
Magazine,  1872,  vol.  xlil.,  p.  401. 

2  Arthur  Phillips,  ibid.  3  n,^ 


238  STRUCTURE   COMMON  TO  ALL  ROCKS. 

gold,  as  already  stated  (p.  231),  exists  as  minute  crystals  and  threads 
of  metal  inclosed  in  the  sulphide  of  iron,  and  must  therefore  have  been 
deposited  from  the  same  solution  as  the  iron.  It  seems  most  probable 
that  the  gold  was  dissolved  in  a  solution  of  sulphate  or  persulphate  of 
iron,  and  that  the  sulphate  was  deoxidized,  and  became  insoluble  sul- 
phide and  precipitated ;  and  that  the  gold  thus  set  free  from  solution 
was  entangled  in  the  sulphide  at  the  moment  of  the  precipitation  of 
the  latter. 

There  are  some  phenomena  connected  with  the  occurrence  of  gold 
in  the  iron  sulphides  of  the  deep  placers  which  seem  to  prove  the  truth 
of  this  view.1  The  deep  placers  of  California  are  gravel-drifts  in  ancient 
river-beds,  covered  up  by  lava-flows  100  to  200  feet  thick.  These  placers 


N  S 

FIG.  217.— Section  across  Table  Mountain,  Tuolumne  County.  California:  Z,  lava;  G,  gravel;  S,  slate; 
£,  old  river-bed ;  R  present  river-bed. 

are  worked  by  running  tunnels  beneath  the  basaltic  lava  until  the  river- 
gravel  is  reached.  Now,  the  waters  percolating  through  these  lava- 
flows  and  reaching  the  subjacent  gravels  are  charged  with  alkali  from 
the  lava.  These  alkaline  waters  are  also  charged  with  silica  from  the 
same  source.  Hence,  the  drift-wood  of  these  ancient  rivers  has  all 
been  silicified  by  these  siliceous  waters.  The  gravels  are  also  in  many 
places  cemented  by  the  same  material.  These  percolating  waters  have 
evidently  also  contained  sulphate  of  iron  ;  for  in  contact  with  the  sili- 
cified wood  is  often  found  iron  sulphide.  Thus,  while  the  wood  decayed 
it  was  partly  replaced  by  silica  and  partly  by  iron  sulphide  produced 
by  deoxidation  of  the  sulphate  by  organic  matter  (p.  192).  The  gravel 
has  also  in  some  places  been  cemented  by  iron  sulphide  reduced  from 
solution  in  a  similar  way.  Now,  both  in  this  petrifying  and  in  this 
cementing  sulphide  of  iron  is  found  (by  solution  in  nitric  acid)  gold: 
sometimes  in  rounded  grains,  and  therefore  simply  inclosed  drift-gold ; 
but  also  sometimes  in  minute  crystals  and  threads,  exactly  as  in 
the  sulphide  of  the  undecomposed  quartz-vein.  Evidently,  this  gold 
has  been  deposited  from  a  solution  of  sulphate  of  iron  at  the  moment 
of  the  reduction  of  the  latter  to  a  sulphide.  The  process  was  probably 
as  follows :  Percolating  water  oxidized  iron  sulphide  and  took  it  into 
solution  as  sulphate.  This  solution  coming  in  contact  with  drift-gold 
dissolved  it,  but,  subsequently,  coming  in  contact  with  decaying  or- 
J  Arthur  Phillips,  ibid. 


THEORY  OF  METALLIFEROUS  VEINS.  239 

ganic  matter,  was  again  deoxidized  and  deposited  as  sulphide  ;  and  the 
gold  crystallizing  at  the  same  moment  is  inclosed.1  If  these  waters 
had  circulated  through  a  fissure,  we  would  have  had  an  auriferous 
quartz-vein.  In  fact,  this  may  be  regarded  as  a  sort  of  horizontal  vein. 

"We  conclude,  therefore,  that  metalliferous  veins  have  been  deposited 
from  hot  alkaline  waters,  circulating  through  fissures,  and  that  in  the 
case  of  auriferous  veins  the  solvent  of  the  gold  was  sulphate  of  iron, 
and  the  sulphate  was  deoxidized  by  organic  matter  in  the  same  solution, 
the  gold  and  the  iron  crystallizing  at  the  same  moment,  one  as  metal, 
the  other  as  sulphide. 

Gold  is  sometimes  found  in  pure  quartz  without  the  sulphide  of  iron. 
In  these  cases  it  may  have  been  in  solution  in  alkaline  water  as  silicate 
of  gold,  as  suggested  by  Bischof.  There  is  a  silicate  of  gold  which 
may  be  made  by  artificial  means.  It  is  slightly  soluble  under  certain 
conditions." 

Nuggets. — It  is  well  known  that,  although  gold  exists  in  the  iron 
sulphide  of  the  unchanged  vein  only  in  minute,  even  microscopic, 
crystals  and  threads,  yet  in  the  changed  upper  portions  of  the  veirn  it 
exists  in  quite  visible  particles,  and  often  in  large  nuggets  weighing- 
several  ounces,  or  even  rarely  several  pounds.  This  fact  is  additional 
evidence  that  sulphate  of  iron  is  the  natural  solvent  of  gold.  There 
can  be  no  doubt  that  these  larger  grains  and  nuggets  result  from  the 
coalescence  of  all  the  minute  particles,  contained  in  a  mass  of  sulphide, 
into  one  or  more  larger  masses.  By  meteoric  agencies,  as  already  ex- 
plained (p.  231),  the  sulphide  is  oxidized  into  sulphate,  and  the  gold 
redissolved.  From  this  solution  it  crystallizes  into  one  mass,  as  the 
solution  concentrates  by  losing  its  sulphuric  acid  and  changing  into 
peroxide.  In  the  case  of  large  nuggets,  the  gold  is  probably  in  some 
way  deposited  constantly  at  the  same  place  from  a  similar  solution 
bringing  gold  for  a  long  time. 

Illustrations  of  the  Law  of  Circulation. — We  have  said  that  the 
iron  sulphate  comes  from  oxidation  of  sulphide,  but  also  the  sulphide 
from  the  deoxidation  of  the  sulphate.  This  is  only  another  example 
of  a  perpetual  cycle  of  changes.  Again,  the  gold  in  the  veins  is  leached 
from  the  strata ;  the  strata  doubtless  received  it  from  the  sea,  for  small 
quantities  of  gold  have  been  detected  in  sea-water ;  but,  again,  doubt- 
less the  sea  received  it  from  the  rocks,  and  this  brings  us  to  another 
perpetual  cycle  of  changes. 

But  in  the  midst  of  all  these  changes  there  has  evidently  been  an 
increasing  concentration  and  availability  of  gold  and  other  metals.  In 
the  strata  the  quantity  is  so  small  as  to  be  undetectible ;  it  is  thence 
carried  and  concentrated  in  veins  in  a  more  available  form ;  it  is  next 
set  free  along  the  backs  of  these  veins  in  a  still  more  available  form; 

1  Arthur  Phillips,  ibid.  2  "  Chemical  and  Physical  Geology,"  vol.  iii.,  p.  535. 


240  STRUCTURE   COMMOX  TO  ALL  ROCKS. 

it  is  last  carried  down  by  currents  along  with  other  materials,  neatly 
sorted,  and  deposited  in  placers  in  a  form  the  most  available  of  all. 

SECTION  3. — MOUNTAIN-CHAINS  :  THEIR  STRUCTURE  AND  ORIGIN. 

Mountains  are  the  glory  of  our  earth,  the  culminating  points  of 
scenic  beauty  and  grandeur.  They  are  so  because  they  are  also  the 
culminating  points,  the  theatres  of  the  greatest  activity,  of  all  geologi- 
cal agencies.  The  study  of  mountain-chains,  therefore,  must  ever  be  of 
absorbing  interest,  not  only  to  the  painter  and  the  poet,  but  also  to  the 
geologist.  A  thorough  knowledge  of  their  structure,  origin,  and  mode 
of  formation,  would  undoubtedly  furnish  a  key  to  the  solution  of  many 
problems  which  now  puzzle  us ;  but  their  structure  is  as  yet  little 
known,  and  their  origin  still  less  so. 

Mountain-  Origin. 

The  general  cause  of  mountain-chains  (as  in  fact  of  all  igneous  phe- 
nomena) is  the  ''reaction  of  the  earth's  hot  interior  upon  its  cooler 
crust."  Mountain-chains  seem  to  be  produced  by  the  secular  cooling, 
and  therefore  contraction,  of  the  earth,  greater  in  the  interior  than  the 
exterior;  in  consequence  of  which,  the  face  of  the  old  earth  is  become 
wrinkled.  Or,  to  express  it  a  little  more  fully,  by  the  greater  interior 
contraction,  the  exterior  crust  is  subjected  to  enormous  lateral  pressure, 
which  crushes  it  together,  and  swells  it  upward  along  certain  lines,  the 
strata,  by  the  pressure,  being  at  the  same  time  thrown  into  more  or  less 
complex  foldings.  These  lines  of  upswelled  and  folded  strata  are 
mountain-chains.  The  first  grand  forms  thus  produced  are  afterward 
chiseled  down  and  sculptured  to  their  present  diversified  condition  by 
means  of  aqueous  agency.  Thus  much  it  was  necessary  to  say  of  the 
origin  of  chains,  in  order  to  make  the  account  of  their  structure  intelli- 
gible. The  theory  of  their  origin  will  be  given  more  fully  hereafter. 

General  Form,  and  how  produced. 

A  mountain-cA«£n  consists  of  a  great  plateau  or  bulge  of  the  earth's 
surface,  often  hundreds  of  miles  wide  and  thousands  of  miles  long. 
This  plateau  or  bulge,  which  is  the  chain,  is  usually  more  or  less  dis- 
tinctly divided  by  great  longitudinal  valleys  into  parallel  ranges;  and 
these  ranges  are  again  often  separated  into  ridges  by  smaller  longitudi- 
nal valleys;  and  these  ridges,  again,  serrated  along  their  crests,  or  di- 
vided into  peaks,  by  transverse  valleys. 

Thus,  the  Appalachian  chain  is  a  great  plateau  or  bulge,  100  miles 
wide,  1,000  miles  long,  and  3,000  feet  high.  It  is  divided  into  three 
ranges,  the  Blue,  the  Alleghany,  and  the  Cumberland,  separated  by  great 
valleys,  such  as  the  Valley  of  Virginia  and  the  Valley  of  East  Tennessee. 


MOUNTAIN-STRUCTURE.  241 

These  ranges  are  again  in  some  places  quite  distinctly  divided  into 
parallel  ridges,  which  are  serrated  into  peaks.  The  American  Cordil- 
leras consist  of  an  enormous  bulge  running  continuously  through  the 
whole  of  South  and  North  America,  nearly  10,000  miles  long,  and  from 
500  to  1,000  miles  wide.  This  great  chain  is  divided  into  parallel  ranges. 
In  North  America  there  are  at  least  three  of  these  very  conspicuous,  the 
Rocky  Mountain,  the  Sierra  Nevada,  and  the  Coast  Range,  separated  by 
the  Great  Salt  Lake  Valley  and  the  Valley  of  Central  California,  respec- 
tively. Each  of  these  ranges  is  separated  more  or  less  perfectly  into 
ridges  and  peaks,  as  already  explained.  These  terms,  chain,  range,  and 
ridge,  are  often  used  interchangeably.  I  have  attempted  to  give  a  more 
definite  meaning. 

Chains  are  evidently  .always  produced  solely  by  the  bulging  of  the 
crust  by  lateral  pressure.  Ganges  are  usually  produced  in  a  similar 
manner,  i.  e.,  by  greater  crushing  together,  and  therefore  greater  bulging 
along  parallel  lines,  within  the  wider  bulge  ;  this  is  the  mode  of  forma- 
tion of  the  ranges  of  the  North  American  Cordilleras.  In  such  cases, 
they  have  been  probably  consecutively  formed.  The  ranges  of  the  Ap- 
palachian chain,  however,  have  been  formed  almost  entirely  by  erosion. 
The  ridges  and  intervening '  longitudinal  valleys  are  usually,  and  the 
peaks,  with  their  intervening  transverse  valleys,  are  always,  produced 
by  erosion. 

Such  is  the  simplest  ideal  of  the  form  of  a  mountain-chain ;  but  in 
most  cases  this  ideal  is  far  from  realized.  In  many  cases  the  chain  is  a 
great  plateau,  composed  of  an  inextricable  tangle  of  ridges  and  valleys 
of  erosion,  running  in  all  directions.  In  all  cases,  however,  the  erosion 
has  been  immense.  Mountain-chains  are  the  great  theatres  of  erosion, 
as  they  are  of  igneous  action.  As  a  general  fact,  all  that  we  see,  when 
we  stand  on  a  mountain-chain — every  peak  and  valley,  every  ridge  and 
canon,  all  that  constitutes  scenery — is  wholly  due  to  erosion. 

3fo  untain-  Structure. 

The  simplest  idea  of  a  mountain-range  is  that  of  a  single  fold  of 
thick  strata.  Such  a  simple  range  is  shown  in  Fig.  218,  which  is  a 
generalized  section  of  the  Uintah  Mountains,  taken  from  Powell.  But, 
more  commonly,  mountains,  even  when  they  are  composed  wholly  of 
stratified  rocks,  consist  of  many  folds,  sometimes  open,  as  in  the  Jura 
Mountains  (Fig.  219),  but  more  often  closely  pressed  together.  This  is 
admirably  illustrated  in  the  following  section  of  the  Coast  Range  of 
California  (Fig.  220),  and  also  in  the  section  of  the  Appalachian,  on 
page  244  (Fig.  225).  The  manner  in  which  mountains  are  formed  is 
very  evident  in  such  cases.  Such  mountains  cannot  be  formed  except 
by  a  mashing  together  of  the  strata  horizontally. 
16 


242 


STRUCTURE  COMMON  TO  ALL  ROCKS. 


FIG.  218.— Ideal  Section  across  the  Uintah  Mountains  (after  Powell). 


FIG.  219.— Section  of  the  Jura  Mountains. 


FIG.  220.— Section  of  Coast  Range,  showing  Plication  by  Horizontal  Pressure. 

But  most  great  mountain-ranges,  as  shown  in  Fig.  221,  consist  of  a 
granite  axis,  g,  coming  up  from  beneath  and  appearing  at  the  surface 

along  the  crest  and  forming  the 
peaks,  flanked  on  either  side  by 
tilted  strata,  a,  a,  usually  of  enor- 
mous thickness,  and  correspond- 
ing on  the  two  sides.  Sometimes 
several  series  of  unconformable  strata  on  the  flanks  show  that  the  range 
has  been  formed  by  successive  upheavals  (Fig.  222).  The  succession 
of  events  represented  by  this  figure  are  :  1.  The  strata  a,  a,  were  de- 
posited ;  2.  a,  a,  were  up- 
tilted  and  the  mountain 
formed  ;  3.  The  strata 
b,  b,  were  deposited  hori- 
zontally, and  therefore 
unconformably,  on  a,  aj  4.  The  mountain-axis  was  pushed  up  higher, 


MOUNTAIN-STRUCTURE. 


243 


FIG.  223.— Ideal  Section  showing  Exposure  of 
Granite  by  Erosion. 


so  as  to  tilt  b,  5,  also  ;  5.  c,  c,  were  then  deposited  unconf ormably  on  5,  b  ; 
and,  finally,  6.  The  whole  was  raised  bodily,  so  as  to  expose  c,  c,  but 
without  tilting  them. 

Now  many  geologists  seem  to  regard  these  two  kinds  of  mountains, 
viz.,  those  composed  only  of  folded  strata,  and  those  with  granite  axis, 
as  essentially  different,  and  formed  in  different  ways.  Mountains  of 
the  latter  class,  they  seem  to  think,  were  formed  by  a  force  acting  ver- 
tically, pushing  the  granite  axis  through  the  broken  strata  to  its  pres- 
ent highest  position  along  the  crest.  .--:::.";•-•.. 
But  it  is  far  more  probable  that  the 
stratified  rocks  and  the  subjacent 
granite  were  all  pushed  up  by  hori- 
zontal pressure  into  a  fold,  and  the 
strata  were  afterward  removed  by 
erosion,  leaving  the  harder  granite  as  a  crest.  This  is  shown  in  the 
ideal  section,  Fig.  223,  in  which  dotted  lines  represent  the  part  removed 
by  erosion.  In  many  ranges,  as,  for  example,  in  the  Sierra,  patches  of 
the  flanking  strata  are  still  left  on  the  summits. 

Thus,  then,  mountain-ranges  are  all  formed  in  the  same  general 
way,  viz.,  by  horizontal  crushing.  Sometimes  they  consist  of  a  single 
fold,  more  often  of  many  close  folds ;  sometimes  the  strata  are  little 
changed,  sometimes  they  are  greatly  metamorphosed  ;  sometimes  they 
are  little  eroded,  sometimes  very  deeply  eroded.  The  combination  of 
these  various  conditions  gives  rise  to  a  great  variety  of  kinds  :  1.  If  it 
consist  of  a  single  fold  and  the  strata  be  unchanged,  then  we  have  the 
simplest  conceivable  range,  as  in  the  Uintah  Range,  Fig.  218.  2.  But  if 
the  strata  be  greatly  changed  and  deeply  eroded,  then  the  upper  part 
of  the  fold  is  removed,  and  the  completely  metamorphic  granite  is  ex- 


FIG.  224.— Ideal  Section  of  a  Mountain-Range. 

posed  along  the  crest,  and  we  have  a  case  like  Figs.  221  and  223. 
3.  If  the  range  consist  of  many  folds  and  the  stratification  be  not  de- 
stroyed by  metamorphism,  then  we  have  cases  like  the  Jura  (Fig.  219), 
the  Coast  Range  (Fig.  220),  and  the  Appalachian  (Fig.  225).  4.  Lastly, 
if  ranges  like  the  last  be  greatly  metamorphosed  and  deeply  eroded, 
then  we  have  the  common  case  represented  by  Fig.  224.  This  may, 
perhaps,  be  regarded  as  the  best  type  of  a  great  mountain-range.  It 
represents  strata  strongly  folded  and  deeply  denuded.  But  the  most- 


244  STRUCTURE   COMMON   TO  ALL  ROCKS. 

folded  and  least-changed  upper  portions  of  the  strata  have  been  re- 
moved, and  the  very  metamorphic  and  less-folded  deeper  portions  are 
exposed  along  the  axis.  The  axis  in  this  case,  also,  is  represented  as 
gneiss  (i.  e.,  a  granite  in  which  the  original  strata  are  still  imperfectlv 
visible),  in  order  the  better  to  bring  out  the  real  structure.  But  carry 
the  process  of  metamorphism  one  step  further,  and  the  foldings  of  this 
part  disappear,  and  we  have  a  range  of  the  type  represented  in  Fig.  221. 
In  fact,  it  is  probable  that  many  mountains  which  consist  only  of  gran- 
ite axes  and  tilted  strata  corresponding  on  each  side,  and  therefore 
seem  to  be  but  one  fold,  were  really  originally  of  many  close  folds, 
only  these  have  been  carried  away  by  erosion. 

In  still  other  ranges  the  constituent  strata  are  overlaid  by  immense 
ejections  of  liquid  matter,  which  conceal  the  true  structure  of  the 
mountain.  The  Cascade  Range  is  perhaps  the  most  remarkable  ex- 
ample of  this. 

As  a  general  rule  the  degree  of  mashing,  and  therefore  of  folding, 
is  greatest  near  the  axis,  and  gradually  passes  into  gentler  and  gentler 
undulations  as  we  leave  this  line.  This  is  strikingly  seen  in  the  Ap- 
palachian. Fig.  225  is  a  simplified  section  of  this  chain,  in  which  each 


7 

FIG.  225.— Appalachian  Chain. 

fold  is  really  composed  of  a  number  of  subordinate  folds.  It  is  seen 
that  the  folds  are  strong  along  the  crest,  but  die  away  in  gentle  un- 
dulations westward  until  the  strata  become  horizontal. 

Rate  Of  Mountain-Formation. — The  uprising  of  a  mountain-range  is 
probably  in  all  cases  extremely  slow,  so  much  so  that  it  may  be  going 
on  now  without  our  observing  it.  Hence,  though  so  deeply  denuded, 
it  is  not  necessary  to  suppose  they  ever  were  higher  than  now.  An 
admirable  proof  of  this  slowness  is  pointed  out  by  Powell  in  the  case  of 
the  Uintah  Mountains.  This  range  consists  of  a  single  great  fold  of 
stratified  rocks,  which  has  risen  right  athwart  the  course  of  the  Green 
River.  But  so  slow  has  been  the  uprising,  that  the  river  has  not  been 
turned  from  its  course,  but  has  cut  through  the  range  to  the  bottom 
(Fig.  218).  The  uprising  has  not  been  faster  than  the  down-cutting  of 
the  river. 

Thickness  of  Mountain-Sediments. — Mountain-chains  seem  always  to 
be  composed  of  sedimentary  rocks  of  enormous  thickness.  The  strata 
composing  the  Appalachian  chain  are  40,000  feet  thick,  while  the  same 
strata  on  the  Mississippi  River  are  only  4,000  feet.  According  to 


MOUNTAIN-SCULPTURE.  245 

Clarence  King,  the  Wahsatch  Mountains  are  composed  of  56,000  feet 
of  conformable  strata.  According  to  Powell,  the  strata  exposed  on 
the  flanks  of  the  Uintah  Range  are  32,000  feet  thick.  According  to 
Whitney,  the  cretaceous  strata  alone  of  the  Coast  Range  are  20,000 
feet  thick.  "We  have  taken  these  examples  from  the  United  States, 
but  the  same  is  true  everywhere.  It  seems  certain  that  the  origin 
of  mountain-chains  is  in  some  way  connected  with  thickness  of  sedi- 
ments ;  that  mountain-chains  are,  in  fact,  formed  by  the  crushing  to- 
gether and  folding  of  lines  of  thick  sediments. 

Foldings  and  Metamorphism. — In  consequence  of  the  foldings,  we 
find  associated  with  mountains  fissures,  dikes,  veins,  etc.  If  any  liquid 
matters  existed  beneath,  these  would  naturally  be  squeezed  out  through 
the  fissures,  and  hence  we  find  outpourings  of  lava  associated  with 
mountain-chains.  In  consequence  of  the  thickness  of  the  sediments, 
and  also  from  the  heat  developed  by  crushing,  mountain-strata,  espe- 
cially those  along  the  crest,  which  are  the  lowest,  are  usually  meta- 
morphic  (p.  213).  In  fact,  thickness,  folding,  and  metamorphism, 
not  only  go  together,  but  seem  to  be  'proportional  to  each  other.  Thus, 
as  already  stated,  the  Appalachian  strata,  which  in  the  Appalachian 
region  are  40,000  feet  thick,  gradually  thin  out  westward  until  they 
become  only  4,000  on  the  Mississippi  River.  Both  the  foldings  and 
the  metamorphism  diminish  and  pass  away  in  the  same  direction. 

Inspection  of  the  figures  given  above  (Figs.  221  and  224)  shows — 
1.  That  mountain-chains  are  necessarily  anticlinal.  This,  however,  is 
far  from  being  true  for  ridges  ;  which,  we  will  show  hereafter,  are  often 
synclinal.  2.  It  shows  that  the  rocks  of  mountain-crests  are  usually 
granitic  or  metamorphic.  '3.  That  the  rocks  of  the  crest  are  usually 
lower  in  the  geologic  series,  i.  e.,  older  than  the  flanking  strata,  these 
lower  rocks  of  the  crest  having  been  exposed  by  enormous  erosion. 
Therefore  mountain-regions  have  been  the  great  theatres — 1.  Of  sedi- 
mentation before  the  mountain  was  formed;  2.  Of  upheaval  in  the 
formation  of  the  chain  ;  and,  3.  Of  erosion  which  determined  the  present 
outline.  Add  to  these  the  metamorphism,  thefissures,  slips,  dikes,  veins, 
and  volcanic  outbursts,  and  it  is  seen  that  all  geological  agencies  con- 
centrate there. 

Moun  tain-  Sc  ulpture. 

All  mountain-chains  have  been  formed  in  the  same  general  way, 
viz.,  by  a  bulging  of  the  earth's  crust  along  certain  lines,  produced  by 
interior  contraction.  But  the  original  mountain-plateau  thus  formed 
has  been  in  all  cases  subsequently  so  enormously  sculptured  by  aqueous 
agencies  as  to  obscure  the  origin  of  the  chain  and  confuse  the  use  of 
the  term  mountain.  This  term  is  loosely  used  to  express  every  con- 
spicuous inequality  of  surface,  whatever  be  its  origin,  from  a  great 
chain,  like  the  Andes  or  Himalayas,  to  isolated  erosion  hills  of  a  few 


2±6  STRUCTURE   COMMON   TO   ALL  ROCKS. 

hundred  feet  altitude.  But  we  should  carefully  distinguish  mountain- 
chains  from  hills,  ridges,  peaks,  formed  by  erosion.  The  one  belongs 
to  mountain-formation,  the  other  to  mountain-sculpture.  The  grand 
forms,  the  chain  always,  the  ranges  usually,  are  produced  by  interior 
or  igneous  agencies,  and  have  only  been  modified  by  exterior  or  aque- 


FIG.  226.— Section  across  the  Valley  of  East  Tennessee  (after  Safford). 

ous  agencies ;  but  in  some  cases  even  what  are  called  ranges,  with  their 
wide  intervening  valleys,  have  been  produced  by  erosion.  The  valley 
of  East  Tennessee,  fifty  miles  wide,  separating  the  Cumberland  from  the 
Blue  Range,  has  been  formed  by  this  cause.  Fig.  226  is  a  section  across 
a  portion  of  the  valley  of  East  Tennessee,  the  length  of  the  section 
being  about  twenty  miles.  It  is  evident  that  it  has  been  swept  out  by 
erosion  alone.  On  account  of  the  immense  work  which  has  in  all  cases 
been  done  by  erosion,  and  the  grand  forms  which  have  often  resulted, 
many  writers  divide  mountains  into  two  classes,  viz.,  mountains  of  up- 
heaval and  mountains  of  denudation.  It  is  better,  however,  to  treat 
the  subject  of  mountains  under  two  heads,  viz.,  mountain-formation 
and  mountain-sculpture. 

Resulting  Forms. — It  is  very  interesting  to  trace  the  laws  of  form 
resulting  from  erosion.  These  laws  have  been  brought  out  chiefly  by 
Lesley.1  We  have  added  some  from  our  own  observations: 

1.  Horizontal  Strata. — Horizontal  or  very  slightly  undulating 
strata  give  rise  by  erosion  to  flat-topped  ridges  or  table-mountains. 


Fig.  227  is  an  ideal  section  of  such  table-mountains.     The  outcrop  of 
harder  strata  on  the  slope  will  often  determine  benches.     This  table- 


FiG.  22S.— Table-Mountains. 


form  is  especially  conspicuous  if  the  eroded  table-land  is  capped  by 

hard  sandstone,  or  by  lava,  as  in  Fig.  228.     Examples  of  this  kind  of 

1  "Manual  of  Coal." 


MOUNTAIN-SCULPTURE. 


247 


erosion  hills  are  found  abundantly  in  Illinois,  Iowa,  Tennessee,  and  in 
Arizona.     We  give  in  Fig.  229  an  actual  section  across  Cumberland 


FIG.  229. — Section  across  Cumberland  Pltiteau  and  Lookout  Mountain,  Tennessee. 

table  ((7),  Sequatchee  Valley  (S),  Walden's  Ridge  (  TF),  Tennessee 
River  (^'),  Lookout  Mountain  Valley  (J!/J7~),  and  Lookout  Mountain 
(1,31),  Tennessee,  in  which  this  structure  is  well  seen. 

On  the  other  hand,  if  the  strata  be  very  soft,  then  erosion  produces 
steep,  rounded  hills,  standing  thickly  together  like  potato-hills  on  a 


FIG.  230.— Bad  Lands,  north  of  Uintah  Mountains  (after  Powell). 

large  scale,  or,  when  somewhat  firmer,  like  crowded  pinnacles  (Figs.  230 
and  231).  The  singular  aspect  of  the  Bad  Lands,  or  soft  tertiary  lake- 
deposits  of  the  Rocky  Mountain  region  and  of  Oregon,  is  thus  produced. 

The  forms  represented  by  Figs.  228  and  229  graduate  insensibly 
into  the  next,  viz. : 

2.  Gently-folded  Strata. — These  by  erosion  usually  produce  syn- 
clinal ridges  and  anticlinal  valleys.  This  is  beautifully  shown  in  the 


248  STRUCTURE  COMMON  TO  ALL  ROCKS. 

subjoined  section  of  the  Appalachian  coal-fields  in  Pennsylvania.     By 
restoring  the  strata  as  in  the  figure,  it  is  seen  that  the  original  ridges 


Fie.  231.— Mauvaises  Terres,  Bad  Lands  (after  Hayden). 

have  become  hollows,  and  the  original  hollows  have  become  ridges. 
The  reason  of  this  seems  to  be  that  the  bending  of  the  strata  in  oppo- 


site directions  crushes  together  and  hardens  them  in  the  synclinals, 
and  stretches  them  and  perhaps  breaks  them  along  the  anticlinals. 
Thus  the  erosion  has  taken  effect  on  the  anticlinals  more  than  the  syn- 
clinals. 

3.  Strongly-folded  or  Highly-inclined   Outcropping  Strata.  —  In 
these  the  ridges  arid  valleys  are  determined  by  the  outcrop  of  harder  and 


S  SK  S  J-fc  $  *A,  -S- 

FIG.  233.— Parallel  Kidges. 

softer  strata  respectively.  In  the  ideal  section  Fig.  233  the  ridges  are 
determined  by  the  outcrop  of  a  succession  of  hard  sandstone  strata 
which  resisted  erosion  more  than  the  intervening  soft  shale,  sh.  Beau- 


MOUNTAIN-SCULPTUKE. 


249 


tiful  examples  of  ridges  and  valleys  formed  in  this  way  are  found  in  the 
Appalachian  chain,  especially  in  Virginia.  Standing  on  the  top  of 
Warm  Springs  Mountain,  a  dozen  or  more  parallel  ridges  may  be 
counted,  each  with  a  longer  slope  on  one  side,  and  a  steeper  slope  on 
the  other,  like  billows  ready  to  break.  The  crest  of  each  ridge  is  deter- 
mined by  an  outcropping  sandstone,  and  the  valleys  by  the  softness 
of  the  intervening  shales.  Fig.  226,  on  p.  246,  shows  the  formation  of 
ridges  in  this  way  in  Tennessee.  A  similar  structure  on  a  magnificent 
scale  is  seen  in  the  hog-backs  of  the  Uintah  Mountains  described  by 
Powell.  In  Fig.  233  I  have  represented  a  single  series  of  strata  con- 
taining several  sandstones  ;  but  sometimes  by  repeated  foldings  the 


Fi«.  234.— Parallel  Eidges,  Infolded  Strata. 


same  sandstone  or  other  hard  strata  may  form  many  ridges.     This  is 
shown  in  Fig.  234. 

In  ridges  determined  by  the  outcrop  of  hard  strata  the  relative 
slope  on  the  two  sides  is  determined  by  the  dip  of  the  strata.  If  the 
strata  are  perpendicular,  the  slopes  on  the  two  sides  are  equal  (Fig.  235, 


a) ;  but  if  the  strata  are  inclined,  the  longer  slope  is  on  the  side  toward 
which  the  strata  dip,  and  the  difference  of  the  slopes  increases  as  the 
angle  of  dip  is  less  (Fig.  235,  b  and  c).  This  case  passes  by  insensible 
gradations  into  the  next,  viz. : 

4.    Gently-inclined  Outcropping  Strata. — These  by  erosion,  perhaps 
under  peculiar  climatic  conditions,  give  rise  to  a'  sucession  of  broad, 


250  STRUCTURE  COMMON  TO  ALL  ROCKS. 

nearly  level  tables,  coincident  with  the  face  of  a  hard  stratum,  termi- 
nated by  parallel  lines  of  cliffs.     Fig.  236  is  an  ideal  section  of  such 


FIG   237.— Bird's-eye  View  of  the  Terrace  Caflon  (after  Powell). 

strata.     This  form  of  sculpture  is  developed  on  a  magnificent  scale  in 
the  region  of  Colorado  Plateau.     Fig.  237,  taken  from  Powell,  shows 


MOUNTAIN-SCULPTURE.  251 

three  such  tables,  twenty  to  sixty  miles  wide,  terminated  by  as  many 
cliffs,  1,200  to  2,000  feet  high.  It  is  evident  that  the  drainage  of  the 
region  Avould  be  against  the  foot  of  the  cliffs,  and  that,  therefore,  all 
the  cliffs  recede  by  erosion. 

5.  Highly-metamorphic  or  Granitic  Rocks. — In  granitic  or  highly- 
metamorphic  regions,  where  the  stratification  is  indistinct  or  wanting, 
the  ridges  and  peaks  can  generally  be  traced  to  the  relative  hardness 
of  lines  or  spots,  or  else  to  some  peculiar  rock-structure.  Thus  the 
domes  and  spires  so  conspicuous  about  Yosemite  have  evidently  been  de- 
termined, the  one  by  a  concentric  structure  on  a  huge  scale,  the  other  by 
a  coarse,  perpendicular  cleavage.  In  all  cases  erosion  inequalities,  once 
commenced,  tend  to  increase  by  the  concentration  of  erosion  in  the 
valleys  first  formed. 

The  Age  Of  Mountain-Chains. — The  time  of  formation  of  a  chain  or  a 
range  is  determined  by  the  age  of  the  strata  which  enter  into  its  struct- 
ure, or  which  lie  inclined  on  its  flanks.  Thus,  the  mountain  represented 
in  Fig.  221  (p.  242)  must  be  younger  than  the  tilted  strata  (a)  on  its 
flanks ;  for  the  strata  must  have  been  first  deposited  in  an  horizontal 
position,  and  afterward  tilted,  when  the  mountain  was  formed;  but  it 
must  be  older  than  the  horizontal  strata  (£),  for  these  are  yet  undis- 
turbed. When  mountain-chains  have  been  gradually  raised  by  successive 
upheavals,  this  fact,  and  the  date  of  the  successive  upheavals,  are  known 
by  the  existence  and  the  age  of  the  several  series  of  tilted  or  folded  strata, 
unconformable  with  each  other.  Thus,  in  Fig.  222  (p.  242),  the  chain 
was  raised  first  between  the  periods  of  deposition  of  a  and  b,  and  again 
higher,  between  the  periods  of  deposition  of  b  and  c.  Thus,  it  is  known 
that  the  Appalachian  was  formed  at  the  end  of  the  Palaeozoic  era ;  for 
all  the  Palaeozoic  strata  enter  into  its  folded  structure,  while  even  the 
oldest  Mesozoic  strata  do  not.  By  similar  means,  it  is  ascertained  that 
the  Sierra  Range  was  formed  at  the  end  of  the  Jurassic^  while  the  Coast 
Range  Avas  not  formed  until  the  end  of  the  Miocene. 

It  seems  most  generally  true  that  the  oldest  chains  are  only  of  mod- 
erate altitude,  while  the  highest  mountains  are  among  the  youngest. 
The  converse  of  these  propositions,  however,  is  by  no  means  true,  for ' 
there  are  many  young  mountains  which  are  also  of  moderate  altitude. 
In  the  United  States,  the  Laurentides  are  the  oldest,  then  the  Appa- 
lachians, and  then  the  Sierra  Nevada.  In  South  America,  the  Brazilian 
mountains  are  older  than  the  higher  Andes.  In  Europe,  the  Ural  Moun- 
tains and  the  Scandinavian  mountains  are  older  than  the  loftier  Alps. 
The  Himalayas,  also,  are  among  the  youngest  of  mountains,  at  least  in 
their  last  development.  This  may  be  due  in  part  to  the  enormous 
erosion  of  the  older  chains,  and  in  part  to  other  causes,  yet  imperfectly 
understood. 


252  STRUCTURE   COMMON   TO  ALL   ROCKS. 

Theory  of  the   Origin  of  Mountain- Chains* 

We  have  already  (p.  78,  et  seq.)  given  reasons  for  believing  that  the 
usual  view  that  the  earth  is  an  incandescent  liquid  globe,  covered  by  a 
solid  shell  twenty-five  to  fifty  miles  thick,  is  untenable,  and  therefore 
that  geological  theories  must  be  reconstructed  on  the  basis  of  a  sub- 
stantially solid  earth.  We  have  also  shown  (p.  168)  how  continents  and 
sea-bottoms  are  probably  formed  by  the  unequal  radial  contraction  of  a 
solid  earth.  We  wish  now  to  show  how  mountain-chains  also  may  be 
formed  on  this  supposition. 

A  cooling,  solid  earth  may  be  regarded  as  composed  of  concentric 
isothermal  shells,  each  cooling  by  conduction  to  the  next  outer,  and  the 
outermost  by  radiation  into  space.  Furthermore,  under  these  conditions, 
at  first  and  for  a  long  time,  the  outermost  shell  would  cool  the  fastest ; 
but  there  would  eventually  come  a  time  when,  the  surface  having  be- 
come substantially  cool,  and  moreover  receiving  heat  from  external 
sources  (sun  and  space)  as  well  as  internal,  its  temperature  would  be- 
come nearly  fixed,  while  the  interior  would  still  continue  to  cool  by  con- 
duction. This  has  probably  been  the  case  during  the  whole  recorded 
history  of  the  earth.  The  interior,  now  cooling  faster,  would  also  con- 
tract faster,  than  the  exterior.  There  is  another  cause  which  would  con- 
tribute to  the  same  result :  The  amount  of  contraction  for  equal  cooling, 
or  the  coefficient  of  contraction,  is  greater  at  high  than  at  low  tempera- 
tures; and  therefore  for  equal,  or  even  slightly  less,  loss  of  heat,  the  hot 
interior  would  contract  more  than  the  cool  exterior.  Now,  therefore, 
the  interior,  for  both  of  these  reasons,  contracting  more  rapidly  than  the 
exterior,  the  latter,  following  down  the  shrinking  interior,  would  be 
subjected  to  powerful  horizontal  pressure,  which  continuing  to  increase 
with  the  progressive  interior  contraction,  the  exterior  must  eventually 
yield  somewhere.  Mountain-chains  are  the  lines  along  which  the 
yielding  of  the  surface  to  horizontal  thrust  has  taken  place.  But,  ob- 
serve :  According  to  our  view,  this  yielding  is  not  by  upbending  into 
an  arch,  leaving  a  hollow  space  beneath,  nor  yet  into  such  an  arch,  filled 
and  supported  by  an  interior  liquid,  as  usually  supposed ;  but  by  mash- 
ing or  crushing  together  horizontally,  like  dough  or  plastic  clay,  icith 
foldings  of  the  strata  and  an  upswelling  and  thickening  of  the  whole 
squeezed  mass. 

The  complex  foldings  so  universal  in  mountain-chains  cannot  be 
accounted  for  except  by  this  crushing  together  bv  horizontal  pressure. 
Simple  inspection  of  the  structure  of  such  ranges  as  the  Coast  Range 
and  the  Appalachian  (Figs.  220  and  225,  pp.  242,  244)  is  sufficient  to 

1  This  subject  is  certainly  best  taken  up  here,  but  some  very  general  knowledge  of 
Part  III.  is  necessary  to  its  full  appreciation.  Those  who  have  not  this  general  knowledge 
had  perhaps  better  put  it  off  to  the  end  of  the  course. 


THEORY   OF   THE   ORIGIN   OF   MOUNTAIN-CHAINS.  253 

convince  one  of  this.  But  there  is  another  phenomenon  which  fur- 
nishes demonstrative  proof  both  of  the  crushing  together  and  the  up- 
swelling  of  mountain-chains,  viz.,  the  phenomenon  of  slaty  cleavage. 

We  have  already  seen  (p.  181,  et  seq.)  that  slaty  cleavage  is  cer- 
tainly produced  by  powerful  pressure  perpendicular  to  the  cleavage- 
planes,  by  which  the  whole  rock-mass  in  which  it  occurs  has  been 
mashed  together  and  shortened  in  that  direction,  and  correspondingly 
extended  in  the  direction  of  these  planes ;  furthermore,  as  the  planes 
are  nearly  or  quite  vertical,  that  the  rock-mass  has  been  crushed  to- 
gether horizontally  and  swollen  up  vertically.  As  a  necessary  qonse- 
quence  of  the  crushing  together,  we  find  associated  the  most  complex 
foldings,  not  only  of  the  strata,  but  also  of  the  layers,  and  even  of 
the  finest  lines  of  lamination.  Thus,  plication  is  always  associated 
icith  cleavage;  and,  vice  versa,  cleavage,  when  the  rock-material  is 
suitable  for  developing  this  structure,  is  always  associated  with  plica- 
tion ;  and  both  are  associated  with  mountain-chains. 

A  mashing  together  horizontally  and  an  extension  vertically  are 
therefore  certain  in  slaty  cleavage,  and  therefore  in  mountain-chains 
where  slaty  cleavage  occurs.  It  only  remains,  therefore,  to  show 
that  the  amount  of  upswelling  absolutely  proved  in  these  cases  is 
fully  adequate  to  account  for  the  upheaval  of  the  greatest  moun- 
tain-chains. 

We  have  seen  (p.  182)  that,  taking  any  ideal  cube  or  sphere  of  the 
original  unsqueezed  mass,  in  the  process  of  mashing,  the  diameter  at 
right  angles  to  cleavage  (horizontal  and  in  the  direction  of  pressure)  has 
been  diminished,  that  in  the  dip  of  the  cleavage  (vertical)  has  been  in- 
creased, while  that  in  the  strike  of  the  cleavage  is  unaffected.  Now,  it 
has  been  shown  that  in  the  case  of  the  first  two  diameters  mentioned,  viz., 
the  horizontal  in  the  direction  of  pressure  and  the  vertical,  their  original 
equality  has  been  changed  into  a  ratio  of  2  :  1,  4  :  1,  6  :  1,  9  :  1,  and 
in  some  cases  even  11  :  1 ;  the  average  being  5  or  6  :  1.  It  follows, 
therefore,  that  the  change  of  each  diameter,  either  in  the  direction  of 
compression  or  of  elongation,  must  be  the  square  roots  of  the  above 
ratios.  Thus,  if  a  cube  of  three  inches'  diameter  be  crushed  together 
horizontally  and  allowed  to  extend  only  vertically,  until  these  previous- 
ly equal  diameters  become  as  9:1,  it  is  evident  that  the  horizontal 
diameter  has  been  diminished  and  the  vertical  diameter  increased,  each 
three  times.  Taking  6  :  1  as  the  ratio,  in  cleaved  slates,  of  diameters 
originally  equal,  we  may  assert  that  in  cleaved  rocks  the  whole  mass  has 
been  stootten  up  two  and  a  half  (2.45)  times  its  original  thickness.  Sup- 
pose, then,  a  mass  of  sediments  10,000  feet  thick  subjected  to  hori- 
zontal pressure  and  crushing  sufficient  to  develop  well-marked  cleavage- 
structure:  a  breadth  of  two  and  a  half  miles  has  been  crushed  into  one 
mile,  and  the  10,000  feet  thickness  swollen  to  25,000  feet,  making  an 


254:  STRUCTURE   COMMON   TO  ALL  ROCKS. 

actual  elevation  of  the  surface  of  15,000  feet.  Now,  we  actually  have 
strata,  not  only  10,000,  but  20,000,  and  even  40,000,  feet  thick. 

We  are  justified,  therefore,  in  asserting  that  the  phenomena  of  pli- 
cation and  of  slaty  cleavage  demonstrate  a  crushing  together  hori- 
zontally and  an  upswelling  of  the  whole  mass  of  mountain-sediments; 
and  that  the  phenomenon  of  slaty  cleavage  demonstrates,  in  addition, 
that  the  amount  of  upswelling  produced  by  this  cause  alone  is  sufficient 
to  account  for  the  elevation  of  the  greatest  mountain-chains.  For  it 
must  be  remembered  that  an  equal  degree  of  crushing  takes  place  in 
mountains  even  when  the  proof  derived  from  slaty  structure  is  wanting. 
No  other  theory  of  mountain-formation  takes  cognizance  of  slaty  cleav- 
age. Some  take  cognizance  of  the  crushing  and  folding,  but  in  all  it  is 
a  subordinate  accompaniment,  instead  of  a  sufficient  cause,  of  the  ele- 
vation. 

Unquestionably,  therefore,  mountain-chains  are  produced  by  hori- 
zontal pressure  crushing  together  the  whole  rock-mass  and  swelling  it 
up  vertically,  the  horizontal  pressure  being  the  necessary  result  of  the 
secular  contraction  of  the  interior  of  the  earth.  It  is  possible  that  even 
continents  may  have  been  formed  by  a  similar  yielding  to  horizontal 
thrust,  and  a  similar  crushing  together  and  upswelling.  If  so,  it  is 
necessary  to  suppose  that  the  amount  of  horizontal  crushing  is  much 
less,  but  the  depth  affected  much  greater,  than  in  the  case  of  mountain- 
chains.  But,  as  we  find  no  unmistakable  relation  between  elevation 
and  amount  of  crushing,  except  in  the  case  of  mountain-chains,  we  have 
preferred  to  attribute  the  formation  of  continents  and  sea-bottoms  to 
unequal  radial  contraction  (p.  168). 

Let  us  now  apply  this  theory  to  the  explanation  of  the  most  con- 
spicuous phenomena  associated  with  mountain-chains. 

1.  Thick  Sediments  of  Mountain-Chains.— It  is  now  generally  ac- 
knowledged that  mountain-chains  consist  either  wholly  or  principally 
of  enormously  thick  sediments  crumpled  together.  But  where  do  such 
great  accumulations  of  sediments  now  take  place  ?  Evidently  off  the 
shores  of  continents  and  in  inland  seas.  Nearly  the  whole  debris  of 
eroded  land  is  deposited  near  shore — only  a  very  small  quantity  of 
very  fine  sediment  reaching  deep-sea  bottom.  Hence  great  accumula- 
tions take  place  only  along  shore.  Mountain-chains,  therefore,  are 
evidently  formed  by  the  crushing  together  and  upswelling  of  sea-bot- 
toms where  great  accumulations  of  sediments  have  taken  place  ;  and  as 
such  accumulations  usually  occur  off  the  shores  of  continents,  mountain- 
chains  are  formed  by  the  up-pressing  of  marginal  sea-bottoms.  The 
proof  of  this  proposition  is  found  in  the  history  of  the  chains  of  the 
North  American  Continent. 

(a.)  Appalachian  Chain. — The  area  now  occupied  by  this  chain  was, 
during  the  whole  Silurian  and  Devonian  ages,  the  eastern  margin  of 


THEORY"   OF   THE   ORIGIN   OF  MOUNTAIN-CHAINS.  355 

the  bed  of  the  great  interior  Palaeozoic  Sea,  which  then  covered  nearly 
the  whole  basin  now  drained  by  the  Mississippi  River.  During  all  this 
time  the  whole  of  this  interior  sea,  but  especially  its  eastern  margin, 
received  sediments  from  a  continental  mass  northward  (the  Laurentian 
area),  and  also  especially  from  a  continental  mass  to  the  eastward. 
Besides  the  marks  of  shore-deposit,  found  abundantly  in  the  Appala- 
chian strata,  other  evidences  are  daily  accumulating,  that  the  area  to 
the  east  of  the  Appalachian  chain — the  so-called  primary  or  gneissic 
region  of  the  Atlantic  slope — is  largely  Laurentian,  and  therefore  was 
land  during  the  Pakeozoic  era.  The  size  of  this  old  eastern  continen- 
tal mass  it  is  now  impossible  for  us  to  know,  since  it  has  been  partly 
covered  by  later  deposits,  and  is  perhaps  even  partly  covered  now  by 
the  sea  ;  but,  judging  by  the  enormous  quantity  of  sediments,  30,000 
feet  thick,  carried  westward  from  it  into  the  Palasozoic  interior  sea,  and 
deposited  along  the  eastern  margin  of  this  sea,  it  must  have  been  very 
large. 

At  the  end  of  the  Devonian  age  much  of  the  middle  portion  of  the 
interior  Palasozoic  Sea  was  upheaved  and  became  land  ;  and  the  Appa- 
lachian region  became  now  alternately  a  coal-marsh,  a  lake,  and  an  in- 
land sea  or  estuary,  emptying  into  the  ocean  southward  (see  map,  page 
278 — the  eastern  black  area).  Into  this  estuary,  or  marsh,  during  the 
Coal  period,  sediments  were  brought  down  from  land  north,  east,  and 
west,  until  10,000  feet  more  were  deposited.  /  During  the  whole  of  this 
immense  time  (Palaeozoic  era),  while  the  40,000  feet  of  sediments  were 
depositing,  this  area — whether  sea-margin  bottom,  or  estuary-bottom, 
or  coal-marsh — slowly  subsided,  so  as  to  maintain  nearly  the  same 
level.  This  is  certain  for  the  Coal  period  (for  every  coal-seam  indi- 
cates a  marsh  nearly  at  sea-level),  and  almost  equally  certain  for  the 
previous  periods,  for  marks  of  shallow-water  shore-deposits  are  found 
throughout.  Besides,  it  seems  to  be  a  general  law  throughout  the 
whole  history  of  the  earth  that  areas  of  great  sedimentation  have  been 
areas  of  slow  subsidence  pari  passu.  The  same  seems  to  be  true  now. 
Nearly  all  great  river-deltas  are  slowly  subsiding  (p.  129).  In  fact,  in 
all  shallow-water  deposits,  and  therefore  in  all  shore-deposits,  the 
accumulation  would  soon  cease,  and  therefore  would  never  become 
thick,  but  for  subsidence,  which  constantly  renews  the  conditions  of 
deposit.  The  subsidence  of  the  Appalachian  area,  therefore,  must  have 
been  40,000  feet  vertical. 

Observe,  then,  that  during  the  whole  Coal  period  the  Appalachian 
region,  so  far  from  being  a  mountain-chain,  was  a  northeast  and  south- 
west trough,  lower  than  the.  land  to  the  east  and  west  of  it.  At  the 
end  of  the  Coal  period  occurred  the  Appalachian  revolution.  The 
great  mass  of  sediments  which  had  been  accumulating  for  so  many 
ages,  yielded  to  the  horizontal  pressure,  was  crushed  together  and 


256  STRUCTURE   COMMON  TO  ALL   ROCKS. 

folded,  and  swollen  upward  to  a  height  proportioned  to  the  horizontal 
crushing.  Thus  was  formed  the  Appalachian  chain.  The  mode  and 
the  date  of  its  formation  are  both  recorded  in  its  structure.  Subse- 
quent sculpturing  has  made  it  what  it  now  is.  It  is  probable  that  in 
the  process  of  up-pushing  of  the  Appalachian  (or  possibly  at  a  later 
time),  the  eastern  continental  mass  was  diminished  both  in  height  and 
in  extent  on  its  eastern  border,  by  subsidence. 

(b.)  Sierras. — There  can  be  no  doubt  that  a  considerable  portion  of 
the  area  now  occupied  by  the  Rocky  Mountains  (the  Basin  Range  re- 
gion) was  land  during  the  Palaeozoic  era.  The  extent  and  height  of  this 
land  we  do  not  know.  We  shall  say  nothing  of  the  mode  of  formation 
of  this  the  oldest  portion  of  the  North  American  Cordilleras,  as  the  his- 
tory of  its  formation  is  little  known.  We  will  commence  with  a  con- 
siderable body  of  land  which  certainly  existed  in  this  region  at  the 
beginning  of  the  Mesozoic  era.  Now,  during  the  whole  Triassic  and 
Jurassic  periods,  the  region  now  occupied  by  the  Sierra  range  was  a 
marginal  sea-bottom  receiving  abundant  sediments  from  a  continental 
mass  to  the  east.  At  the  end  of  the  Jurassic  this  line  of  enormously 
thick  off-shore  deposits,  yielding  to  the  horizontal  thrust,  was  crushed 
together  and  swollen  up  into  the  Sierra  range.  All.  the  ridges,  peaks, 
and  caiions — all  that  constitutes  the  grand  scenery  of  these  mountains 
— are  the  result  of  an  almost  inconceivable  subsequent  erosion. 

(<?.)  Coast  Range. — The  up-squeezing  of  the  Sierras,  of  course,  trans- 
ferred the  coast-line  farther  westward,  and  the  region  now  occupied  by 
the  Coast  Range  became  the  marginal  sea-bottom.  This  in  its  turn 
received  abundant  sediments  from  the  now  greatly-enlarged  continent, 
until  the  end  of  the  Miocene,  and  then  it  also  yielded  in  a  similar  man- 
ner, and  formed  the  Coast  Range. 

(d.)  Alps. — Mr.  Judd  has  recently  shown  that  the  region  of  the  Alps, 
during  the  whole  Mesozoic  and  early  Tertiary,  was  a  marginal  sea-bot- 
tom (probably  a  mediterranean),  receiving  sediments  until  a  thickness 
was  attained  not  less  than  that  of  the  Appalachian  strata.  At  the  end 
of  the  Eocene  these  enormously  thick  sediments  were  crushed  together 
with  complicated  foldings,  and  swollen  upward  to  form  these  mountains, 
and  subsequently  sculptured  to  their  present  forms.1 

Thus,  then,  it  is  quite  certain  that  the  places  now  occupied  by 
mountain-chains  have  been,  previous  to  their  formation,  places  of  great 
sedimentary  deposit,  and  therefore  most  usually  marginal  sea-bottoms. 
In  some  cases,  however,  perhaps  in  many  cases,  the  deposits  in  interior 
seas  or  mediterraneans  may  have  yielded  in  a  similar  manner,  giving 
rise  to  more  irregular  chains  or  groups  of  mountains. 

2,  Position  of  Mountain-Chains  along  the  Borders  of  Continents. — 
The  view  that  mountain-chains  are  the  up-squeezed  sediments  of  mar- 
1  Geological  Magazine,  1876,  vol.  iii ,  p.  337. 


THEORY   OF   THE   ORIGIN  OF  MOUNTAIN-CHAINS.  257 

ginal  sea-bottoms  completely  explains  the  well-known  law  of  conti- 
nental form,  viz.,  that  continents  consist  of  interior  basins,  with  coast/- 
chain rims.  In  fact,  the  theory  necessitates  this  as  a  general  form  of 
continents,  but  at  the  same  time  prepares  us  for  exceptions  in  the  case 
of  mountains  formed  from  mediterranean  sediments. 

3.  Parallel  Ranges. — Whitney  has  drawn  attention  '  to  the  fact  that 
"  parallel  ranges  of  the  same  system  are  formed  successively"  and  we 
would  add,  most  usually  formed  successively  coastward.     An  example 
is  found  in  the  North  American  Cordilleras,  the  three  parallel  ranges  of 
which  were  successively  formed — first  the  Rocky  Mountains,  then  the 
Sierras,  and  last  the  Coast  chain.     The  same  is  probably  true  of  many 
other  mountains.      Both  the   general  parallelism  and  the  successive 
formation,  and  the  successive  formation  coastward,  are  explained  by  the 
theory. 

4.  Metamorpbism  of  Mountain-Chains. — Admitting,  then,  as  quite 
certain,  that  mountains  are  formed  by  the  squeezing  together  and  the 
upswelling  of  lines  of  off-shore    sediments,  the  question  still  occurs, 
"  Why  does  the  yielding  to  horizontal  pressure  take  place  along  these 
lines  in  preference  to  any  others  ?  "     The  answer  to  this  question  is 
found  in  the  metamorphic  changes  and  the  aqueo-igneous  softening  of 
deeply-juried  sediments.     Taking  the  increase  of  heat  as  we  descend 
into  the  interior  of  the  earth  to  be  1°  for  every  50  feet,  and  adding  the 
mean  surface  temperature,  60°,  the  lower  portion  of  10,000  feet  of  strata 
must  have  a  temperature  of  about  260°,  and  of  40,000  feet  of  strata 
860°  Fahr.     Even  the  former  moderate  temperature,  long  continued  in 
the  presence  of  the  included  water  of  sediments,  would  probably  pro- 
duce incipient  change,  especially  if  the  included  waters  be  at  all  alka- 
line.    The  latter  temperature,  we  know  from  Daubree's  experiments, 
would  certainly  produce  aqueo-igneous  pastiness  or  even  aqueo-igneous 
fusion.     Now,  this  aqueo-igneous  softening  would  affect  not  only  the 
sediments,  but  also  the  crust  beneath  on  which  the  sediments  were  de- 
posited.   Thus  would  be  produced  a  line  of  weakness,  and  therefore  a  line 
of  yielding  to  the  horizontal  crushing.     Thus  we  fully  account  for  the 
formation  of  the  chain  along  the  line  of  thick  sediments,  and  at  the 
same  time  for  the  metamorphism  of  the  strata,  especially  the  lower 
strata,  involved  in  mountain-structure.     By  this  view,  of  course,  the  ex- 
posure of  metamorphic  rocks  on  the  surface,  as  already  stated  (p.  217), 
is  the  result  of  erosion.     Even  the  granite  axis,  in  most  if  not  in  all 
cases,  is  but  the  lowermost,  and  therefore  the  most  changed,  portion  of 
the  squeezed  mass  exposed  by  erosion ;  although  it  is  possible  that  in 
some  cases  the  granite  may  have  been  squeezed  out  as  a  pasty  mass 
through  a  rupture  at  the  top  of  the  swelling  mass  of  strata. 

Thus  it  will  be  seen  that  the  thickness  of  mountain-strata,  the  nor- 
1  Whitney  on  "  Mountain-Building." 


258  STRUCTURE   COMMON   TO  ALL  ROCKS. 

mal  position  of  chains  on  the  borders  of  continents,  the  successive  for- 
mation coastward  of  parallel  ranges,  and  the  metamorphism  of  the 
strata  of  great  chains,  are  all  accounted  for,  and  shown  to  be  neces- 
sarily connected  with  each  other. 

5.  Fissures  and  Slips,  and  Earthquakes. — The  enormous  foldings 
of  strata  which  must  always  occur  in  the  formation  of  a  mountain- 
chain  by  lateral  thrust  would  of  necessity  often  produce  fractures  at 
right  angles  to  the  thrust,  or  parallel  to  the  folds,  i.  e.,  to  the  range. 
The  walls  of  such  fissures  would  often  slip  by  readjustment  by  the  force 
of  gravity,  or  else,  in  cases  of  great  mashing  together,  might  be  pushed 
one  over  the  other  by  the  sheer  force  of  the  horizontal  thrust.    The  for- 
mer case  would  give  rise  to  those  slips  in  which  the  hanging  wall  has 
dropped  down,  which  are  by  far  the  most  common  slips  in  gently -folded 
strata  (Figs.  204,  205,  pp.  224,  225).     The  latter  would  give  rise  to 
those  cases  often  found  in  strongly-folded  strata,  as  in  the  Appalachian 
(Fig.  199,  p.  222),  in  which  the  hanging  wall  has  been  pushed  upward, 
and  slidden  over  the  foot-wall.     The  sudden  rupture  of  the  earth's  crust 
under  accumulating  horizontal  forces,  or  the  sudden  slipping  of  the 
broken  strata,  sufficiently  accounts  for  the  phenomena  of  earthquakes. 

6.  Fissure-Eruptions. — It  will  be  observed  that,  according  to  our 
view,  beneath  every  thick  mass  of  sediments  there  is  a  layer  of  aqueo- 
igneously  softened   matter.     This  it  is  which  determines  the  line  of 
yielding,  and  therefore  the  place  of  the  mountain-chain.     Perhaps  this 
aqueo-igneous  softening  may  be  sufficient  to  account  for  some  cases  of 
semi-fused  lavas  and  hot  volcanic  muds  ;   although  the  intense  heat  of 
ordinary  fused  lavas  cannot  be  thus  accounted  for.     But  as  soon  as  the 
yielding  commences,  mechanical  energy,  by  means  of  the  friction  of 
the  crushed  strata,  is  converted  into  heat.     Mr.  Mallet  believes '  that  the 
heat  thus  produced  is   sufficient  to  fuse  the  rocks.     Beneath  every 
chain,  therefore,  there  must  be,  or  has  been,  a  mass  of  fused  matter. 
Now,  in  the  progressive  crushing  together  of  the  mountain-strata,  it 
follows  inevitably  that  this  fused  matter  is  squeezed  into  fissures  of  the 
folded  strata,  forming  dikes,  or  squeezed  out  through  such  fissures,  and 
outpoured  upon  the  surface  as  great  sheets  of  lava.     Thus  the  associa- 
tion of  these  lava-floods  with  mountain-chains  is  also  completely  ac- 
counted for ;  and  it  is  simply  impossible  to  account  for  them  in  any 
other  way,  unless,  indeed,  by  Fisher's  view  of  superheated  steam  issu- 
ing from  the  fissures. 

7.  Volcanoes. — No  doubt  the  study  of  causes  now  in  operation  forms 
the  only  true  foundation  of  a  scientific  geology.     Nevertheless,  the 
assimilation  of  agencies  in  previous  geological  epochs  to  those  now  in 
operation  may  be  carried  too  far.     For  instance,  there  is  a  strong  ten- 
dency among  the  best  geologists  to  make  volcanoes  or  crater-eruptions 

1  "  Philosophical  Transactions  "  for  18"72. 


THEORY   OF   THE   ORIGIN   OF  MOUNTAIN-CHAINS.  259 

(the  only  form  of  eruption  now  going  on)  the  type  of  all  igneous  erup- 
tions in  all  times.  But  the  attentive  study  of  the  mode  of  occurrence 
of  eruptive  rocks  will  show  that  by  far  the  larger  quantity  have  come 
through  fissures,  as  explained  above,  and  not  through  craters.  No  one 
who  has  examined  the  eruptive  rocks  of  the  Pacific  coast  can  for  a 
moment  believe  that  these  immense  floods  of  lava  have  issued  from 
craters.  The  lava-flood  of  the  Sierra  and  Cascade  ranges  is  certainly 
among  the  most  extraordinary  in  the  world.  Commencing  in  Middle 
California  as  separate  lava-streams  (which,  however,  cannot  be  traced 
in  any  case  to  craters),  in  Northern  California  it  becomes  an  almost  con- 
tinuous sheet,  several  hundred  feet  thick;  and  in  Oregon  an  overwhelm- 
ing flood,  at  least  2,000  feet  thick.  In  apparently  un diminished  thick- 
ness it  then  stretches  through  Washington  Territory  to  the  borders  of 
British  Columbia.  An  area  800  miles  long  and  100  miles  wide  is 
apparently  entirely  covered  with  a  universal  lava-flood,  which,  in  the 
thickest  part,  where  it  is  cut  through  by  the  Columbia  River,  is  certainly 
not  less  than  3.000  feet  thick.  Over  this  enormous  area  there  are  scat- 
tered about  a  dozen  extinct  volcanoes — mere  pimples  on  its  face.  It  is 
incredible  that  all  this  flood  should  have  issued  from  these  craters. 
There  is  no  proportion  between  the  cause  and  the  effect.  We  therefore 
unhesitatingly  adopt  the  view  of  Richthofen,1  that  these  immense 
floods  of  lava,  so  often  associated  with  mountain-chains,  and  often  form- 
ing, as  in  this  case,  the  great  mass  of  the  chain  itself,  have  issued,  not 
from  craters,  but  from  fissures  /  and  that  volcanoes  or  crater-eruptions 
are  secondary  phenomena,  arising  from  the  access  of  water  to  the  hot 
interior  portions  of  great  fissure-eruptions.  Thus,  as  monticules  are 
parasites  on  volcanoes,  so  are  volcanoes  parasites  on  fissure-eruptions, 
and  fissure-eruptions  themselves  parasites  on  an  interior  fluid  mass. 
This  interior  fluid  mass,  however,  according  to  Richthofen,  is  the  sup- 
posed universal  liquid  interior  /  while,  according  to  our  view,  it  is  the 
sub-mountain  reservoir,  locally  formed,  as  above  explained. 

By  this  theory  it  is  necessary  to  suppose  that  there  have  been,  in 
the  history  of  the  earth,  periods  of  comparative  quiet,  during  which  the 
forces  of  change  were  gathering  strength ;  and  periods  of  revolutionary 
change — periods  of  gradually-increasing  horizontal  pressure,  and  peri- 
ods of  'yielding  and  consequent  mountain-formation.  These  latter 
would  also  be  periods  of  great  fissure-eruptions,  and  would  be  followed 
during  the  period  of  comparative  quiet  by  volcanoes  gradually  decreas- 
ing in  activity.  The  last  of  these  great  fissure-eruption  periods  in  the 
United  States  occurred  in  the  later  Tertiary.  Since  then  we  have  been 
in  a  crater-eruption  period,  which  has  been  steadily  decreasing  in  activ- 
ity, until  only  geysers  and  hot  springs  remain  to  tell  us  of  the  still  hot 
interior  masses  of  the  great  fissure-erupted  lavas.  The  periods  of 
1  "  Natural  History  of  Volcanic  Rocks,"  Memoirs  of  California  Academy  of  Science. 


260  DENUDATION,   OR   GENERAL  EROSION. 

revolution  separate  the  great  eras  and  ages  of  geological  history,  and 
are  marked  by  unconformity r,  because  the  sea-margin  sediments,  upon 
which  the  sediments  of  the  next  period  are  necessarily  dep9sited, 
are  crumpled  up y  and  also  by  change  of  species,  because  changes  of 
physical  geography  determine  changes  of  climate,  and  therefore  en- 
forced migration  of  species. 

The  theory  here  presented  accounts  for  all  the  principal  facts  asso- 
ciated in  mountain-chains.  This  is  the  true  test  of  its  general  truth. 
It  explains  satisfactorily  the  following  facts :  1.  The  most  usual  position 
of  mountain-chains  on  the  borders  of  continents.  2.  When  there  are 
several  ranges  belonging  to  one  system,  these  have  been  formed  suc- 
cessively coastward.  3.  Mountain-chains  are  masses  of  immensely 
thick  sediments.  4.  The  strata  of  which  mountain-chains  are  composed, 
are  strongly  folded,  and,  where  the  materials  are  suitable,  are  affected 
with  slaty  cleavage ;  both  the  folds  and  the  cleavage  being  usually  par- 
allel to  the  chain.  5.  The  strata  of  mountain-chains  are  usually  affected 
with  metamorphism,  which  is  great  in  proportion  to  the  height  of  the 
chain  and  the  complexity  of  the  foldings.  6.  Great  fissure-eruptions  and 
volcanoes  are  usually  associated  with  mountain-chains.  7.  Many  other 
minor  phenomena,  such  as  fissures,  slips,  and  earthquakes,  it  equally 
accounts  for. 

Rev.  O.  Fisher  and  Captain  Button 1  have  objected  to  the  above 
view,  that  at  the  calculable  rate  at  which  the  earth  is  now  cooling,  the 
amount  of  contraction  is  wholly  inadequate  to  produce  the  supposed 
effect.  But  even  if  this  be  true,  the  objection  does  not  touch  the  fact 
of  contraction,  which  is  certain,  but  only  the  cause  of  contraction,  viz., 
by  cooling.  Other  causes  of  contraction  are  conceivable,  for  example, 
loss  of  interior  vapors  and  gases,  according  to  Fisher's  theory  of  volca- 
noes (p.  93).  • 


CHAPTER    VI. 
DENUDATION,  OR    GENERAL  EROSION. 

THE  term  denudation  is  used  by  geologists  to  express  the  general 
erosion  which  the  earth-surface  has  suffered  in  geological  times.  The 
correlative  of  denudation  is  sedimentation,  and  the  amount  of  denudation 
is  measured  by  the  amount  of  stratified  rocks. 

Agents  of  Denudation. — The  agents  of  erosion,  as  we  have  already 

seen  in  Part  I.,  are :    1.  Rivers,  including  under  this  head  the  whole 

course   of  rainfall   on   its  way  back  to  the   ocean  whence   it   came ; 

1  Fisher,  "  Cambridge  Philosophical  Transactions,"  vol.  xii. ;  Dutton,  Pcnn  Monthly^ 

May  and  June,  1876. 


DENUDATION,   OR   GENERAL  EROSION.  261 

2.  Glaciers,  including  under  this  head  not  only  glaciers  proper,  but 
moving  ice-sheets,  such  as  now  exist  in  polar  regions  and  in  the  Glacial 
epoch  extended  far  into  now  temperate  regions,  and  also  moving  snow- 
fields,  for  it  is  probable  that  all  extensive  snow-fields  and  snow-caps  are 
in  motion ;  3.  Waves  and  tides  ;  and,  possibly,  4.  Oceanic  currents. 

Oceanic  currents  usually  run  on  a  bed  and  between  banks  of  still 
water,  and  therefore  produce  no  erosion.  It  is  possible,  however,  that 
a  rising  sea-bottom  may  be  eroded  by  this  agent ;  but  as  we  have  no 
knowledge  of  such  effects,  we  are  compelled  to  omit  this  from  our  esti- 
mate of  the  probable  rate  of  denudation.  The  action  of  waves  and  tides 
is  violent  and  conspicuous;  yet  these  agents  are  so  entirely  confined  to 
the  shore-line  that  their  aggregate  effect  is  but  a  small  fraction  of  the 
whole  erosion.  Prof.  Phillips  has  shown  *  that,  taking  the  coast-lines  of 
the  world  as  100,000  miles,  and  making  the  extravagant  estimate  that 
the  average  erosion  along  this  whole  line  is  equal  to  that  of  the  English 
coast,  or  one  foot  per  annum  of  a  cliff  one  hundred  feet  high,  still  the 
aggregate  wave-erosion  is  far  less  than  river-erosion,  being  equivalent 
to  a  general  land-surface  erosion  of  only  z^00  of  an  inch  per  annum,  or 
-jJg-  of  that  which  is  now  going  on  over  the  hydrographical  basin  of  the 
Ganges,  and  ^  of  that  going  on  in  the  basin  of  the  Mississippi.  Glaciers 
and  rivers,  therefore,  are  the  great  agents  of  erosion.  The  one  takes  the 
place  of  the  other,  according  as  falling  water  takes  the  form  of  rain  or 
snow;  both  come  under  the  general  head  of  circulating  meteoric  water. 
In  a  general  estimate  of  the  rate  of  denudation  we  may,  therefore,  with- 
out sensible  error,  regard  it  as  the  work  of  circulating  meteoric  water. 

Again,  although  it  is  probable  that  the  erosive  power  of  glaciers 
is  far  greater  than  that  of  rivers,  yet  their  action  is  so  much  more  local, 
both  in  time  and  space,  that  we  believe  we  may  take  the  average  rate  of 
river-erosion  as  a  fair  representative  of  the  average  rate  of  denudation. 

Amount  of  Denudation. — A  mere  glance  at  the  figures  below  will 
show  in  a  general  way  the  manner  in  which  geologists  estimate  the 
amount  of  denudation  in  certain  regions.  In  almost  all  countries,  espe- 
cially in  mountain-regions,  we  find  slips  varying  from  a  few  feet  to 
many  thousands  of  feet  perpendicular  (Fig.  238).  There  are  slips  in 
the  Appalachian  chain  which  are  estimated  to  be  8,000,  and  even  one 
20,000,  feet  perpendicular.  And  yet  in  most  cases  the  escarpment, 
which  would  otherwise  exist,  is  completely 
cut  away,  so  that  no  surface-indication  of 
the  slip  exists.  Evidently  in  such  cases  there 
must  have  been  erosion  on  the  elevated  side, 
at  least  equal  to  the  amount  of  slip,  and 
probably  much  greater.  The  dotted  line 
represents  the  probable  original  surface.  FIG.  238. 

1  "Life,  its  Origin  and  Succession,"  p.  130. 


262 


DENUDATION,  OR  GENERAL  EROSION. 


Sometimes  the  horizontal  strata  of  isolated  mountain-peaks  corre- 
sponding to  each  other  (mountains  of  erosion)  show  that  these  are  but 
scattered  fragments  of  a  once  high  plateau,  which  has  been  removed  by 


FIG.  239.— Denudation  of  Red  Sandstone,  Northwest  Coast  of  Boss-shire,  Scotland. 

erosion,  as  shown  in  the  annexed  figure  (Fig.  239),  and  in  the  sections 
of  the  Appalachian,  on  pages  246, 247  (Figs.  226, 229).  In  such  cases  the 
erosion  must  have  been  at  least  equal  to  the  height  of  the  peaks,  and 


FIG.  240. — Section  across  Middle  Tennessee.    The  dotted  lines  show  the  amount  of  matter  removed. 

may  have  been  to  any  extent  greater.  The  accompanying  section  across 
Middle  Tennessee  shows  a  vertical  erosion  of  1,200  to  2,400  feet,  over 
the  whole  valley  of  Middle  Tennessee,  which  is  sixty  miles  across, 


FIG.  241.— Section  through  Portions  of  England. 


and  one  hundred  miles  long.  In  most  cases  the  removed  matter  is  not 
so  easily  estimated  as  in  those  mentioned.  The  strata  in  mountain- 
chains  are  usually  folded  in  a  very  complex  way,  and  then  denuded. 


FIG.  242.-Section  through  Portions  of  England. 

But  the  ideal  restoration  of  these  may  be  effected,  and  the  amount  of 
erosion  approximately  estimated.  Figs.  241  and  242  are  sections  across 
the  mountainous  parts  of  England,  as  restored  by  Prof.  Ramsay. 


DENUDATION,   OR   GENERAL   EROSION,  263 

Average  Erosion. — By  these  methods  Prof.  Ramsay  estimates  the 
denudation  over  many  portions  of  England  to  be  not  less  than  10,000 
to  11,000  feet  in  thickness.1  Over  the  whole  Appalachian  region  the 
denudation  has  probably  been  enormous,  in  some  places  8,000  to  20,000 
feet.  Over  the  whole  region  of  the  high  Sierra  Range,  as  we  have 
shown,2  erosion  has  removed  the  whole  of  the  Jurassic  and  Triassic 
slates,  and  bitten  deep  into  the  underlying  granite.  The  thickness  of 
these  slates  is  not  known,  but  it  must  be  many  thousand  feet.  In 
the  TJintah  Mountain  region,  according  to  Powell,  over  an  area  of 
2,000  square  miles,  an  average  thickness  of  three  and  a  half  miles 


}.  243.— Uintah  Mountains— Upper  Part  restored,  showing  Fault;  Lower  Part  showing  the  Present 
Condition  as  produced  by  Erosion"  (after  Powell). 

has  been  taken  away  (Fig.  243),  the  extreme  thickness  removed  being 
nearly  five  miles.  Over  the  whole  Colorado  Plateau  region  the  succes- 
sion of  cliffs,  separated  by  broad  tables  (Fig.  237),  shows  enormous 
erosion. 

It  seems  impossible  to  avoid  the  conclusion,  therefore,  that  the 
average  erosion  over  all  present  land-surfaces  has  been  at  least  several 
thousand  feet. 

There  is  another  mode  of  estimating  the  average  erosion,  viz.,  by 
the  average  thickness  of  stratified  rocks.  The  debris  of  erosion  is  car- 
ried down  into  seas  and  lakes,  and  forms  strata,  and  the  amount  of  strat- 
ified rocks  becomes  thus  the  measure  of  the  erosion ;  the  average  thick- 
ness of  sediments,  if  they  had  been  spread  over  an  equal  area,  Avould 
be  an  accurate  measure  of  the  average  thickness  removed  by  erosion. 
Now,  the  stratified  rocks  are  in  some  localities  10,000  feet,  20,000  feet, 
and  sometimes  40,000  and  50,000  feet  thick.  They  are  scarcely  ever 
found  less  than  2,000  or  3,000  feet.  It  is  certain,  therefore,  that  the 
average  thickness  of  strata  over  the  whole  known  sui'face  of  the  earth  is 
not  less  than  several  thousand  feet.  Let  us  take  it  at  only  2,000  feet. 
But  the  area  of  sedimentation,  the  sea-bottom,  is  now,  and  has  proba- 

1  Geological  Observer,  p.  819. 

2  American  Journal  of  Science  and  Arts,  voL  v.,  p.  325. 


264  DENUDATION,   OR   GENERAL   EROSION. 

bly  alwa3Ts  been,  at  least  three  times  the  area  of  erosion,  the  land-surface. 
Thus  an  average  of  2,000  feet  of  strata  would  require  an  average  erosion 
of  6,000  feet. 

Estimate  of  Geological  Times. — There  are  many  facts  connected 
with  geology,  especially  the  facts  of  evolution,  which  cannot  be  under- 
stood without  the  admission  of  inconceivable  lapse  of  time.  It  is  im- 
portant, therefore,  that  the  mind  should  become  familiarized  with  this 
idea.  It  will  not  be  out  of  place,  therefore,  to  make  a  rough  estimate 
of  time  based  upon  the  amount  of  erosion. 

We  have  already  seen  (p.  11)  that,  taking  the  Mississippi  as  an  aver- 
age river  in  erosive  power  (it  is  probably  much  more  than  an  average), 
the  rate  of  continental  erosion  is  now  about  one  foot  in  5,000  years. 
At  this  rate,  to  remove  an  average  thickness  cf  6,000  feet  would  re- 
quire 30,000,000  years. 

Some  may  object  to  this  estimate,  on  the  ground  that  geological 
agencies  were  once  much  more  active  than  now.  It  is  probable  that 
this  is  true  of  igneous  agencies,  since  these  are  determined  by  the  in- 
terior heat  of  the  earth,  and  this  has  evidently  been  decreasing.  It  is 
probable  also  that  this  is  true  of  the  chemical  agencies  of  water  in  dis- 
integrating rocks  and  forming  soils,  since  chemical  effects  are  also  usu- 
ally increased  by  heat.  But  there  is  good  reason  to  believe  that  the 
mechanical  agencies  cf  water,  i.  e.,  its  erosive  powery  have  been  con- 
stantly increasing  with  the  course  of  time,  and  are  greater  now  than 
at  any  previous  epoch  except  the  Glacial  epoch. 

For  observe :  The  erosive  power  of  water  is  determined  entirely  by 
the  rapidity  of  circulation  of  air  and  water,  and  this  is  determined  by 
the  diversity  of  temperature  in  different  portions,  and  this  in  its  turn  by 
the  size  of  continents  and  the  height  of  mountains.  Continents  and 
seas  are  two  poles  of  a  circulating  apparatus — at  one  pole  is  condensa- 
tion, at  the  other  evaporation.  In  proportion  to  condensation  are  also 
evaporation  and  circulation.  Now,  there  is  good  reason  to  believe  that, 
amid  many  oscillations,  there  has  been  throughout  all  geological  times  a 
constant  increase  in  the  size  of  continents  and  the  height  of  mountains. 
If  so,  then  the  circulation  of  air  and  water  has  been  becoming  swifter 
and  swifter ;  the  life-pulse  of  our  earth  has  beaten  quicker  and  quicker, 
and  therefore  the  waste  and  supply  (erosion  and  sedimentation)  have 
been  greater  and  greater. 

We  therefore  return  to  our  estimate  of  30,000,000  years  with 
greater  confidence  that  it  is  even  far  within  limits  of  probability.  For, 
1.  We  have  taken  the  average  thickness  of  strata  at  2,000  feet,  while 
it  is  probably  much  more.  2.  We  have  taken  the  Mississippi  as  an 
average  river,  and  therefore  the  present  rate  of  general  erosion  as  one 
foot  in  5,000  years :  it  is  probably  much  less.  3.  We  have  taken  the 
rate  of  erosion  in  previous  epochs  as  the  same  as  now,  while  it  is  prob- 


DENUDATION,   OR   GENERAL   EROSION.  £65 

ably  much  less,  for  two  reasons  :  1.  The  land-surface  to  be  eroded  was 
smaller  ;  and,  2.  The  erosive  power  of  water  was  less.  Taking  all  these 
things  into  consideration,  the  time  necessary  to  produce  the  structure 
which  we  actually  find,  is  enormously  increased. 

But  even  this  gives  us  no  adequate  conception  of  the  time  involved 
in  the  geological  history  of  the  earth.  For  rocks  disintegrated  into 
soils,  and  deposited  as  sediments,  are  again  reconsolidated  into  rocks, 
lifted  into  land-surfaces  to  be  again  disintegrated  into  soils,  transported 
and  deposited  as  sediments.  And  thus  the  same  materials  have  been 
worked  over  and  over  again,  perhaps  many  times.  Thus  the  history  of 
the  earth,  recorded  in  stratified  rocks,  stretches  out  in  apparently  end- 
less vista.  And  still  beyond  this,  beyond  the  recorded  history,  is  the 
infinite  unknown  abyss  of  the  unrecorded.  The  domain  of  Geology  is 
nothing  less  than  (to  us)  inconceivable  or  infinite  time. 


PAET  III. 
HISTORICAL  GEOLOGY; 


THE  HISTORY  OF   THE  EVOLUTION  OF  EARTH -STRUCTURE  AND    OF    THE 
ORGANIC  KINGDOM. 


CHAPTER  I. 
GENERAL    PRINCIPLES. 

THBEE  are  certain  laws  underlying  all  development — certain  general 
principles  common  to  all  history,  whether  of  the  individual,  the  race,  or 
the  earth.  We  wish  to  illustrate  these  general  principles  in  the  more 
unfamiliar  field  of  geology  by  running  a  parallel  between  the  history 
of  the  earth  and  other  more  familiar  forms  of  history. 

1.  All  history  is  divided  into  eras,  ages,  periods,  epochs,  separated 
from  each  other  more  or  less  trenchantly  by  great  events  producing 
great  changes.     In  written  history  these  are  treated,  according  to  their 
importance,  in  separate  volumes,   or  separate  chapters,  sections,  etc. 
So  earth-history  is  similarly  divided  into  geological  eras,  ages,  periods, 
etc. ;  and  these  have  been  recorded  by  Nature  in  separate  rock-systems, 
rock-series,  rock-formations,  and  rock-strata.     In  geology  these  terms, 
both  those  referring  to  divisions  of  time  and  those  referring  to  divisions 
of  record,  are  unfortunately  loosely  and  interchangeably  used.     We 
shall  strive  to  use  them  as  definitely  as  possible,  the  eras  and  the  cor- 
responding rock-systems  being  the  primary  divisions,  and  the  others 
subdivisions  in  the  order  mentioned. 

2.  In  all  history  successive  eras,  ages,  periods,  etc.,  usually  graduate 
insensibly  into  each  other,  though  sometimes  the  change  is  more  rapid 
and  revolutionary.     In  individual  history  childhood  usually  graduates 
into  youth,  and  vouth   into  manhood  ;   yet  sometimes  a  remarkable 


GENERAL   PRINCIPLES.  267" 

event  determines  a  more  rapid  change.  In  social  and  political  life,  too, 
successive  phases  of  civilization  embodying'  successive  dominant  prin- 
ciples usually  graduate  into  each  other  ;  yet  great  events  have  some- 
times determined  exceptionally  rapid  changes  in  the  direction  or  the 
rate  of  movement.  So  also  is  it  in  geological  history.  The  eras, 
periods,  etc.,  usually  shade  more  or  less  insensibly  into  each  other ;  yet 
there  have  been  times  of  comparatively  rapid  or  revolutionary  change. 
In  all  history  there  are  periods  of  comparative  quiet,  during  which 
forces  of  change  are  gathering  strength,  separated  by  periods  of  more 
rapid  change,  during  which  the  accumulated  forces  produce  conspicuous 
effects. 

3.  Ages,  periods,  etc.,  in  all  history,  whether  individual,  political, 
or  geological,  are  determined  by  the  rise,  culmination,  and  decline,  of 
certain  dominant  forces,  principles,  ideas,  functions.     Thus,  in  individual 
development,  we  have  the  culmination,  first,  of  the  nutritive  functions; 
then  of  the  reproductive  and  muscular  functions ;  and,  last,  of  the  cere- 
bral functions.     And  in  mental  development,  also,  we  have  the  culmina- 
tion, first,  of  the  perceptive  faculties,  and  memory;  then,  the  imaginative 
and  aesthetic  faculties ;  and,  last,  the  reflective  faculties ;  the  first  gath- 
ering and  storing  material,  the  second  vivifying  it,  the  third  using  it  in 
productive  mason-work  of  science.     In  social  history,  too,  the  succes- 
sive culminations  of  different  phases  of  civilization  have  been  the  result 
of  the  introduction  and  culmination  of  successive  dominant  principles 
or  ideas — of  successive  social  forces  or  functions.     So  has  it  been  in 
geological  history.     The  great  divisions  of  time,  especially  what  are 
called  ages,  are  characterized  by  the  introduction  and  culmination  of 
successive  dominant  classes  of  organisms,  for  these  are  the  highest 
expression  of  earth-life.     Thus,  in  geology,  we  have  an  age  of  mollusks, 
an  age  of  fishes,  an  age  of  reptiles,  in  which  these  were  successively 
the  dominant  class. 

But  since  (Law  2)  successive  ages  graduate  more  or  less  into  and 
overlap  each  other,  we  might  expect,  and  do  indeed  find,  that  the  char- 
acteristics of  each  age  commence  in  the  preceding  age.  Each  age  is 
foreshadowed  in  the  previous  age.  The  same  is  true  of  all  history. 

4.  In  all  history,  at  the  close  of  an  age,  the  characteristic  dominant 
principle  or  class  declines,  but  does  not  perish.     It  only  becomes  sub- 
ordinate to  the  succeeding  dominant  class  or  principle.     Thus,  to  illus- 
trate from  individual  history,  in  youth,  the  characteristic  faculties  of 
childhood,  viz.,  perception  and  memory,  decline,  and  become  subordinate 
to  the  higher  faculty  of  imagination,  and  this  in  turn  becomes  subordi- 
nate to  the  still  higher  faculty  of  productive  thought ;  and  thus  the 
whole  organism  becomes  higher  and  more  complex,  each  stage  of  devel- 
opment including  not  only  its  own  characteristic,  but  also,  in  a  subordi- 
nate degree,  those  of  all  preceding  stages.     The  same  is  true  of  social 


268  GENERAL   PRINCIPLES. 

history.  Each  stage  of  social  development  absorbs  and  includes  the 
social  principles  and  forces  characteristic  of  previous  stages,  but  subor- 
dinates them  to  the  higher  principles  which  form  its  own  characteristic, 
and  thus  the  social  organism  becomes  ever  higher,  more  complex,  and 
varied. 

So  is  it  also  in  geologic  history.  When  the  dominance  of  any  class  de- 
clines at  the  end  of  an  age,  the  class  does  not  disappear,  but  remains  sub- 
ordinate to  the  next  succeeding  and  higher  dominant  class,  and  the  organ- 
ic kingdom,  as  a  whole,  becomes  successively  more  and  more  complex  and 
varied.  This  is  graphically  represented  by  the  accompanying,  diagram, 


FIG.  244.— Diagram  illustrating  Successive  Culminations  of  Classes. 

in  which  1,  2,  3,  4,  represent  four   successive  ages  determined  by  the 
culmination  of  successive  dominant  classes. 

5.  There  are  two  modes  of  determining  and  limiting  eras,  ages,  peri- 
ods, etc.,  in  geology,  viz.,  unconformity  of  the  rock-system  and  change 
in  the  life-system.     In  written  human  history,  the  divisions  of  time  are 
recorded  in  separate  volumes,  chapters,  sections,  with  boards  or  blanks 
between.     These  divisions  in  the  record  ought  to  correspond  to  con- 
spicuous changes  in  the  character  of  the  most  important  contents.     So, 
in  the  history  of  the  earth,  the  rock-systems,  rock-series,  rock-forma- 
tions, are  volumes,  chapters,  sections,  respectively,  more  or  less  com- 
pletely separated  from  each  other  by  unconformity,  indicating  blanks 
in  the  known  record;  and  the  most  important  changes  in  the  contents, 
i.  e.,  in  the  life-system,  ought  to,  and  usually  do,  correspond  with  the 
unconformity  of  the  rock-system.     But  if  there  should  be  (as  there  is  in 
some  limited  localities)  a  discordance  between  these  two,  we  should  fol- 
low the  life-system  rather  than  the  rock-system,  the  contents  rather 
than  the  artificial  divisions  of  the  record. 

6.  As  in  human  history  there  is  a  general  onward  movement  of  the 
race,  and  yet  special  modifications  in  character  and  rate  in  each  coun- 
try, so  in  geology  there  has  been  a  general  march  of  evolution  of  the 
whole  earth  and  the  organic  kingdom,  and  yet  special  modifications  in 
character  and  rale  in  each  continent,  and  to  a  less  extent  in  different 
portions  of  the  same  continent.     The  great  eras,  ages,  and  periods,  be- 
long to  the  whole  earth  alike,  and  are  the  same  in  all  countries,  but  the 
epochs  and  the  smaller  divisions  of  time,  though   similar,  are  probably 
not  contemporaneous  in  different  countries.     This   fact  has  probably 
been  too  much  overlooked  by  geologists. 

Great  Divisions  and  Subdivisions  of  Time.— Eras.— It  is  upon  these 


GENERAL  PRINCIPLES.  269 

principles  that  geologists  have  established  the  divisions  of  time  and 
the  corresponding  divisions  of  strata. 

The  whole  history  of  the  earth  is  divided  into  five  eras,  with  corre- 
sponding rock-systems.  These  are  :  1.  Archaean  or  Eozoic1  era,  em- 
bodied in  the  Laurentian  system  ;  2.  Palaeozoic a  era,  embodied  in  the 
Paleozoic  or  Primary  system  ;  3.  Mesozoic  *  era,  recorded  in  the  Sec- 
ondary system ;  4.  Cenozoic*  recorded  in  the  Tertiary  and  Quaternary 
systems  ;  and,  5.  The  Psychozoic  era,  or  era  of  Mind,  recorded  in  the 
recent  system. 

These  grand  divisions,  with  the  exception  of  the  last,  are  founded  on 
an  almost  universal  unconformity  of  the  rock-system,  and  a  very  great 
and  apparently  sudden  change  in  the  life-system,  a  change  affecting  not 
only  species  but  also  genera,  families,  and  even  orders.  Between  the 
last  and  the  preceding,  it  is  true,  neither  the  unconformity  of  the  rock- 
system  nor  the  change  in  the  life-system  is  so  great  as  in  the  others ; 
but  the  introduction  of  man  upon  the  scene  is  deemed  sufficient  to  make 
this  one  of  the  grand  divisions  of  time. 

We  have  already  seen  (p.  179)  that  unconformity  is  the  result  of 
deposit  of  strata  on  old  eroded  land-surfaces,  and  that  it  therefore  always 
indicates  an  oscillation  of  the  crust,  and  an  emergence  and  submergence 
of  land.  In  every  such  case,  as  already  explained,  a  portion  of  the 
record  is  lost,  which  may  or  may  not  be  recovered  elsewhere.  It  is 
certain  that  if  the  lost  leaves  could  be  all  recovered,  and  the  record  made 
complete,  the  suddenness  of  the  break  in  the  life-system  would  disap- 
pear. Nevertheless,  it  is  also  certain  that  these  general  unconformities 
indicate  times  of  great  change  in  physical  geography,  and  therefore  of 
climate,  and  therefore  of  rapid  changes  of  organic  forms ;  and  therefore, 
also,  they  mark  the  natural  boundaries  of  the  great  divisions  of  time. 

Ages. — Again,  the  whole  history  of  the  earth  is  otherwise  divided 
into  seven  ages,  founded,  with  perhaps  the  exception  of  the  first,  on  the 
culmination  of  certain  great  classes  of  organisms.  These  are :  1.  The 
Archaean  or  Eozoic  Age,  represented  by  the  Laurentian  system  of 
rocks;  2.  The  Age  of  MollusJcs,  represented  by  the  Silurian  series  of 
rocks ;  3.  The  Age  of  Fishes,  represented  by  the  Devonian  rocks  ; 
4.  The  Age  of  Acrogens,  or  sometimes  called  the  Age  of  Amphibians, 
represented  by  the  Carboniferous  rocks ;  5.  The  Age  of  Reptiles,  repre- 
sented by  the  Secondary  rocks  ;  6.  The  Age  of  Mammals,  by  the  Ter- 
tiary and  Quaternary;  and,  7.  The  Age  of  Man,  by  the  recent  rocks. 

In  the  accompanying  diagram  (Fig.  245),  vertical  height  represents 
time,  the  strong  horizontal  lines  divide  the  whole  into  eras,  while  the 
lighter  lines,  where  necessary,  separate  the  ages.  The  shaded  spaces 
represent  the  origin,  the  increase  and  decrease,  in  the  course  of  time,  of 
the  great  dominant  classes  of  animals  and  plants.  To  illustrate  :  The 
1  Dawn  of  animal  life.  *  Old  life.  3  Middle  life.  *  Recent  life. 


270 


GENERAL  PRINCIPLES. 


class  of  reptiles  commenced  in  the  Carboniferous  increased  to  a  maxi- 
mum in  the  Secondary,  and  again  decreased  to  the  present  time.     It 


Age  of  Man . 
Age  of  Mam- 


Age  of  Rep- 
tiles. 


Age  of  Aero- 
gens. 


Age  of  Fishes 


Age  of  Inver- 
tebrates. 


Archaean  Age. 


HIM!  11! I II  I 


III  14M-M1: 

LI,  I    i'-l  ^asSS  "III 

ii  i  > 


IBONIFEROUS 
DEVON  lf\N 
S  I  L  U  R  JAN 


LAUREN  T  I  A  N 


SYSTEM 


S  YS  T  E  M 


Psychozoic. 

Cenozoic. 

Mesozoic. 


Archaean,  or 
Eozoic. 


Fio.  245. 


will  be  seen  that  the  ages  correspond  with  the  eras,  except  in  the  case 
of  the  Palaeozoic  era.  This  long  and  diversified  era  is  clearly  divisible 
into  three  ages. 

Subdivisions. — The  subdivisions  of  eras  and  ages  into  periods  and 
epochs  are  founded,  as  already  explained  (p.  201),  on  less  general  un- 
conformity in  the  rock-system,  and  less  conspicuous  changes  in  the  life- 
system.  The  names  of  periods  are  often,  and  of  epochs  are  nearly 
always,  local,  and  therefore  different  in  different  countries.  We  will, 
of  course,  use  those  appropriate  to  American  geology.  The  table  on 
page  201  represents,  as  far  as  periods,  the  classification  used  in  this 
country.  We  have  added  epochs  only  in  the  uppermost  part,  viz.,  in 
the  Tertiary  and  Quaternary. 

We  give,  also  (Fig.  246),  an  ideal  diagram  of  the  principal  groups 
of  strata,  which  we  shall  notice  in  the  order  of  their  superposition,  indi- 
cating also  the  principal  places  of  general  unconformity. 

Order  of  Discussion. — Many  geologists  take  up  the  several  epochs 
and  periods  of  the  history  of  the  earth  in  the  inverse  order  of  their  oc- 
currence. Commencing  with  a  thorough  discussion  of  "  causes  now  in 
operation"  i.  e.,  geological  history  of  the  present  time,  as  that  which 
is  best  known,  they  make  this  the  basis  for  the  study  of  the  epoch 
immediately  preceding,  and  which,  therefore,  is  most  like  it.  Having 
acquired  a  knowledge  of  this,  the  studsnt  passes  to  the  preceding,  and 
so  on.  This  has  the  great  advantage  of  passing  ever  from  the  better 
known  to  the  less  known,  which  is  the  order  of  induction.  Other 


GENERAL   PRINCIPLES. 


2T1 


geologists  prefer  to  follow  the  natural  order  of  events.     This  has  the 
great  advantage  of  bringing  out 
the  philosophy  of  the  history — 
the  law  of  evolution.     The  first       EK' 
method  is  the  best  method  of  in-    '    sy°  ° 
vestigation ;  the  second  method 
is  the  best  method  of  presenta- 
tion.     In    common    with    most  4  cenozoie. 
geologists,  I  shall,  therefore,  fol- 
low the  order  of  history. 

As  in  human  history,  so  in  the 
geological  history,  the  recorded 
events  of  the  earliest  times  are 
very  few  and  meagre,  but  become  3;  Mesozoic..., 
more  and  more  numerous  and  in- 
teresting as  we  approach  the 
present  time.  Our  account  of 
the  Archaean  era  will,  therefore, 
be  quite  general,  and  will  not  en- 
ter into  any  subdivisions,  al- 
though this  era  was  very  long. 
In  the  next  era  we  will  go  into 
the  description  of  the  several 
ages,  in  the  next  into  the  pe- 
riods, and  in  the  next  even  into  2.  Palaeozoic., 
the  epochs. 

Prehistoric  Eras.  —  Previous 
to  even  the  dimmest  and  most 
imperfect  records  of  the  history 
of  the  earth  there  is,  as  already 
said  (p.  265),  an  infinite  abyss  of 
the  unrecorded.  This,  however, 
hardly  belongs  strictly  to  geol- 
ogy, but  rather  to  cosmic  philos- 
ophy. We  approach  it  not  by  i-  Archrean... 
written  records,  but  by  means  of 
more  or  less  probable  general 
scientific  reasoning.  We  pass  on, 
therefore,  without  pause  to  the 

lowest    system    of    rocks   contain-  FIG.  246.— Ideal  General  Section  of  the  Whole  Series  of 
J  Strata,  showing  the  Principal  Divisions  and  Subdi- 

ing  the  record  of  the  earliest  era.      visions. 


Carboniferous. 


Silurian. 


272  LAURENTIAN  SYSTEM  OF  ROCKS 


CHAPTER   II. 

LAURENTIAN  SYSTEM  OF  ROCKS  AND  ARCHAEAN  ERA. 

IT  is  one  of  the  chief  glories  of  American  geology  to  have  estab- 
lished this  as  a  distinct  system  of  rocks  and  a  distinct  era. 

It  had  been  long  known  that  beneath  the  lowest  Palaeozoic  rocks 
there  still  existed  strata  of  unknown  thickness,  highly  metamorphic 
and  apparently  destitute  of  fossils.'  These  had  been  usually  regarded 
as  lowermost  Palaeozoic — as  the  earliest  defaced  leaves  of  the  Palgeo- 
zoic  volume.  But  the  study  of  the  Canadian  rocks,  by  Sir  William 
Logan,  revealed  the  existence  of  an  enormous  thickness  of  highly-con- 
torted, metamorphic  strata,  everywhere  unconformable  with  the  over- 
lying Potsdam  or  lowest  Silurian.  More  recent  observations  show  this 
relation  not  only  in  Canada,  but  also  in  New  York,  on  Lake  Superior, 
in  Nebraska,  Montana,  Idaho,  Wyoming,  Colorado,  Utah,  Nevada, 
Texas,  New  Mexico,  and  Arizona.  Nor  is  it  confined  to  our  own  coun- 
try, for  the  same  unconformable  relation  has  been  found  by  Murchison 
on  the  west  coast  of  Scotland,  between  the  lowest  Silurian  (Cambrian) 
and  an  underlying  gneiss,  evidently  corresponding  to  the  Laurentian 
of  Canada.  Similar  rocks,  and  in  similar  unconformable  relation,  have 
been  found  underlying  the  lowest  Silurian  in  Bohemia,  and  also  in  Swe- 
den and  Bavaria.  Such  general  unconformity  shows  great  and  wide- 
spread changes  of  physical  geography  at  this  time.  There  seems  no 
longer  any  doubt,  therefore,  that  it  should  be  regarded  as  a  distinct 
system. 

The  following  figures  give  the  relation  between  the  Palaeozoic  and 
the  Laurentian  in  New  Mexico,  in  Canada,  and  in  Scotland. 


FIG.  247.— Section  across  Santarita  Mountain,  New  Mexico:  c.  Carboniferous;  S,  Silurian;  A,  Archscan  ; 
m,  metalliferous  vein  (after  Gilbert). 

These,  then,  are  the  oldest  known  rocks.  They  form  the  first  vol- 
ume of  the  recorded  history  of  the  earth.  Yet  they  evidently  are  not 
the  absolute  oldest ;  evidently  they  do  not  constitute  any  part  of  the 
primitive  crust.  For  they  are  themselves  stratified  or  fragmental 


AND   ARCILEAN  ERA. 


273 


rocks,  and  therefore  formed  from  the  debris  of  other  rocks  still  older 
than  themselves ;  and  these  last  possibly  from  still  older  rocks.     Thus, 


FIG.  '248.— Section  showing  Primordial  unconformable  on  the  Archaean :  1,  Archaean  or  Laurentian ; 
Primordial  or  Lowest  Silurian  (after  Logan). 


FIG.  249.— Diagram  Section,  showing  the  Structure  of  the  North 
dial ;  c,  Lower  Silurian  (Jukes). 


:  a,  Laurentian ;  6,  Primor- 


we  search  in  vain  for  the  so-called  primary  rocks  of  the  original  crust. 
Thus  is  it  with  all  history.  No  history  is  able  to  write  its  own  beginning. 

Rocks. — There  is  nothing  very  characteristic  in  the  rocks  of  the 
Laurentian  system.  They  do  not  differ  very  conspicuously  from  those 
of  other  periods ;  consisting,  in  fact,  only  of  altered  sandstones,  lime- 
stones, and  clays.  They  are  all,  however,  very  much  contorted  and 
very  highly  metamorphic.  In  Canada  they  consist  mainly  of  the  schist 
series,  passing  on  the  one  hand  into  gneiss  and  granite,  and  on  the 
other  into  hornblehdic  gneiss,  syenites,  and  diorites;  of  sandstones, 
passing  into  qnartzites;  and  of  limestones,  passing  into  marbles,  which 
are  sometimes  even  intrusive.  These  together,  in  Canada,  form  a 
series  of  rocks  at  least  40,000  feet  thick. 

Interstratified  with  these  are  found  immense  beds  of  iron-ore  100  or 
more  feet  thick,  and  great  quantities  of  graphite,  sometimes  impreg- 
nating the  rocks,  and  sometimes  in  pure  seams.  In  rocks  of  this  age 
occur  the  great  iron-beds  of  Missouri,  of  New  Jersey,  of  Lake  Superior, 
and  of  Sweden.  The  quantity  of  iron  found  in  these  strata  is  far 
greater  than  in  any  other.  It  may  be  well  called  the  Age  of  Iron. 

The  following  figures  show  the  contortion  of  the  strata  (Fig.  250), 
and  the  mode  of  occurrence  of  the  iron  (Figs.  251,  252). 


FiQ.  252. 


£74  LAURENTIAN  SYSTEM   OF  ROCKS 

Area  in  North  America. — 1.  These  strata  cover  the  greater  portion 
of  Labrador  and  Canada,  and  then,  turning  northwestward,  extend  to 
an  unknown  distance,  but  probably  to  the  Arctic  Ocean.  The  area  forms 
a  broad  V,  within  the  arms  of  which  is  inclosed  Hudson's  Bay.  It  may 
be  seen  on  map,  p.  278.  This  is  the  only  extensive  area  known  on  the 
continent.  2.  On  the  eastern  slopes  of  the  Appalachian  chain  un- 
doubted patches  are  found  as  far  south  as  Virginia,  and  a  considerable 
area  in  this  region  is  referred  with  much  probability  to  the  same.  This 
is  shown  on  map,  p.  278,  as  the  area  left  blank.  Its  further  extension 
southward  along  the  chain  is  still  doubtful,  though  probable.  3.  In 
the  Rocky  Mountain  region  extensive  lines  and  areas  of  outcrop  are 
known,  trending  in  the  general  direction  of  the  chain,  especially  a  large 
area  in  the  Basin  region.  4.  Several  small  patches  are  also  found  scat- 
tered about  in  the  basin  of  the  Mississipi,  apparently  exposed  by  erosion. 

Doubtless  the  Laurentian  rocks  are  far  more  extended,  but  covered 
and  concealed  by  other  and  later  rocks.  The  area  mentioned  is  the 
area  of  surface-exposure.  It  represents  so  much  of  Archaean  sea-bot- 
tom as  was  subsequently  raised  into  land,  and  not  afterward  again 
covered  by  sediments;  or,  if  so  covered,  again  exposed  by  erosion. 

Physical  Geography  of  Archaean  Times. — As  these  are  the  oldest 
known  rocks,  we  know  nothing  of  the  land  from  which  they  were 
formed.  But  since,  during  the  rest  of  the  geological  history,  the  con- 
tinent has  developed  from  the  north  toward  the  south,  it  seems  most 
probable  that  this  earliest  land  lay  still  farther  north,  and  disappeared 
when  the  Laurentian  area  was  elevated  into  land. 

Time  represented. — The  enormous  thickness  of  these  rocks  (40,000 
feet  in  Canada,  and  still  greater  in  Bohemia  and  Bavaria)  certainly  in- 
dicates a  very  great  lapse  of  time.  It  is  probable  that  the  Archaean 
era  is  longer  than  all  the  rest  of  the  recorded  history  of  the  earth  put 
together ;  and  yet,  precisely  as  in  the  beginnings  of  human  history,  the 
record  is  almost  a  blank.  The  events  are  few,  and  imperfectly  re- 
corded. 

Evidences  of  Life. — We  have  already  explained  (p.  136)  how  iron- 
ore  is  ^present  accumulated.  We  have  there  shown  that  all  accumu- 
lations or  this  kind  now  going  on  are  formed  by  the  agency  of  organic 
matter.  It  is  almost  certain  that  the  same  is  true  for  all  times,  and 
therefore  that  iron-ore  accumulations  are  the  sign  of  the  existence  of 
organic  matter,  and  quantity  of  the  ore  accumulated  is  a  measure  of  the 
amount  of  organic  matter  consumed  in  doing  the  work.  The  immense 
beds  of  iron-ore  found  in  the  Laurentian  rocks  are,  therefore,  evidence 
of  the  existence  of  organisms  in  great  abundance.  That  these  organ- 
isms were  chiefly  vegetable,  we  have  the  further  evidence  derived  from 
the  great  beds  of  graphite ;  for  graphite,  as  we  shall  see  hereafter  is 
only  the  extreme  term  of  the  metamorphism  of  coal. 


AND   ARCHAEAN  ERA. 


275 


Of  the  existence  of  animal  organisms  the  evidence  is  not  yet  com- 
plete, although  it  is  probable  that  the  lowest  forms  of  Protozoa,  such  as 
Rhizopods,  were  abundant.  We  have  seen  that  limestones  are  abun- 
dant among  the  Laurentian  rocks.  Now,  the  limestones  of  subsequent 


FIG.  253. — Fragment  of  Eozoon,  of  the  Natural  Size,  showing  Alternate  Lamince  of  Loganite  and  Dolomite 
(after  Dawson.) 

geological  epochs  are  almost  wholly  composed  of  the  accumulated  shelly 
remains  of  lower  organisms,  especially  nullipores  and  coccolites  among 
plants,  and  rhizopods  among  animals. 

The  existence  of  rhizopods  is  believed  by  many  to  have  been  demon- 
strated. There  have  been  found  abundantly,  in  the  Laurentian  lime- 
stones of  Canada,  of  Bohemia,  of  Bavaria,  and  elsewhere,  large,  irreg- 
ular, cellular  masses,  which  are  fwmur-/'-'-'  "> 

believed  by  the   best   authori-  iglliii  i,...,,,( 

ties  to  be  the  remains  of  a  gi- 
gantic foraminiferous  rhizopod. 
The  supposed  species  has  been 
called  Eozoon l  Canadense. 
Fig.  253  is  a  section  of  an 
Eozoonal  mass,  natural  size, 
in  which  the  white  is  the  cal- 
careous matter  secreted  by  the 
rhizopod,  and  the  dark  corre- 
sponds to  the  animal  matter  of 

,.  ,  .  .,  Fro.  254,-Diagram  of  a  Portion  of  Eozoon  cut  vertically: 

the  rhizopod  itself  ;  and  Flff.  A,  B,  <?,  three  tiers  of  chambers  communicating  with 

,1C,,  „  ,.  _  ..  one  another  by  slightly  constricted  apertures ;  a,  a, 

/i04  a  Small  portion  Ot  the  Same,  the  true  shell-wall,  perforated  by  numerous  delicate 

•fl-j  .  i  ,,  tubes;  b,  b,  the  main  calcareous  skeleton  ("interme- 

magnineu  SO  as  to  SHOW  the  diate  skeleton ");  c,  passage  of  communication  ("sto- 

a+m-in+ui.**  ^f  -(-V.^  11  Ion-passage")  from  one  tier  of  chambers  to  another : 

Structure  Ot  the  Cells.  ^  ramifying  tubes  in  the  calcareous  skeleton  (after 

There  has  been,  and  is  still,       CarPenter>- 

much  discussion  as  to  the  organic  or  mineral  nature  of  these  curious 

structures.     If  these  irregular  masses  be  indeed  of  animal  origin,  as 

1  Dawn  animal. 


276  PALAEOZOIC   SYSTEM   OF  ROCKS. 

seems  most  likely,  it  is  evident  that  they  belong  to  the  lowest  forms  of 
compound  protozoa — lower  far  than  the  symmetrically-formed  forami- 
nifera  of  later  times.  It  is  precisely  in  such  almost  amorphous  masses 
of  protoplasmic  matter  that,  according  to  the  evolution  hypothesis,  the 
animal  kingdom  might  be  expected  to  originate. 

Some  very  obscure  tracings,  suggesting  the  possible  existence  of 
marine  worms,  have  been  found  both  in  Canada  and  in  Bohemia ;  but 
as  yet  we  have  no  reliable  evidence  of  any  animals  higher  than  the  pro- 
tozoa. It  is  impossible  to  say  that  other  animals  of  low  form  did  not 
exist ;  yet  the  absence  of  any  reliable  trace  in  rocks  not  more  metamor- 
phic  than  some  of  the  next  era,  which  are  crowded  with  fossils  of  many 
kinds,  seems  to  indicate  a  paucity,  if  not  an  entire  absence,  at  this  time, 
of  such  animals. 


CHAPTER  III. 

PRIMARY  OR  PALAEOZOIC  SYSTEM  OF  ROCKS  AND  PALAEOZOIC 

ERA. 

General  Description. 

THIS  is  a  distinct  system  of  rocks,  revealing  a  distinct  time-world — 
a  distinct  rock-system,  containing  the  record  of  a  distinct  life-system. 
The  rock-system  is  distinct,  being  everywhere  unconformed  to  the 
Laurentian  below  and  the  Secondary  above — a  bound  volume — volume 
second  of  the  Book  of  Time.  The  life-system  is  also  equally  distinct, 
being  conspicuously  different  from  that  which  precedes  and  that  which 
follows.  Whatever  of  life  existed  before,  its  record  is  too  imperfect  to 
give  us  a  clear  conception  of  its  character.  But  in  the  Palaeozoic  the 
evidences  of  abundant  and  very  varied  life  are  clear ;  about  20,000  spe- 
cies having  been  described.  It  stands  out  the  most  distinct  era  in  the 
whole  history  of  the  earth.  The  Archaean  must  be  regarded  as  the 
mythical  period.  Here,  with  the  Palaeozoic,  commences  the  true  dawn 
of  history. 

Rocks— Thickness,  etc. — The  rocks  of  this  system,  although  less 
powerful  than  the  preceding,  are  also  of  enormous  thickness  compared 
with  those  of  later  geological  times,  being  in  the  Appalachian  region 
about  40,000  feet.  It  is  believed  that  we  are  safe  in  saying  that  the 
time  represented  by  them  is  equal  to  all  subsequent  time  to  the  present. 

There  is  nothing  very  characteristic  in  the  rocks  composing  Palaeo- 
zoic strata,  though  the  practised  eye  may  often  distinguish  them  by 
their  lithological  character.  Though  strongly  folded  and  highly  meta- 


GENERAL  DESCRIPTION.  277 

morphic  in  some  regions,  these  characters  are  not  so  universal  as  in  the 
Laurentian. 

In  the  United  States  the  rocks  of  the  whole  system  are  conformable. 
In  Europe,  on  the  contrary,  the  principal  divisions  are  usually  uncon- 
formable.  In  this  country,  therefore,  the  subdivisions  are  founded 
almost  wholly  on  change  in  the  life-system  ;  while  in  Europe  the  same 
subdivisions  are  founded  on  unconformity  of  the  rock-system,  as  well  as 
change  in  the  life-system.  Further,  in  this  country,  in  passing  from 
Pennsylvania,  through  New  York,  into  Canada,  we  pass  over  the  out- 
cropping edges  of  the  whole  system,  from  the  highest  to  the  lowest ; 
and  finally  into  the  Laurentian  (Fig.  255).  This,  taken  in  connection 
with  the  conformity  of  the  rocks,  shows  that  during  the  Palaeozoic  the 
continent  in  this  part  was  successively  developed,  from  the  north  toward 
the  south,  by  bodily  upheaval  of  the  Laurentian  area  and  successive  ex- 
posure of  contiguous  sea-bottom.  In  Europe  the  oscillations  seem  to 
have  been  more  frequent  and  violent. 

Fig.  255  is  a  section  from  Pennsylvania  to  Canada,  showing  the 


JJS  '  li  S       I          A. 


FIG.  255.— Ideal  Section  north  and  south  from  Canada  to  Pennsylvania :  A,  Archsean ;  L  S  and  US, 
Silurian ;  Z>,  Devonian  ;  C,  Carboniferous. 

relation  of  the  subdivisions  to  each  other,  and  the  manner  in  which  they 
lie  on  the  Archaean.  This  will  be  better  understood  if  the  map  on  page 
284  be  at  the  same  time  carefully  examined. 

Area  in  the  United  States. — The  area  in  the  Eastern  United  States 
in  which  the  country  rock  belongs  to  this  system  is  seen  in  the  map  given 
on  next  page  (Fig.  256).  It  may  be  stated  roughly  to  embrace  all  that 
part  included  between  the  Great  Lakes  on  the  north,  the  Blue  Ridge  of 
the  Appalachian  chain  on  the  east,  the  Prairies  on  the  west,  and  Middle 
Alabama  and  Southern  Arkansas  on  the  south.  It  includes  the  richest 
portion  of  our  country.  Besides  this  great  continuous  area  there  are 
also  areas  of  imperfectly  known  size  and  shape  in  the  Rocky  Mountain 
region,  and  on  either  side  of  the  Sierra  Nevada. 

If  we  compare  the  Palaeozoic  rocks  of  the  Appalachian  region 
with  the  same  in  the  central  portion  of  the  Mississippi  basin,  we  ob- 
serve the  following  changes  as  we  go  westward :  (a.)  The  rocks  in  the 
Appalachian  region  are  highly  metamorphic  ;  as  we  go  westward,  they 
become  less  and  less  so,  until  in  the  region  about  the  Mississippi  River 
they  are  wholly  unchanged,  (b.)  In  the  Appalachian  region  they  are 
strongly  and  complexly  folded;  as  we  go  west,  these  folds  pass  into 


278 


PALAEOZOIC   SYSTEM   OF  ROCKS. 


gentle    undulations,  which  die   away  into  horizontally  (see    section 
on  p.  244).     (c.)  In  the  Appalachian  region  they  are  about  40,000  feet 


thick  ;  as  we  go  west,  they  thin  out  until  the  whole  series  is  only  4,000 
feet  at  the  Mississippi,    (d.)  In  the  Appalachian  region  grits  and  sand- 


GENERAL   DESCRIPTION.  279 

stones  and  shales  predominate  greatly  over  limestones ;  as  we  go  west, 
the  proportion  of  limestone  increases,  until  these  are  the  predominating 
rocks.  These  four  changes  are  closely  connected  with  each  other,  and 
all  with  the  formation  of  the  Appalachian  chain,  as  we  have  already 
explained  in  the  chapter  on  Mountain-Formation  (p.  252,  et  seq.). 

Physical  Geography  of  the  American  Continent. — At  the  beginning 
of  the  Palaeozoic  era  the  land  was  the  Laurentian  area,  already  described, 
excepting  Archaean  areas  exposed  by  erosion.  From  this  nucleus,  dur- 
ing Palaeozoic  times,  the  continent  was  developed  southward,  until, 
at  the  end,  it  included  also  the  Palceozoic  area  just  described.  The 
accompanying  map  (Fig.  257) '  gives  approximately  the  area  of  land  at 


FIG.  257.— Existing  Seas.  Lakes,  etc..  shaded  black  ;  Portions  of  Continent  then  covered,  lighter;  Land  of 
that  Time  left  white ;  where  Outline  known,  surrounded  with  Full  Line ;  when  doubtful,  by  Dotted  Line. 

the  beginning.  The  map  of  the  physical  geography  of  Cretaceous  times 
(p.  452)  gives  somewhat  less  approximately  its  area  at  the  end.  It 
will  be  seen  that  the  continent  was  already  sketched  out  at  the  begin- 
ning, and  steadily  developed  throughout  its  continuance.  There  is 
much  reason  to  believe  that  a  considerable  body  of  land  existed  at  this 

1  A  map  similar  to  the  above,  but  containing  also  small  scattered  patches  of  Archaean 
exposures,  is  sometimes  spoken  of  as  an  Archaean  map  of  North  America,  or  map  of 
Archaean  land.  It  must  be  borne  in  mind,  however,  that  it  represents  indeed  land  of 
Archaean  strata,  but,  for  that  very  reason,  not  of  Archaean  time,  but  of  Silurian  time. 


280  PALAEOZOIC   SYSTEM   OF   ROCKS. 

time  to  the  east  of  the  Appalachian  region,  much  of  which  afterward 
disappeared  by  subsidence.  It  is  only  thus  that  we  can  explain  the 
thick  strata  of  this  region. 

Subdivisions. — The  Palaeozoic  era  is  divided  into  three  ages,  which 
are  embodied  in  three  distinct  subordinate  rock-systems.  These  ages 
are  each  characterized  by  the  dominance  of  a  great  class  of  organisms. 
They  are  :  1.  The  Silurian  System,  or  Age  of  Invertebrates,  or  some- 
times called  Age  of  Mollusks ;  2.  The  Devonian  System,  or  Age  of 
Fishes;  and,  3.  The  Carboniferous  System,  or  Age  of  Acrogens  and 
Amphibians.  These  are  three  chapters  in  the  Palaeozoic  volume. 

These  three  systems  are  generally  conformable  with  each  other  in 
the  Palaeozoics  of  the  United  States,  as  we  have  already  shown,  but 
elsewhere  they  are  often  unconformable.  Before  taking  up  the  first  in 
the  order  of  time,  viz.,  the  Silurian,  it  is  necessary  to  say  something  of 
the  interval  which  in  our  record  separates  the  Archaean  from  the  Palaeo- 
zoic era. 

The  Interval. — We  have  already  seen  that  the  lowest  Silurian  lies 
unconformably  on  the  upturned  and  eroded  edges  of  the  crumpled  strata 
of  the  Lauren tian.  We  have  also  shown  (p.  179)  that  unconform- 
ability  indicates  always  an  oscillation  of  the  earth's  crust  at  the  ob- 
served place.  More  definitely  it  indicates  an  upheaval,  by  which  the 
lower  series  of  rocks  became  land-surface,  and  were  at  the  same  time, 
perhaps,  crumpled  ;  then  a  long  period  unrecorded  at  that  place,  during 
which  the  land  was  eroded  and  the  edges  of  the  crumpled  rocks  were  ex- 
posed; then  a  subsidence,  and  the  deposit  of  the  upper  series  of  rocks  on 
these  exposed  edges.  Now,  oscillation  necessitates  increase  and  decrease 
of  land-surface.  Evidently,  therefore,  such  increase  and  decrease  of  land- 
surface  took  place  in  the  unrecorded  interval  between  the  Archaean  and 
Palaeozoic  eras  ;  and  the  length  of  this  unrecorded  interval  is  measured 
by  the  amount  of  erosion  which  the  Laurentian  underlying  the  lowest 
Palaeozoic  has  suffered.  We  have  stated  that  the  land  at  the  beginning 
of  the  Silurian  age  was  approximately  the  Laurentian  area.  The  shore- 
line of  the  earliest  Palaeozoic  sea  was  the  line  of  junction  between  the 
Silurian  and  Laurentian  (see  map,  p.  278).  But  this  was  not  the  shore- 
line at  the  end  of  the  Archaean  time.  ,  Evidently  this  shore-line  was 
much  farther  south ;  evidently  the  land-area  was  much  greater  at  the 
end  of  the  Archaean  than  at  the  beginning  of  the  Silurian.  The  Archae- 
an era  was  closed  by  the  upheaval  into  land-surface  and  the  crumpling 
of  the  strata  of  the  whole  Laurentian  area,  and  much  more.  Then  fol- 
lowed an  interval  of  which  we  know  nothing,  except  that  it  was  of  long 
duration,  during  which  the  crumpled  Laurentian  strata  forming  the  then 
land-surface  were  deeply  eroded.  Then,  at  the  end  of  this  interval  came 
a  subsidence  down  to  the  shore-line  already  indicated  as  the  Silurian 
shore-line,  and  the  Silurian  age  commenced,  its  first  sediments  being 


GENERAL   DESCRIPTION. 


281 


of  course  deposited  on  the  exposed  edges  of  the  submerged  Laurentian 
rocks. 

I  have  attempted  to  illustrate  these  facts  by  the  following  diagrams 
(Fig.  258),  in  which  c  represents  a  section  north  and  south  through  the 
Laurentian  and  Palaeozoic  rocks.  The  crumpled  Laurentian  strata,  with 
their  outcropping  eroded  edges,  are  seen  to  underlie  the  lowest  Silurian 
to  some  distance.  This  is  the  actual  condition  of  things.  The  manner 


FIG.  258.— Ideal  Sections,  showing  how  Unconformity  is  produced. 

in  which  this  condition  was  brought  about  is  shown  in  a  and  b.  In  a 
we  have  represented  the  supposed  condition  of  things  during  the  inter- 
val, s  I  being  the  sea-level,  and  s  the  shore  line  ;  in  b  the  condition  of 
things  at  the  end  of  the  interval  or  beginning  of  the  Silurian,  when  by 
subsidence  the  shore-line  had  been  shifted  northward  to  s',  and  on  the 
exposed  edges  of  the  strata  of  the  previous  land-surface,  from  s  to  s', 
Silurian  sediments  had  begun  to  deposit. 

We  have  spoken  thus  far  only  of  the  unconformity  of  the  New 
York  rocks  on  the  Canadian  rocks.  This  phenomenon  may  be  explained, 
as  we  have  seen,  by  local  oscillations,  with  increase  and  decrease  of  land- 
area  during  the  lost  interval.  But,  when  we  remember  that  the  same 
unconformity  is  found  in  the  most  widely-separated  localities,  over 
the  whole  area  of  the  United  States,  we  are  forced  to  the  conclusion 
that  the  lost  interval,  as  compared  with  the  Silurian,  was  probably  a 
continental  period — a  period  of  widely-extended  land  composed  of 


282  PALAEOZOIC  SYSTEM  OF   ROCKS. 

Laurentian  rocks.  The  whole  of  this  land  disappeared  by  submergence 
at  the  beginning  of  the  Silurian,  except  the  Canadian  area,  and  prob- 
ably a  considerable  area  in  the  Basin  region,  and  perhaps  a  few  islands 
or  larger  areas  in  Silurian  seas  between. 

In  all  speculations  on  the  origin  of  the  animal  kingdom  by  evolu- 
tion, it  is  very  necessary  to  bear  in  mind  this  lost  interval,  for  it  was 
evidently  of  great  duration. 

SECTION  1. — SILURIAN  SYSTEM  :  AGE  OF  INVERTEBRATES. 

The  Rock-System. — The  rocks  of  this  age  have  been  carefully  studied 
in  England,  by  Sedgwick  and  Murchison ;  in  Russia  and  Sweden,  by 
Murchison  ;  in  Bohemia,  by  Barrande  ;  and  in  New  York,  by  Hall.  The 
divisions  and  subdivisions  established  by  these  geologists  have  become 
the  standard  of  comparison  elsewhere.  The  system  was  first  clearly 
defined  by  Murchison  in  Wales.  The  name  Silurian  (from  Silures,  the 
Roman  name  for  the  inhabitants  of  Wales)  was  given  by  him,  and  is 
now  universally  adopted.  But  the  most  perfect  examples  are,  perhaps, 
those  found  in  Bohemia  and  in  New  York.  We  have  already  given 
(Fig.  255)  a  section  of  the  Palaeozoics  of  New  York,  including,  of  Course, 
the  Silurian.  Some  geologists  call  the  lower  portion  Cambrian — a 
name  given  bv  Sedgwick. 

Subdivisions. — The  following  table  gives  the  divisions  and  subdi- 
visions of  the  Silurian  system  and  the  corresponding  periods  of  this 
age  in  this  country  : 

rOriskany  Period. 

TT          r,.,    .        I  Lower  Helderbere 
Upper  Silurian.  jSa]ina 

[Niagara 

(  Trenton 

Lower  Silurian.  -I  Canada 

[Primordial 

The  larger  divisions,  viz.,  Lower  and  Upper  Silurian,  are  generally 
recognized  ;  also,  the  Primordial  is  generally  recognized  ;  by  some  as  a 
subdivision  of  the  Silurian,  by  others  as  more  distinct  than  the  other 
periods  and  as  synonymous  with  Cambrian.  The  subdivisions  are, 
with  this  exception,  local,  each  country  having  its  own  ;  but  they  are 
synchronized,  as  far  as  possible,  by  comparison. 

Character  of  the  Rocks. — The  Silurian,  like  nearly  all  rocks,  are 
greatly  disturbed  and  metamorphosed  in  mountain-regions,  though  less 
so  than  the  Laurentian ;  but  in  Sweden  and  Russia,  and  in  the  valley 
of  the  Mississippi,  Silurian  rocks  are  found  in  their  original  horizon- 
tal position,  and  not  greatly  changed  from  their  original  sedimentary 
condition. 

Area  in  America. — By  turning  to  the  map  (p.  278)  it  will  be  seen : 
1.  That  the  Silurian  is  attached  to  the  Laurentian  nucleus  as  an  irregu- 


SILURIAN   SYSTEM:  AGE   OF  INVERTEBRATES.  283 

lar  border  on  the  outer  side  of  the  V-shaped  area ;  2.  Again,  the  Ap- 
palachian Laurentian  region  is  also  bordered  on  the  west  side  by 
Silurian  ;  3.  Also  we  observe  large  patches  in  the  interior — one  about 
Cincinnati,  another  occupying  the  southern  portion  of  Missouri  and 
northeastern  portion  of  Arkansas,  and  one  in  Middle  Tennessee ;  4. 
Also,  large  areas  are  known  to  occur  in  the  Rocky  Mountain  region  and 
in  the  Basin  region  between  the  Wahsatch  Mountain  and  the  Sierra  ; 
but  their  outlines  are  yet  too  little  known  to  describe  them  accurately. 

Physical  Geography. — At  the  beginning  of  the  Silurian,  as  already 
said,  the  land  was  approximately  the  Laurentian  area  (Fig.  257).  The 
Silurian,  which  embraces  the  great  V-shaped  Laurentian  area  on  the 
southeast,  south,  and  southwest,  was  then  the  sea-bottom  border  of  the 
coast  of  that  Silurian  continent.  The  Silurian  bordering  the  Appalachian 
Laurentian  was  also  then  a  sea-bottom  bordering  the  Silurian  continent 
in  that  region.  It  is  probable,  also,  that  the  Silurian  of  the  Rocky 
Mountain  region  also  borders  Laurentian  areas,  and  these  areas  repre- 
sent Silurian  continents,  and  the  Silurian  border  the  marginal  sea-bot- 
tom of  that  time.  The  other  patches  mentioned  in  the  interior  were 
probably  bottoms  of  open  seas. 

Now,  the  Silurian  area  represents  so  much  of  Silurian  sea-bottoms 
as  were  raised  into  land-surfaces  during  or  at  the  end  of  Silurian  times, 
and  not  subsequently  covered  by  sea.1  Therefore,  at  the  beginning  of 
Silurian  times  the  land  was  the  Laurentian  area ;  while  at  the  end  of 
the  Silurian  times  the  land  was  increased  by  the  addition  of  the  Silurian 
area.  This  addition  was  not  all  made  at  once,  but  very  gradually.  The 
steps  of  this  increase  have  been  carefully  studied  in  New  York.  The 
following  map  (Fig.  259)  shows  the  principal  successive  steps,  as  does 
also  the  section  (Fig.  255)  with  which  it  should  be  compared.  Inspec- 
tion of  these  figures  shows  not  only  the  Silurian  bordering  the  Laurentian, 
but  the  rocks  of  the  several  periods  bordering  each  other  successively  ; 
so  that  in  walking  from  Pennsylvania  to  Canada,  or  to  the  Adirondack 
Mountains  of  New  York,  we  successively  walk  over  the  Carboniferous, 
the  Devonian,  the  Silurian,  and  the  Laurentian ;  and  in  the  Silurian 
over  rocks  of  the  successive  periods,  from  the  highest  to  the  lowest. 
This  plainly  shows  that  during  Silurian  times  the  continent  (Laurentian 
area)  was  slowly  upheaved,  and  contiguous  sea-bottoms  successively 
added  to  the  land,  and  the  shore-line  gradually  pushed  southward  from 
the  Canadian  region,  and  probably  westward  from  the  land-mass  along 
the  Appalachian.  Of  course,  therefore,  the  oldest  Silurian  shore-line 
was  the  most  northern  and  eastern.  This  is  the  primordial  beach. 

Primordial  Beach  and  its  Fossils.— As  already  stated,  the  element- 
ary character  of  this  treatise  renders  it  impossible  to  take  the  several 

1  This  is  true  as  a  broad,  general  fact ;  but  patches  of  Silurian  may  also  be  exposed 
by  removal  of  later  deposits  by  erosion. 


284 


PALEOZOIC  SYSTEM  OF  ROCKS. 


periods  of  this  age.  We  must  confine  ourselves  to  a  general  descrip- 
tion of  the  age  only.  But  there  is  so  peculiar  and  special  an  interest 
connected  with  the  dawn  of  life  on  the  earth,  that,  before  taking  up 
the  life-system  of  the  whole  age,  it  seems  necessary  to  say  something 
of  the  earliest  fauna. 

We  have  seen  that  at  the  beginning  of  Silurian  times  a  large  V- 
shaped  mass  of  land  occupied  the  region  now  embraced  by  Canada  and 
Labrador,  and  stretched  northwestward  to  an  unknown  distance,  the  two 
arms  of  the  V  being  nearly  parallel  to  the  two  present  shores  of  the 
American  Continent  ;  further,  that  a  land-mass  of  extent  unknown 
occupied  the  position  of  the  eastern  slope  of  the  Appalachian  chain  ; 
also,  that  land  of  unknown  extent  occupied  the  position  of  the  Rocky 
Mountains  ;  and  the  continent  was  thus  early  sketched  out.  Now, 
southward  of  the  first-mentioned  land-area  and  betwreen  the  other  two 


FIG.  259.— Oeolojrical  Map  of  New  York :    o,  Archaean  ;   PS,  Primordial ;  LS,  Lower  Silurian  ; 
US,  Upper  Silurian ;  d,  Devonian ;   SC,  Subcarboniferous ;  C,  Coal-measures. 

there  was  a  great  interior  sea,  which  we  will  call  the  Interior  Palceozoic 
Sea.  The  shores  of  that  sea  beat  upon  the  continental  masses  north, 
east,  and  west,  and  accumulated,  on  exposed  places,  a  beach.  Patches 
of  that  earliest  beach  still  remain.  They  are  found,  of  course,  closely 
bordering  the  Laurentian  rocks,  Canadian  and  Appalachian,  and  lying  un- 
conformably  upon  them.  They  are  the  primordial  sandstones  and  slates 
of  Canada,  New  York,  Pennsylvania,  Virginia,  and  probably  Tennes- 


SILURIAN  SYSTEM:   AGE  OF  INVERTEBRATES.  285 


FIGS.  260-69.  — AMEBICAX  PRIMORDIAL  FOSSILS:  260.  Plant:  Scolithus  linearis  (after  Hall).  261. 
Brachiopod  :  a  and  6,  Linsrula  acmninata  (after  Lo^an).  262.  Lineula  antiqua  (after  Hall).  _  263. 
Gasteropod  :  Ophileta  compacta.  264.  Cephalopod :  Orthoceras.  265.  Pteropod  :  Hyahthis  primor- 
dialis  (after  White).  266.  Tracks:  Crustacean  (after  White).  267.  Trail  of  Marine  Worm  (after 
Logan).  263.  Conocoryphe  Kingii  (after  White).  269.  Agnostus  interstrictus  (after  White). 


286 


PALEOZOIC  SYSTEM  OF  ROCKS. 


see,  and  possibly  Georgia.  The  fact  that  these  are  indeed  remnants  of 
a  beach  is  proved  by  the  existence,  in  almost  every  part,  of  shore-marks 
of  all  kinds — such  as  ripple-marks,  sun-cracks,  worm-tracks,  worm-bor- 
ings, broken  shells,  etc. 

This,  then,  is  the  old  primordial  beach.  It  is  of  the  extremest  inter- 
est to  the  geologist  because  it  marks  the  outline  of  the  earliest  Silurian 
sea,  and  contains  the  remains  of  the  earliest  Silurian  fauna.  Indeed, 
we  may  say  it  contains  the  remains  of  the  earliest  known  fauna.  It  is 
true,  the  lowest  Rhizopods  probably  existed  in  Archaean  times,  but 
these  cannot  be  said  to  constitute  a  fauna.  With  the  very  commence- 
ment of  Silurian  times,  however,  we  find  at  once  a  considerable  variety 
of  animal  forms. 

What,  then,  was  the  character  of  this  earliest  fauna  and  flora  ?  If 
we  could  have  walked  along  that  beach  when  it  was  washed  by  pri- 
mordial seas,  what  would  we  have  found  cast  ashore  ?  We  would  have 
found  the  representatives  of  all  the  great  types  of  animals  except  the 
vertebrata.  The  Protozoa  were  then  represented  by  sponges  and 
Rhizopods  ;  the  Radiates  by  Hydrozoa  (graptolites)  and  Cystidean 

Crinoids  ;  the  mollusks  by  Brachiopods,  Gasteropods  (Pleurotomaria), 
Pteropods,  and  even  Cephalopoda  (orthoceras)  ;  and  the  Articulates  by 

Crustaceans  (trilobites,  etc.)  and  Worms.  Plants  are  represented  by 
Fucoids.  These  widely-distinct  classes  are  already  clearly  differentiated 
and  somewhat  highly  organized.  Nor  is  the  fauna  a  meagre  one  in 
number  of  species.  In  the  United  States  and  Canada  alone  about  200 
species  are  already  known,  of  which  nearly  100  are  trilobites.  About 
a  dozen  species  of  plants  are  also  known.  When  we  recollect  the  great 


FIGS.  270-278—FoRKiGN  PRIMORDIAL  FOSSILS:  270.  Oldhamia  antiqua.  probably  a  plant.    271    Arenico- 
272.   Linpulclla  ferrufrinea.     273.    Theca  Davidii.     274.  Modiolopsis 


lites  didymus.  Worm-tubes, 
solvensis.     275.   Orthis  Hicksii. 
Olenus  macrurus. 


270.  Obolella   sagittalis.     277.  Hymenocaris   vermicauda.     278. 


SILURIAN   SYSTEM:   AGE   OF  INVERTEBRATES. 


287 


age  of  these  rocks  and  their  usual  metamorphism,  and  the  fragmentary 
character  of  all  fossil  fauna,  it  seems  certain  that  great  abundance  and 
variety  of  life  existed  already  in  these  early  seas.  Of  this  life  the  tri- 
lobites,  by  their  size,  their  abundance,  their  variety,  and  their  high 
organization,  must  be  regarded  as  the  dominant  type.  Among  the 
largest  trilobites  known  at  all  are  some  from  this  period.  The  Para- 
doxides, represented  in  Figs.  279  and  280,  attained  a  length  of  twenty 


FIG.  279.— Paradoxides  Bohemicus, 
Foreign. 


FIG.  280.— Paradoxides  Hsrlani, 
x  J  (after  Kogers),  American. 


inches.  English  beds  of  the  same  age  furnish  specimens  of  the  same 
genus  two  feet  long. 

We  give  in  the  above  figures  a  few  of  the  more  remarkable  primor- 
dial forms  taken  from  the  rocks  of  this  country,  and  of  foreign  coun- 
tries. They  are  intended  only  to  give  a  general  idea  of  the  fullness 
and  variety  of  the  primordial  life  ;  the  affinities  of  these  fossils  will  be 
discussed  hereafter. 

General  Remarks  on  First  Distinct  Fauna. — There  are  several 
points  of  great  philosophic  interest  suggested  by  the  nature  of  these 
first  organisms  : 

1.  Plants  in  this,  and  in  all  other  geological  periods,  are  far  less 
numerously  represented  in  a  fossil  state  than  animals.     This  cannot  be 
because  animals  were  more  abundant  than  plants,  for  since  the  animal 
kingdom  subsists  on  the  vegetable  kingdom,  and  since  every  animal 
consumes  many  times  its  own  weight  of  food,  plants  must  have  been 
always  more  abundant  than  animals.     The  true  reason  of  the  greater 
abundance  of  animal  remains  is  to  be  found  in  the  fact  that  the  hard 
parts  of  animals  are  far  more  indestructible  than  any  portion  of  vege- 
table tissue.    . 

2.  At  the  end  of  the  Archa3an  times — when  the  Archaean  volume 


288  PALAEOZOIC  SYSTEM  OF  ROCKS. 

closed — we  find  only  the  lowest  Protozoan  life.  But  with  the  opening 
of  the  next  era,  apparently  with  the  first  pages  of  the  next  volume,  we 
find  already  all  the  great  types  of  structure  except  the  vertebrata.  And 
these  not  the  lowest  of  each  type,  as  might  have  been  expected,  but  al- 
ready trilobites  among  Articulata,  and  Cephalopods  among  Mollusca — 
animals  which  can  hardly  be  regarded  as  lower  than  the  middle  of  the 
animal  scale. 

We  must  not  hastily  conclude,  however,  that  these  widely-divergent 
and  highly-organized  types  originated  together  at  once.  We  must  re- 
member that  between  the  Archaean  and  Palaeozoic  there  is  a  lost  interval 
of  enormous  duration.  Evidently,  therefore,  the  Primordial  fauna  is  not 
the  actual  first  fauna.  Evidently  we  have  not  yet  recovered  the  leaves 
in  which  is  recorded  the  gradual  differentiation  of  these  widely-distinct 
types.  All  this  must  have  taken  place  during  the  lost  interval. 

But  if,  on  the  other  hand,  we  suppose,  as  many  do,  that  evolution 
proceeds  always  "  with  equal  steps,"  then  we  are  forced  to  the  very  im- 
probable conclusion  that  the  lost  interval  is  equal  to  all  geological  times 
which  followed  to  the  present ;  for  the  differentiation  of  types  which 
occurred  during  that  interval  is  equal  in  value  to  all  that  has  taken 
place  since. 

Therefore,  we  are  compelled  to  admit  that  there  have  been  in  the 
history  of  the  earth  periods  of  rapid  change  in  physical  geography,  and 
periods  of  comparative  quiet  in  this  respect ;  that,  corresponding  with 
these,  there  have  been  also  periods  of  rapid  evolution  of  the  organic 
kingdom,  developing  new  forms,  and  periods  in  which  forms  are  more 
stationary.  The  periods  of  rapid  change  are  marked  by  unconformity, 
and  are  therefore  unfortunately  often  lost. 

As  we  proceed,  we  will  probably  find  many  examples  of  rapid  change 
which  must  be  accounted  for  in  a  similar  manner. 

General  Life- System  of  the  Silurian  Age. 

There  were  evidently  extraordinary  abundance  and  variety  of  life  in 
the  Silurian.  These  early  seas  literally  swarmed  with  living  beings. 
The  quantity  and  variety  of  life — the  number  of  individuals  and  of 
species — were  probably  not  less  than  at  the  present  time ;  though  orders, 
classes,  and  departments,  were  less  diversified.  Over  10,000  species  have 
been  described  from  the  Silurian  alone  (Barrande)  ;  and  these  must  be 
regarded  as  only  a  small  fragment  of  the  actual  fauna  of  the  age.  In  cer- 
tain favored  localities,  the  number  of  species  found  in  a  given  area  of  a 
single  stratum  will  compare  favorably  with  the  number  now  existing  in 
an  equal  area  of  our  present  sea-bottoms.  Yet,  in  all  this  teeming  life 
there  is  not  a  single  species  similar  to  any  found  in  any  other  geological 
time.  And  not  only  are  the  species  peculiar,  but  even  the  genera,  the 
families,  and  the  orders,  are  different  from  those  now  existing. 


GENERAL  LIFE-SYSTEM  OF  THE  SILURIAN  AGE.  £89 


FIG.  2S4.  r,G.  285. 

FIGS.  281-286.— SILURIAN  PLANTS:  2S1.  Sphenothallus  an<nistifolius  (after  Hall).  2S2.  Buthotrephis 
succulens  (aftor  Hall).  -.'S3.  tt  and  b,  Buthotrc-phis  gracUis  (after  Hall).  2S1.  Arthrophycus  H?rlani 
Cafter  Hall).  2S5.  Ouziana  bilobata  (after  Hall) 


290  PALEOZOIC  SYSTEM  OF  ROCKS. 

"We  can  give  only  a  very  brief  sketch  of  this  early  life,  touching  only 
the  most  salient  points,  especially  such  as  throw  light  on  the  great 
question  of  evolution. 

Plants. 

The  only  plants  yet  found  are  the  lowest  forms  of  cellular  crypto- 
gams, viz.,  marine  algae,  or  sea-weeds.  It  is  difficult,  from  the  impres- 
sions left  by  these,  to  determine  genera,  much  more  species,  with  any 
degree  of  certainty.  We  shall,  therefore,  call  them  by  the  general  some- 
what indefinite  name  of  Fucoids  (Fucus,  tangle  or  kelp),  or  Fucus-like 
plants.  As  already  stated,  plants  are  far  less  abundantly  and  perfectly 
preserved  than  animals,  on  account  of  their  want  of  a  skeleton. 

"We  give  above  some  of  the  more  characteristic  Fucoids  of  the  Silu- 
rian age. 

Animals. 

Protozoans. — The  large,  irregular  masses  which  are  called  EozoOn 
seem  entirely  characteristic  of  Archaean  times.  They  are  replaced  in 
the  Silurian  age  by  the  more  regular  sponges.  Of  these,  the  most 
characteristic  Silurian  genera  are  Stromatopora  and  Receptaculitis. 
They  seem  to  have  formed  large  coralline  masses,  which  are  regarded 
either  as  calcareous  sponges,  or  as  compound  Rhizopods  like  EozoOn. 


FIG.  286. — Stromatopora  rugosa. 


Radiates,  Corals. — Corals  were  very  abundant,  forming  often  whole 
rock-masses,  as  if  they,  while  living,  formed  reefs.     These,  if  they  in- 


ANIMALS. 


291 


dicate  warm  seas,  show  a  great  uniformity  of  temperature,  since  they 
are  found  in  all  portions  of  the  earth  alike. 

The  corals  of  the  Silurian  age  belong  principally  to  three  families, 


Fia.  290. 

FIGS.  2S7-290.— SILURIAN  PROTOZOANS:  28T.  a,  Stromatopora  concentrica ;  5,  section  of  same;  c,  view 
from  above  (after  Hall).  2SB.  Keceptaculitis  formosus  (after  Worthen).  2S9.  Diagram  showing 
Strucure  of  Keceptaculitis  (after  Nicholson).  290.  Brachiospongia  Euemerana  x  i  (after  Marsh). 


292  PALEOZOIC  SYSTEM  OF  ROCKS. 

viz.,  Cyathophylloids,  or  cup-corals ;  Favositidce,  or  honey-combed 
corals  ;  and  Halysitidce,  or  chain-corals.  They  are  remarkable  in  not 
usually  being  profusely  and  widely  branched  like  most  modern  corals, 
but  consisting  mostly  of  masses  of  parallel  or  nearly  parallel  columns.  In 
Cyathopliylloids  the  corals  are  sometimes  separate  and  of  a  horn-like 
form,  and  sometimes  aggregated  in  large,  rough,  columnar  masses  (Ru- 
gosa).  Their  upper  portions  are  cup-shaped,  and  the  radiating  laminae 
are  very  distinct.  In  Favositids  the  hexagonal  parallel  columns  are  di- 
vided somewhat  minutely  b}r  horizontal  plates  (Tabulatse),  giving  a 
cellular  structure  which  may  be  finer  or  coarser.  The  Halysitids  seem 
to  be  made  up  of  small,  hollow,  flattened  columns  with  imperfect 
septa,  united  to  form  inosculating  plates  which  on  section  "have  the 
appearance  of  chains  crossing  in  all  directions.  These  are  also  minutely 
tabulated.  The  Syringoporoids  are  similar  to  the  Halysitids,  except 
that  the  hollow  columns  are  cylindrical  and  connect  with  each  other 
only  in  places. 

The  following  are  some  of  the  more  characteristic  species  of  these 
families. 


FIG.  292. 


FIGS.  291-293.— CTATHOPHTLLOrD  CORALS  :   291.  Lonsdaiia  floriformis  (after  Nicholson).    292.  a  and  b, 
Zaphrentis  bilateralis  (after  Hall).    293.  Stroinbodes  pentagonus  (after  Hall). 


ANIMALS. 


293 


FIGS.  294-290.— FA VOSITID  AND  HALYSITID  CORALS:    294.  Columnaria  alveolata:  a,  vertical;  5,  cross- 
section  (after  Hall).    295.  Syringopora  verticillata.  296.  Halysites  catenulata  (after  Hall). 

There  are  many  other  forms  than  those  mentioned  above,  but  their 
affinities  are  little  understood,  and  many  are  not  true  corals,  but  Polyzoa 
and  sponges.  Nearly  all  the  corals  of  Silurian,  in  fact  of  Palaeo- 
zoic times,  fall  under  two  orders — Rugosa  and  Tabulata.  The  Cyatho- 
phylloids  are  Rugosa,  the  other  families  mentioned  are  Tabulata.  The 
Rugosa  are  characteristic  of  the  Palaeozoic ;  the  Tabulata  are  also  near- 
ly extinct :  they  have  only  one  family  living,  viz.,  the  millipores.  The 
Rugosa  dift'er  from  modern  star-corals  in  having  their  radiating  septa 
in  multiples  of  four,  while  modern  star-corals  have  theirs  in  multiples 
of  five  or  six.  Hence 'star-corals  have  been  divided  into  two  types — a 
Palaeozoic  and  a  Neozoic — the  one  four-parted  (quadripartita),  the  other 
six-parted  (sexpartita). 

Hydrozoa—  The  perfect  forms  of  this  class,  viz.,  Medusae,  or  jelly- 
fishes,  are  so  soft  and  perishable  that,  with  one  or  two  exceptions  in 
the  Mesozoic  rocks,  they  are  not  found  preserved  at  all  in  the  strata  of 


294 


PALAEOZOIC  SYSTEM  OF  ROCKS. 


any  geological  period.  They  may  or  may  not  have  existed  at  this  time ; 
probably  they  did  not.  But  the  larval  form  of  most,  if  not  all,  Medusre 
is  a  compound  polypoid  animal,  forming  a  minutely-branching,  horny, 
or  coralline  axis.  These  minutely-branching  axes  are  strung  on  each 
side  with  cells,  in  which  are  inclosed  little  polypoid  animals.  They 
grow  in  still,  quiet  waters,  and  are.  often  mistaken  by  the  unscientific 
for  sea-weed.  These,  by  their  composition,  are  well  adapted  for  preser- 
vation, and  it  is  this  larval  form,  therefore,  only  that  we  might  expect 
to  find. 


Fro.  298  a. 


FIG.  298  b. 


FIGS.  297-299.— LIVING  HYDROZOA :  297.  Sertularia  pinnata:   a,  natural  size;  &,  enlarged.    298.  «and&, 
Different  Forms  of  Sertularia.    299.  Pluinularia. 

Now,  in  very  fine  shales  of  Silurian  age,  especially  of  Lower  Silu- 
rian, are  found  abundantly  beautiful  impressions  of  an  organism  which 
is  most  probably  a  compound  Hydrozoan  allied  to  Sertularia  of  the 
present  day.  They  are  called  graptolites.  Sometimes  the  cells  are 
arranged  on  one  side  of  the  axis,  sometimes  on  both  sides,  sometimes 
the  axis  is  divided.  Whatever  be  their  affinities,  they  are  of  great 
importance,  inasmuch  as  they  are  entirely  characteristic  of  the  Silurian 


ANIMALS. 


295 


age,  and  those  with  cells  on  both  sides,  of  the  Lower  Silurian.  The 
twin  graptolites  (Fig.  302)  are  also  wholly  characteristic  of  Lower 
Silurian. 


FIG.  300. 


FIG.  801. 


FIG.  302. 


FIG.  303.  Fifl.  804. 

FIGS.  300-304.-GRAPTOLITES:  300.  Diplograptus  pristis  (after  Nicholson).  301.  Phyllograptus  typus 
(after  Hall).  302.  Didymograptus  V-fractus  (after  Hall).  803.  Graptolithus  Logani  (after  Hall).  304. 
Monograptus  priodon  :  a,  side-view  ;  Z>,  back-view;  c,  front-view,  showing  opening  (after  Nicholson). 


PALEOZOIC  SYSTEM  OF  ROCKS. 


FIGS.  305.  306.— GBAPTOLITES  :  305.  Dendrograptus  Ilallianus  (after  Hall).    306.  Graptolites  Clintonensis 
(after  Hall). 

Polyzoa. — There  are  many  kinds  of  compound  coralline  animals, 
probably  allied  to  the  Bryozoa  (sea-mats)  of  our  present  seas,  found  in 
the  Silurian.  The  doubtful  affinities  of  these  Palseozoic  forms,  and  the 
difficulty  of  separating  them  sharply  from  certain  forms  of  true  corals  on 


ffl 


FIG.  807. — Living  Polyzoa :  Flustra  truncata :  a,  natural  size ;  &,  enlarged  to  show  the  cells. 


the  one  hand,  and  from  certain  forms  of  graptolites  on  the  other,  seem 
to  require  their  notice  in  this  connection,  although  their  affinities  are 
probably  molluscoid.  Two  of  the  Silurian  forms  are  represented  on 
page  297,  Figs.  308  and  309. 

EcMnoderms. — During  Silurian  times  the  class  of  Echinoderms  was 
represented  principally  by  Crinoids,  A  Crinoid  is  a  stemmed  Echino- 
derm,  usually  with  branching  arms.  The  animal  consists  of  a  long 
jointed  stalk,  rooted  to  the  sea-bottom,  and  bearing  atop  a  rounded  or 


ANIMALS. 


297 


pear-shaped  body,  covered  with  calcareous  plates  (calyx),  from  the  mar- 
gin of  which  spring  the  arms,  which  may  be  long  and  profusely  branched, 


FIGS.  80S  and  309.— SIIXRIAN  POLYZOA  :  308.  Fenestella  elegans  (after  Hall).    809.  Alecto  auloporoides 
(after  Hall). 

or  short  and  simple,  or  absent  altogether.  In  the  middle  of  the  calyx, 
between  the  bases  of  the  arms,  is  placed  the  mouth.  Their  general 
structure  and  appearance  will  be  better  understood  by  examination 
of  the  following  figures  of  living  Crinoids. 


FIG.  810.  FIG.  811. 

FIGS.  310  and  311.— LIVING  CRIXOIDS:   310.  Rhizocrimis  Lofotensis  (after  Thompson).    811.  Pentacrinus 
Caput-Medus.i?. 

.  At  present,  leaving  out  the  Holothurians,  or  sea-cucumbers,  which, 
having  no  shell,  are  little  apt  to  be  preserved  as  fossils,  the  class  of 


298  PALEOZOIC  SYSTEM  OF  ROCKS. 

Echinoderms  may  be  conveniently  divided  into  three  orders,  viz. :  the 
Echinoids,  or  sea-urchins ;  the  Asteroids,  or  starfishes ;  and  the  Cri- 
noids.  The  members  of  the  first  and  second  orders  are  free  moving, 
while  those  of  the  third  are  stemmed.  Of  these  orders  the  Crinoids  are 
the  lowest,  as  proved  not  only  by  their  simpler  organization,  but  also 
by  the  fact  that  a  living  Crinoid,  the  Comatula,  is  attached  when 
young,  but  free  when  mature. 


FIG.  812.— A  Living  Free  Crinoid— Comatula  rosacea.  the  Feather-Star :   «,  free  adult ;  b,  fixed  young 
(after  Forbes). 

Now,  in  Silurian  times,  the  stemmed  Echinoderms  are  very  abun- 
dant, while  the  free  are  very  rare :  at  the  present  time,  on  the  contrary, 
the  reverse  is  the  case.  Thus,  in  the  course  of  time,  the  former  de- 
creased until  they  are  now  almost  extinct,  while  the  latter  increased 
until  they  are  now  very  abundant.  If  we  take  the  abundance  of  Echino- 
derms during  geological  times  as  constant,  and  represent  the  course  of 


FIG.  813.— Diagram  showing  the  General  Distribution  in  Time  of  Stemmed  and  Free  Echinoderms. 


time  by  the  absciss  A  B  (Fig.  313),  and  the  abundance  by  distance 
from  A  JB  to  C  D,  then  the  parallelogram  would  represent  this  fact. 
If,  now,  we  draw  the  diagonal,  C  J3,  then  the  shaded  triangle  would 


ANIMALS. 


299 


represent  the  stemmed,  and  the  unshaded  the  free,  and  the  diagonal  the 
line  of  decrease  of  the  one  and  increase  of  the  other;  and  the  whole 
figure  the  general  relations  of  the  two  sub-classes  throughout  time. 
In  the  Palaeozoic  the  stemmed  predominate ;  in  the  Mesozoic  the  two 
are  equally  represented ;  in  modern  times  the  free  predominate. 


FIG.  318  a. 


FIG.  319. 


FIGS.  314-819.— SILURIAN  CRINOIDS:  314.  Caryocrinus  ornatus.  315.  Pleurocystitis  squamosus.  816. 
Pseudocrinus  bifasciatus.  317.  Lepadocrinus  Gebhardii.  318.  Glyptocrinus  deeadactylus  (after 
Hall) :  a,  specimen  with  arms ;  6,  larger  specimens  without  the  arms.  319.  Ichtbyocrinus  sublaevis 
(after  Hall). 

Stemmed  Echinoderms,  or  Crinoids,  may  be  divided  into  three  fami- 
lies, viz.:  1.  Crinids ;  2.  Cystids ;  3.  Blastids.  Crinids  are  the  typi- 
cal Crinoids,  with  branching  arms,  already  illustrated  from  living  exam- 


300 


PALAEOZOIC  SYSTEM  OF  ROCKS. 


FIGS.  320-824— SiLtmrAN  CRINOFDS  ANT>  ASTEROIDS:  820.  Mnriacrinus  noWlissiaius  (after  Hall).  321. 
Ilomocrimis  sroparius  (after  Hall).  322.  Heterocrinus  simplex  (after  Meek).  323.  Protaster  Scdg- 
wickii.  S24.  1'aliuaster  Sba'fferi  (after  Hall). 


ANIMALS. 


301 


pies  (Figs.  310-312).  Cystids  are  of  a  bladder-like  form  (hence  the 
name),  and  are  either  without  arms,  or  else  have  few,  short,  simple 
arms  springing  from  near  the  centre  of  the  upper  part  of  the  body,  the 
mouth  being  probably  on  one  side.  The  radiated  structure  in  these  is 
imperfect.  Blastids  (Gr.  0/la<7TOC,  a  bud}  had  a  bud-shaped  body,  with 
five  petalloid  spaces  (ambulacra)  radiating  from  the  top  and  reaching 
half-way  down  the  body  (see  Figs.  510-513,  p.  382).  If  Crinids  are  com- 
parable to  inverted  Starfishes  with  many  arms  and  set  upon  a  stalk,  the 
Cystids  and  Blastids  may  be  compared  to  Sea-urchins  similarly  set. 
All  these  families  are  found  in  the  Silurian.  The  Cystids  pass  away 
with  the  Silurian,  and  are  therefore  characteristic  of  that  age.  The 
Blastids  pass  away  before  the  end  of  the  Carboniferous  age,  and  are 
therefore  characteristic  of  the  Palaeozoic  era.  The  Crinids  continue, 
though  in  diminished  numbers,  to  the  present  day.  Figures  of  Blastids 
are  given  under  the  Carboniferous,  where  they  were  far  more  abundant. 

Mollusks — AcepTials  or  Bivalves. — Bivalves  may  be  divided  into 
two  great  sub-classes,  viz.,  Lamellibranchs  (leaf-gills) 
and  Brachiopods  (arm-feet).  The  valves  of  Lamelli- 
branchs are  right  and  left ;  those  of  Brachiopods  are 
upper  and  lower,  or  dorsal  and  ventral.  Brachiopods 
'are  much  less  highly  organized  than  the  other  sub-class, 
and  differ  so  essentially  in  their  organization  that  some 
of  the  best  naturalists  remove  them  not  only  from  the 
class  of  Acephals,  but  from  the  department  of  Mollusca, 
and  ally  them  rather  with  the  Worms.  Their  general 
resemblance  in  external  form  to  bivalves  makes  it  more 
convenient  to  treat  them  under  that  head,  until  the 
question  of  their  affinity  is  more  definitely  settled. 

General  Description  of  a  Brachiopod.— A  Brachiopod 
shell  consists  of  two  valves,  a  dorsal  and  a  ventral.  The 
ventral  is  the  larger,  and  usually  projects  beyond  the 
dorsal,  at  the  hinge,  as  a  prominent  beak.  This  pro- 
jecting portion  is  often  perforated  to  give  passage  to  a 
muscular  peduncle,  by  which  the  shell  is  attached  in  the 
living  animal.  The  following  figures  (Figs.  325,  326)  of 
Brachiopods,  living  and  extinct,  will  make  these  points 
clear. 

The  viscera  of  a  Brachiopod  fill  but  a  small  space 
in  the  shell,  this  cavity  being  occupied  principally  by 
two  long  spiral  arms  (hence  the  name),  which  probably  subserve  the 
functions  of  respiration  and  alimentation.  These  arms  are  attached 
to  a  curious  bony  apparatus,  sometimes  itself  spiral  in  form.  Figs. 
327-329  show  the  internal  structure  described  above. 

In  the  present  seas  the  Lamellibranchs  are  extremely  abundant, 


anatina,  showing 
the  muscular  ped- 
uncle bv  which  the 

shell  is  attached. 


302 


PALAEOZOIC  SYSTEM  OF  ROCKS. 


FIG.  326.— Rhynchonella  sulcata:  side-view,  dorsal  view,  and  showing  suture 


FIG.  329. 

FIGS.  827-329.— SHOWING  THE  STBrcrrRE  OF  BKACHTOPODS  :  827.  Spirifer  striatus  (Carboniferous) :  a, 
dorsal  surface ;  b,  interior,  showing  the  bony  spirals.  828.  Tercbratula  llavrsrcns  diving  species): 
«.  exterior  surface ;  6,  showing  bony  structure  for  attachment  of  spiral  arms.  329.  Spirifer  hysterica 
(Carboniferous) :  a,  exterior;  6,  showing  bony  spires. 

while  the  Brachiopods  are  nearly  extinct,  being  represented  by  very 
few  species.  In  Silurian  times,  on  the  contrary,  the  very  reverse  is  the 
case,  bivalve  shells  being  represented  mostly  by  Brachiopods.  Taking 
the  number  of  bivalve  species  throughout  geological  times  as  constant, 
then  the  general  relation  of  these  two  sub-classes  to  each  in  time  may 
be  roughly  represented  by  the  following  diagram,  in  which  the  lower 


ANIMALS. 


303 


triangle  represents  Brachiopods,  the  upper  Lamellibranchs,  and  the  com- 
mon diagonal  the  line  of  decrease  of  one  and  increase  of  the  other. 


FIG.  330. — Diagram  showing  the  General  Alteration  of  Brachiopods  to  Lamellibranchs. 


The  abundance  of  individuals  and  the  number  of  species  of  this 
order  in  Silurian  times  are  almost  incredible.  The  following  figures 
represent  some  of  the  common  and  characteristic  forms. 


FIGS.  331-334.-Sii.cRiAX  BRACIUOPODS  :   331.  Orthis  Davidsonii.     332.  Orthis  porcata.     833.  Spirifer 
Cumberlandiaa:  a,  ventral  valve;  6,  dorsal  valve;  c,  suture.    334.  Pentamerus  Knightii. 

It  is  very  difficult  to  give  any  general  distinctive  mark  of  Silurian 
Brachiopods,  although,  of  course,  the  species  and  even  the  genera  are 
peculiar,  and  may  be  recognized  by  the  paleontologist.  It  may  be  said, 
however,  that  the  straight-hinged  or  square-shouldered  Brachiopods, 
including  the  Spirifer  family,  the  Strophomena  or  Leptena  family,  and 


304 


PALAEOZOIC  SYSTEM   OF  ROCKS. 


the  Productus  family,  are  characteristic  of  the  Palaeozoic,  though  not  of 
the  Silurian. 

Lamellibranchs. — We  have  said  that  Lamellibranchs  are  also  found 
in  the  Silurian,  but  not  so  abundantly  as  the  Brachiopods.  Lamelli- 
branchs are  divided  into  Siphonates  and  Asiphonates,  i.  e.,  those  with 


FIG.  336. 


FIG.  337.  FIG.  338.  FIG.  389. 

FIGS.  335-339. — SILURIAN  LAMELLIBRANCHS:  335.  Orthonota  parallela.  336.  Cardiola  interrupta  (after 
Hall).  837.  Avicula  Trentonensis  (after  Ilall).  338.  Ambonychia  belli  striatus  (after  Hall).  3S9. 
Tellenomya  curta  (after  Hall). 

and  those  without  breathing-siphons  behind.  The  Siphonates  are  the 
higher.  At  present  the  Siphonates  are  the  more  abundant — in  Palaso- 
zoic  times  the  Asiphonates.  We  give  seme  figures  above. 


FIG.  340. 


FIG.  841.  FIG.  342. 

FIGS    340-342  —SILURIAN  GASTEROPODS:  840.  Pleurotomaria  dryope.    341.  Pleurotoiuaria  agave.    842. 
Murchisoni  gracilis. 


ANIMALS. 


305 


Gasteropods —  Univalves. — Land  and  fresh-water  Gasteropods  have 
not  been  found  in  the  Silurian.  If  we  divide  marine  Gasteropods  or 
univalves  into  those  having  beaked  shells  and  those  having  smooth- 
mouthed  or  beakless  shells,  the  former  being  carnivorous  and  the  latter 
herbivorous,  then  only  the  smooth-mouthed  or  beakless  shells  have 


FIG.  345. 


FIG.  344. 


FIG.  346. 


FIGS.  343-346.— SILURIAN  GASTEROPODS  AND  PTEROPODS  :  843  Cyrtolites  compressus  (after  Hall).  844. 
Cyrtolites  Trentonensis  (.after  Hall).  345.  Cyrtolites  Dyeri  (after  Meek).  346.  Conularia  Trentonensis 
(after  Hall),  a  Pteropod. 

been  found  in  the  Silurian.  The  beaked-shelled  are  usually  regarded 
as  the  more  highly-organized  class.  The  affinities  of  Conularia  (Fig. 
346)  and  Tentaculites  are  little  understood.  They  are  usually  placed 
among  Pteropods. 

Cephalopods—  Chambered  Shells.— These  are  by  far  the  most  high- 
ly organized  of  Mollusks,  and  the  most  powerful  among  Invertebrates. 
They  are  represented  in  the  present  seas 
by  the  Nautilus,  the  Squids,  and  the  Cut- 
tle-fishes.   If  we  divide  all  known  Cepha- 
lopods into  Dibranchs  (two-gilled)  and  f ... 
Tetrabranchs  (four-gilled),  the  former  be- 
ing naked  and  the  latter  shelled,  then,  at 
the  present  time,  the  Dibranchs,  or  naked, 
vastly  predominate,  there  being  only  a 
single  genus  of  shelled  or  Tetrabranchs 
known,   viz.,   the   Nautilus,  and  of  this 
genus  only  three  or  four  species.     In  the 
Silurian  age,  and.  for  many  ages   after- 
ward, only  the  shelled  existed.     The  naked  or  Dibranchs  are  decidedly 
the  higher  in  organization. 
20 


FIG.  347.— P 

pilius) :  c 
c,  hood ; 
nel. 


arly  Nautilus  (Nautilus  pom- 
mantle  ;  b,  its  dorsal  fold ; 
9,  eye ;  t,  tentacles ;  /,  fun- 


306 


PALAEOZOIC   SYSTEM   OF   ROCKS. 


Again,  if  we  divide  chambered  shells  into  those  having  simple 
septa  and  central  or  subcentral  tube  or  siphon  (Nautilus  tribe],  and 
those  having  septa  plaited  at  their  junction  with  the  shell  (plaited 
suture)  and  dorsal  tube  (ammonite  tribe),  then  in  the  Silurian  age  the 
former  only  were  represented. 

Again,  if  we  divide  the  Nautilus  tribe  into  straight-shelled  and 
coiled-shelled,  then  the  straight-chambered  shells  greatly  predominated. 
Straight-chambered  shells  are  called  Orthoceratites  (opflo^,  straight ; 
wepaf,  horn).  The  Orthoceratites,  therefore,  are  a  very  striking  feat- 
ure of  the  Silurian  age.  They  may  be  defined  as  straight-chambered 


a,  Onnoceras. 


b,  Actinoceras. 


c,  Huronto.  d,  Section  of  Siphuncle  of  Huronia. 

FIG.  848.— «,  ft,  c,  d,  Showing  Structure  of  Orthoceratite. 

shells,  with  simple  partitions  and  a  central  or  subcentral  siphon-tube 
(siphuncle).  The  siphuncle  of  the  family  was  large  in  proportion  to 
the  shell,  and  had  often  a  beaded  structure  (Fig.  348,  a,  £,  c,  d).  The 
genera  are  founded  largely  on  the  form  of  this  part. 

They  existed  in  great  numbers,  and  attained  very  great  size.  Speci- 
mens have  been  found  fifteen  feet  long,  and  eight  to  ten  inches  in  diam- 
eter. They  were,  without  doubt,  the  most  powerful  animals  of  that 


ANIMALS. 


307 


time,  the  tyrants  and  scavengers  of  these  early  seas.     We  give,  in  Fig. 
357,  a  restoration  of  the  creature.     They  are  entirely  characteristic  of 


FIG.  850. 


FIG.  851. 


' 

JK^niignM  i^MMnBHBMBMHBBHMWfflSflffl^Bffil 


FIG.  853. 

FIGS.  849-353.— SILTJRIAN  CEPIIALOPODS  :  $49.  Orthoceras  medullare  (after  Meekl.  850.  Ormoeeras 
tenufilum,  showing  chambers  and  siphimele  (after  Hall).  351.  Orthoceras  vertebrale  (after  Hall). 
852.  Orthoceras  multicarneratum  (after  Hall).  353.  Orthoceras  Duseri  (after  Hall). 


308 


PALAEOZOIC   SYSTEM   OF   ROCKS. 


the  Palaeozoic ;  commencing  in  the  Primordial,  extending  through  into 
the  Carboniferous,  and  passing  out  there.  They  attained  their  maxi- 
mum of  development  in  size  and  number  in  the  Silurian. 

Although  straight-chambered  shells  (Orthoceratites)  are  most  abun- 
dant and  characteristic,  coiled  shells  of  the  same  tribe  are  also  found, 
and  some  of  them  of  considerable  size.  Some  of  these  are  close-coiled 


FIG.  354. 


FIG.  855.  FIG.  856. 

FIGS.  854-856.— SILURIAN  CKMIALOPODS  :  854.  Trocholites  Ammonias  (after  Hall):  a,  exterior;/;,  cast, 
showing  septa,    855.  Lituites  Graftonensis  (Meek  and  Worthen).    356.  Lituites  cornu-anetis. 

shells,  true  Nautilus  family ;  others  open-coiled,  and  more  nearly  allied 
to  the  straight.  Barrande  gives  1,622  species  of  Cephalopods  in  the 
Silurian. 

Articulates—  Worms. — These  are  fleshy  animals  without  skeletons, 
and  are  therefore  not  preserved.  They  are  known  only  by  their  tracks, 
their  borings,  and  their  tubes.  Nevertheless,  some  185  species,  accord- 
ing to  Barrande,  have  been  described  from  the  Silurian  of  different 


ANIMALS. 


309 


countries.  Fig.  358  represents  worm-tubes,  and  Fig.  359  worm-tracks, 
from  Upper  Silurian. 

Crustacea — Trilobites. — The  principal  representatives  of  the  articu- 
late department  in  Silurian  times  were  Crustaceans,  but  mostly  of  a 
very  characteristic  order  of  that  class,  now  long  extinct,  viz.,  Trilobites. 

General  Description. — The  carapace  or  shell  of  these  curious  creat- 
ures was  convex  and  usually  smooth  above,  and  flat  or  concave  below, 
and  divided  transversely,  like  most  Crustacea,  into  a  number  of  movable 


FIG.  3n7.— Restoration  of  Orthoeeras,  the 
shell  bein<r  supposed  to  be  divided  ver- 
tically, and  only  its  upper  part  being 
shown  :  a,  arms ;  /;  muscular  tube 
("funnel")  by  which  water  is  expelled 
from  the  mantle-chamber;  c,  air-cham- 
bers, s,  siphuncle  (after  Nicholson). 


PIG.  359. 

FIGS.  353,  359.— SILURIAN  ANNELIDS  :  358.  Cor- 
nulitis  serpentarius  (Worm  -  Tube).  359. 
Trail  of  an  Annelid  (after  Hall). 


joints.  Several  of  the  front  joints  are  always  consolidated  to  form  a 
head-shield  or  Buckler,  and  sometimes  a  number  of  the  posterior  joints 
are  similarly  consolidated  to  form  a  tail-shield  or  Pygidium.  The  whole 
shell  or  carapace  is  divided  longitudinally,  more  or  less  distinctly,  into 
three  lobes  (hence  the  name) — a  middle,  a  right,  and  a  left.  Well-or- 
ganized compound  eyes  are  distinctly  seen  in  well-preserved  specimens 


310 


PALJ30ZOIC  SYSTEM  OF  ROCKS. 


on  the  lateral  lobes  of  the  head-shields  (cheeks)  (Fig.  360).  The  under 
side  of  the  animal  has  never  been  distinctly  seen,  and  therefore  the 
character  of  the  locomotive  organs  is  not  certainly  known.  But  it  is 
believed  that,  like  some  of  the  lower  Crustaceans  of  the  present  day 
(Phyllopods),  their  limbs  were  thin,  flat,  soft,  leaf-like  swimmers.  On 
this  view  it  is  easy  to  see  why  the  under  side  is  never  exposed ;  for  the 
mud,  in  which  they  were  entombed,  would  become  entangled  among 
these  leaf-like  swimmers,  and  in  breaking  the  rock  this  would  determine 


FIG.  360.— Structure  of  the  Eye  of  Trilobites:  o,  Dalmania  pleuroptoryx ;  b,  eye  slightly  magnified; 
c,  eye  more  highly  magnified ;  d,  small  portion  still  more  highly  magnified  (after  Hall). 

the  line  of  fracture  over  the  smooth  back,  and  leave  the  creature  firmly 
attached  by  its  ventral  surface  to  the  lower  piece.  Not  uncommonly 
Trilobites  are  found  folded  up  on  their  ventral  surface,  so  as  to  bring 
head  and  tail  together  and  form  a  kind  of  ball.  This  was  probably  a 
position  of  defense.  In  such  cases  the  Trilobite  may  be  gotten  out  of 
the  rocky  matrix  complete ;  but  none  the  less  are  the  feet  completely 
hidden  (Fig.  361  b). 

The  great  number  of  genera  into  which  this  large  order  is  divided  is 
founded  principally  on  the  form  and  sculpturing  of  the  Buckler,  the  size 
and  form  of  the  Pygidium,  the  number  of  the  movable  segments,  etc. 
The  figures  below  will  give  an  idea  of  some  of  these  forms. 

It  is  very  interesting  to  observe  that  a  complex  mechanism,  the 
compound  eye  like  that  of  crustaceans  and  insects  of  the  present  day, 
an  exquisite  instrument  for  the  formation  of  an  image  on  the  retina, 
was  already  developed  even  in  the  earliest  Primordial  times. 

Trilobites  commenced,  as  already  stated,  in  the  earliest  Primordial, 
continued  through  the  whole  Palaeozoic,  and  then  became  extinct  for- 
ever. They  are  therefore  entirely  characteristic  of  the  Palaeozoic.  They 
reached  their  maximum  of  development,  in  size,  number,  and  variety,  in 
the  Silurian.  Barrande  gives  the  number  of  species  described  in  the  Silu- 
rian alone  as  1,579.  They  reached  in  some  cases  a  size  equal  to  any  crus- 
taceans now  living.  The  Asaphus  (Isotelus)  gigas,  from  the  Lower 


ANIMALS. 


311 


FIGS.  361—305. — SILCTITAN  TRILOBITES  :  361.  Calymene  Blumenbachii :  b,  same  In  folded  condition. 
362.  Trinucleus  Ponsrerardi.  3i>3.  Lichas  Boltoni  (alter  Hall).  364.  Acidaspis  crosotus  (after  Meek). 
805.  Isotelus  gigar,  reduced  (after  Hall) ;  365  a,  same,  side-view. 

Silurian  (Fig.  365),  was  sometimes  twenty  inches  in  length  and  thir- 
teen wide.     Paradoxides  (Fig.  280,  p.  287),  of  the  earliest  Primordial, 


312 


PALAEOZOIC   SYSTEM   OF  ROCKS. 


attained  a  length  of  twenty-two  inches.  On  account  of  their  great 
abundance  and  fine  preservation,  their  embryonic  development  has  been 
carefully  studied  by  Barrande,  who  has  described  and  figured  thirty 
steps  in  the  development  of  some  species.  According  to  Agassiz,  we 
know  more  of  the  development  of  trilobites  than  of  any  living  crustacean. 


FIG.  3G6.— Dalmania  limulurns. 


FIG.  31'tf.— </,  Larva  of  a  Trilobite ; 
ft,  Larva  of  a  King-Crab. 


3.-Limulus  before  hatching,  Trilobite  Stage :  a,  Bide  view.    Limulns  beYore  hatching,  Trilobite 
Stage :  ft,  dorsal  view. 

Affinities  Of  Trilobites. — The  affinities  of  this  very  distinct  order  are 
imperfectly  understood.  Crustaceans  are  divided  into  two  sub-classes,  a 
higher,  Malacostraca  (mollusk-shelled  or  calcareous-shelled),  and  a  lower, 
JE/ntomostraca  (insect-shelled).  Now,  Trilobites,  though  belonging  to  the 


ANIMALS. 


313 


lower  division,  or  Entomostraca,  occupy  a  position  near  the  confines  of 
the  two  divisions.  More  definitely,  they  probably  stand  between  the 
Isopocls  (tetradecapod  Malacostracans),  on  the  one  hand,  and  the  Phyl- 
lopods  and  Limuloids  (Entomostracans),  on  the  other.  In  general  ap- 
pearance they  certainly  approach  Limuloids  (horseshoe-crabs  or  king- 
crabs),  and  these  seem  to  have  replaced  them  in  the  process  of  evolution. 
They  are  by  no  means  very  low  in  the  scale  of  crustaceans ;  their  position 
being  near  the  middle.  The  larvag  of  Crustaceans,  especially  of  Limu- 
loids, greatly  resemble  some  forms  of  Trilobites,  and  especially  the  larva? 
of  Trilobites.  From  the  generalized  forms  represented  by  Figs.  367  and 
368  have  been  probably  differentiated,  in  one  direction  the  more  per- 
fect Trilobites,  and,  in  the  other,  the  Limuloids. 


FIG.  369. 

FIGS.  869-371.-- SILUKIAN  EURTPTEBTDS  :  369.  Pterygotus  Anxious,  viewed  from  the  under  side,  re- 
duced in  size,  and  restored  :  c  o,  the  feelers  (antennae),  terminating  in  nippintr-claws  ;  o  o.  eyes:  m  m, 
three  pairs  of  jointed  limbs,  with  pointed  extremities  ;  n  n,  swimming-paddles,  the  bases  of  which 
are  spiny  and  act  as  jaws— Upper  Silurian,  Lanarkshire  (after  Henrv  Woodward).  870.  Eurypterus 
remipes,  greatly  reduced.  871.  Same  restored:  a,  dorsal  view;  b,  ventral  view  (after  Hall). 


FIG. 


Eurypterids. — In  the  Upper  Silurian  was  introduced  and  continued 
to  exist  along  with  Trilobites  during  the  rest  of  the  Palaeozoic,  another 


314:  PALAEOZOIC   SYSTEM   OF  ROCKS. 

family  of  huge  entomostracans  probably  in  advance  of  Trilobites  in 
organization,  viz.,  Eurypterids.  The  family  includes  the  two  genera 
Eurypterus  (broad  wing)  and  Pterygotus  (winged  ear).  Some  of  the 
latter  are  the  largest  crustaceans  known.  The  huge  Inachus  Koemp- 
feri  (Japan  crab),  with  carapace  sixteen  inches  in  diameter,  and  legs 
four  feet  long,  and  the  Moluccas  king-crab  (Limulus  Moluccanus), 
three  feet  long  and  eighteen  inches  across  the  carapace,  are  the  largest 
crustaceans  now  living.  But  the  Eurypterids  were  some  of  them  far 
greater.  The  Pterygotus  Anglicus  (Fig.  369)  was  six  feet  long  and 
one  foot  wide,  and  the  Pterygotus  Gigas,  seven  feet  long  and  propor- 
tionately wide.  The  above  figures  represent  some  species  of  these  two 
genera  from  the  American  and  English  rocks. 

Anticipations  of  the  Next  Age. — Higher  animals  and  plants  than 
those  already  mentioned  do  not  strictly  belong  to  this  age.  Neverthe- 
less, in  the  uppermost  beds  of  the  Upper  Silurian,  or  passage-beds  into 
the  Devonian,  are  found  several  anticipations  of  the  next  age.  Land- 
plants  are  there  introduced  in  the  form  of  a  few  small  club-mosses 
(Psilophyton)  and  vertebrates  in  the  form  of  fishes.  The  latter  have 
not  yet  been  found  in  this  country.  Such  anticipations  are  in  accord- 
ance with  the  law  already  mentioned  (p.  267),  that  the  characteristics 
of  an  age  often  commence  in  the  preceding  age.  It  is  better,  however, 
to  treat  of  these  classes  in  connection  with  the  age  in  which  they  cul- 
minate, or  at  least  become  a  striking  feature. 

The  Silurian  was,  therefore,  essentially  an  age  of  Invertebrates.  In 
number,  size,  and  variety,  these  have  scarcely  been  surpassed  in  any 
subsequent  period.  The  most  characteristic  orders  were  :  Among  plants, 
Fucoids ;  among  animals,  Cyathophylloid  and  Tabulate  corals,  Grapto- 
lites,  Cystidean  crinoids,  Square-shouldered  brachiopods,  Beakless  gaster- 
opods,  Orthoceratites,  and  Trilobites.  Orthoceratites  and  trilobites  were 
the  highest  animals  of  the  age,  and  the  former  were  the  rulers  and  scav- 
engers of  these  early  seas.  We  give  below  a  table  showing,  according 
to  Barrande,  the  number  of  Silurian  species  described  up  to  1872  : 


Sponges  and  other  Protozoans.. .     153 

Corals 718 

Echinoderms 588 

Worms 185 

Trilobites 1,679 


Brachiopods 1,567 

Lamellibranchs 1,086 

Heteropods  )  OQA 

Pteropods    \ 89° 

Gasteropods 1,306 


Other  Crustaceans 348    Cephalopods 1,622 

Bryozoans 478  Irishes 40 

"Which,  with  four  of  uncertain  relatives,  make  10,074  species. 

SECTION  2. — DEVONIAN  SYSTEM  AND  AGE  OF  FISHES. 

The  name  Devonian  was  given  to  these  rocks  by  Murchison  and 
Sedgwick,  because  in  Devonshire  the  system  occurs  well  developed, 


LIFE-SYSTEM   OF  DEVONIAN  AGE— PLANTS.  3^5 

and  abounds  in  fossils.  In  England  the  system  is  usually  uneonform- 
able  with  the  underlying  Silurian,  and  sometimes  with  the  overlying 
Carboniferous,  as  in  Fig.  372.  But  in  the  Eastern  United  States,  as 
already  stated,  the  Palaeozoics  are  conformable  throughout  (Fig.  255). 


d 

FIG.  872.—*,  Silurian ;  d,  Devonian ;  c,  Carboniferous  (after  Phillips). 

Area  in  United  States. — The  area  over  which  the  Devonian  appears 
as  a  country  rock  is  shown  in  map,  page  278.  It  borders  generally 
the  Silurian  on  the  south  and  southwest,  extending  with  it  far  south- 
ward in  the  middle  region,  viz.,  in  Indiana,  Western  Ohio  and  Ken- 
tucky. In  the  Basin  Range  region,  especially  about  White  Pine, 
Nevada,  Devonian  is  known  to  exist,  but  the  limits  of  these  areas  are 
too  imperfectly  known  to  be  described. 

Physical  Geography. — In  the  eastern  portion  of  the  United  States 
the  land  of  the  Devonian  age  was  approximately  that  of  the  Silurian 
age  already  described,  increased  by  the  addition  of  the  Silurian  area, 
which  Silurian  was  of  course  so  much  marginal  sea-bottom  exposed  by 
upheaval  during  and  at  the  end  of  Silurian  times.  At  the  end  of  De- 
vonian times  the  Devonian  area  was  added  to  the  existing  land,  and 
the  continental  mass  thus  further  increased. 

Subdivision  into  Periods. — In  the  United  States  the  following  four 
periods  are  recognized  by  Dana: 

4.  Catskill  period. 

3.  Chemung  period. 

2.  Hamilton  period. 

1.  Corniferous  period. 

We  shall,  however,  neglect  these  subdivisions  in  our  general  de- 
scription of  the  life  of  the  age. 

^Life-System  of  Devonian  Age — Plants. 

It  will  be  remembered  that  during  the  Silurian  age,  except  in  the 
very  last  part  where  a  few  small  club-mosses  were  introduced,  the  only 
plants  found  were  Fucoids.  These,  of  course,  continued  in  Devonian 
times.  But,  in  addition  to  these,  were  now  introduced  land-plants  in 
considerable  numbers  and  variety,  and  decided  complexity  of  organiza- 
tion. They  included  all  the  orders  of  vascular  cryptogams,  viz.,  ferns, 
Lycopods,  and  JSquisetce ;  and  also  Conifers  among  gymnospermous 
Pha3nogams ;  and  by  their  great  size  and  numbers  probably  formed  for 
the  first  time  in  the  history  of  the  earth  a  true  forest  vegetation. 

The  Ferns  were  represented  by  several  genera,  such  as  Cyclopteris 
and  Neuropteris  ;  the  Lycopods  (club-mosses)  not  only  by  the  Psilophy- 


316 


PALAEOZOIC   SYSTEM   OF  ROCKS. 


ton,  which  had  been  already  introduced  in  the  uppermost  Silurian,  but 
also  now  by  gigantic  Lepidodendrids  and  Sigillarids,  and  the  Equi- 
setas  by  Calamites  and  Asterophyllites.  The  Conifers  were  represented 


FIG.  373.— Microscopic  Section  of  the  Silicifled 
Wood  of  a  Conifer  (Sequoia},  cut  in  the  long 
direction  of  the  fibres.  Post-tertiary?  Colorado. 
(After  Nicholson.) 


FIG.  874.— Microscopic  Section  of  the  Wood  of  the 
Common  Larch  (Abies  larix).  cut  in  the  long 
direction  of  the  fibres.  In  both  the  fresh  and 
the  fossil  wood  (Fig.  867)  are  seen  the  disks 
characteristic  of  coniferous  wood.  (After 
Nicholson.) 


by  the  genus  Protaxites,  allied  to  the  yew  (Taxus).  They  are  known 
to  be  conifers  by  their  concentric  rings  of  growth  and  gymnospermous 
tissue,  i.  e.,  the  elliptic  disk-like  markings  on  the  walls  of  the  wood- 
cells  on  longitudinal  section  (Figs.  373  and  374), 
and  the  entire  absence  on  cross-section  of  the 
visible  pores  so  characteristic  of  dycolytedonous 
Exogens  (Fig.  375).  Some  of  these  conifers  have 
f  T*f  f'  [frf1  been  found  by  Dawson  eighteen  inches,  and  one 
FIG.  375.— Pine-Wood,  Cross-  three  feet,  in  diameter.  There  have  been  fifty 
species  of  land-plants  of  these  various  orders  found 

by  Dawson  in  the  Devonian  of  Nova  Scotia  alone.     On  pages  317  and 
318  we  give  the  most  characteristic  Devonian  land-plants. 

General  Remarks  on  Devonian  Land-Plants. — We  will  not  at  present 
discuss  the  affinities  of  these  plants,  and  their  relations  to  evolution, 
because  they  are  similar  to  those  found  in  the  coal,  where  they  exist 
in  far  greater  variety  and  abundance,  and  the  subject  will  be  discussed 
under  that  head.  There  are,  however,  some  thoughts  suggested  by 
the  first  appearance  of  highly-organized  plants  which  ought  not  to  be 
omitted : 

1;  The  ringed  structure  of  Devonian  conifers  shows  that,  at  that 
time,  there  was  a  growing  season  and  a  season  of  rest,  and  therefore, 
probably,  a  warm  and  a  cold  season.  In  one  trunk  the  number  of  rings 
counted  was  150,  indicating  a  considerable  age. 

2.  What  were  the  precursors  of  this  highly-organized  forest  vegeta- 
tion ?  That  there  were  precursors,  from  which  these  were  derived,  there 


LIFE-SYSTEM   OF  DEVONIAN  AGE— PLANTS. 


317 


can  be  little  doubt,  and  we  shall  probably  some  day  find  them  in  the 
Upper  Silurian  ;  but  that  the  steps  of  evolution  were  just  at  this  point 
somewhat  rapid,  seems  also  certain.  It  is  impossible  to  account  for 
this  comparatively  sudden  appearance  of  .so  highly-organized  a  vege- 


FIG.  379. 


Fio.  8SO. 


FIGS.  376-380.— DEVONIAN  PLANTS  (after  Dawson) :  37C.  Psilophyton  princeps,  restored.  377.  a,  Lopido- 
dendron  Gaspianum;  ft,  same  enlarged.  37S.  a,  Asterophillites  latifolia;  b,  fruit  of  same.  379.  Cy- 
clopteris  obtusa,  a  fern.  3SO.  Neuropteris  polymorpha,-  a  fern. 

tation  bj'  evolution,  unless  we  admit  that  there  have  been  periods  of 
rapid  evolution,  as  explained  on  page  288.  When  all  the  conditions 
are  favorable  for  a  great  advance,  the  advance  takes  place  at  once,  i.  e., 
with  great  comparative  rapidity. 

3.  We  have  seen  that  the  coal  vegetation  is  to  a  large  extent  an- 
ticipated in  the  Devonian.  So,  also,  to  some  extent,  were  the  condi- 
tions necessary  to  the  preservation  of  this  vegetation  and  the  formation 


318 


PALEOZOIC   SYSTEM   OF   ROCKS. 


of  coal.  In  the  Devonian,  for  the  first  time,  we  find  dark  bands  between 
the  strata,  impregnated  with  carbonaceous  matter.  We  find,  also,  thin 
seams  of  coal,  with  under-clajs  filled  with  ramifying  rootlets,  such  as  \ve 
shall  find  in  the  coal ;  in  other  words,  we  find  ancient  dirt-beds,  sub- 


Fia.  884. 


FIG.  886. 


FIGS.  881-386. — DEVONIAN  PLANTS  (after  Dawson)  :  881.  Cyclopteris  Jacksoni,  a  Fern.  882.  Dadoxylon 
Quangondianum,  a  Conifer:  a,  Pith;  ft,  Pith-Sheath;  c.  Wood.  883.  Sections  of  same;  a;,  Longi- 
tudinal ;  y.  Transverse,  enlarged— z,  greatly  magnified,  showing  disk-like  markings.  884.  Cardio- 
carpum  Baileyi,  a  Fruit.  885.  Anthophyllitis  Devonicus.  886.  Uordaites  Kobbii,  a  Group  of  Leaves. 

merged  forest-grounds,  and  peat-bogs.  All  the  phenomena  of  the  coal- 
measures,  therefore,  are  here  found,  though  imperfectly  developed,  and 
the  coal  not  workable.  The  Carboniferous  day  is  already  dawning. 


ANIMALS. 
Animals. 


319 


In  accordance  with  our  prescribed  plan,  all  we  can  do  in  describing 
Devonian  animals  is  to  touch  prominent  points — to  notice  what  is  going 
out,  what  is  coming  in,  and  to  dwell  only  on  what  bears  on  evolution. 


FIGS.    3S7-300.— DEVONIAN  CORALS:  3ST.  Acervularia  Davidsoni  (after  Hall).     388.  Favosites  hemi- 
spherica.    339.  Crepidopkyllum  Archiaci.    390.  Zaphrentis  Wortheni  (after  Meek). 

Radiates. — Among  corals,  the  chain-corals  (Halysitids)  have  disap- 
peared ;  the  other  orders  continue  under  different  species.  Among 
hydrozoa,  the  Graptolites  are  gone ;  among  Crinoids,  the  Cystids  are 
gone,  but  in  their  place  the  Blastids  (bud-like),  those  curious  armless 
crinoids,  with  petalloid  markings  already  spoken  of  as  rare  in  the  Silu- 
rian, become  more  abundant.  The  Crinids,  or  plumose-armed  crinoids, 


320 


PALEOZOIC  SYSTEM  OF  ROCKS. 


continue  undiminished.  The  Blastids,  however,  are  far  more  character- 
istic of  the  Carboniferous.  We  therefore  defer  their  illustration  to  that 
period. 

BracMopods. — Brachiopods  are  still  very  abundant,  and  still  many 
of  them  of  the  characteristic  Palaeozoic,  square-shouldered  type.  Among 
spirifers,  the  long-winged  species  (Fig.  392)  are  very  abundant  and 


Fio.  893. 

FIGS.  391-394.— DEVONIAN  BRACHIOPODS:  891.  Spirifer  fornacula  (after  Meek  and  \VortheiO:  a,  ven- 
tral valve;  6,  suture.  392.  Spirifer  perextensus  (after  Meek).  393.  Orthis  Livia:  a,  dorsal;  6, 
side-view.  394.  Strophomena  rhomboidalis. 

characteristic.  "We  give  a  few  figures  of  Devonian  bivalves,  both 
brachiopods  and  lamellibranchs,  and  a  few  univalves.  It  is  worthy  of 
remark  that  many  of  these  univalves  are  fresh-water  species. 

Gephalopods. — The  characteristic  Palaeozoic  Cephalopods,  or  Ortho- 
ceratites,  continue,  but  in  greatly-diminished  numbers  and  size;  but  the 
Groniatites,  a  coiled-chambered  shell,  which  seems  to  be  the  beginning 
of  the  Ammonite  family,  are  introduced  first  here.  This  family,  as 
already  explained,  is  distinguished  by  the  complexity  of  the  junction  of 


ANIMALS. 


321 


FIG.  402. 


FIG.  401 


FIGS.  395-402. — DEVONIAN  LAMELLIBRANCHS  AND  GASTEROPODS:  895.  Conocardium  trigonale  (after 
Logan).  896.  Aviculopecten  parilis  (after  Meek).  897.  Otenopistha  antiqua  (after  Meek).  898. 
Lucina  Ohioensis  (after  Meek).  399.  Spirorbis  omphalodes,  enlarged.  400.  Spirorbis  Arkonensis. 
401.  Orthonema  Newberryi  (after  Meek).  402.  Bellerophon  Newberryi  (after  Meek). 

the  septa  and  the  shell  (suture),  and  by  the  dorsal  position  of  the  si- 
phuncle.  In  the  Goniatites  the  sutures  are  not  yet  very  complex. 
They  are  only  zigzag.  This  is  shown  in  the  figure. 


FIG.  403.— Goniatites  lamellosus  (after  Pictet). 


322  PALAEOZOIC  SYSTEM   OF   ROCKS. 

Crustacea. — The  very  characteristic  Palaeozoic  order  Trilobites  is  still 
abundantly  represented,  although  it  has  already  passed  its  prime,  and  is 
diminishing'  in  number  and  size  of  species.  The  Eurypterids  introduced 
in  the  Upper  Silurian  maintain  their  place  through  the  Devonian. 


FIG.  404.  FIG.  4U5. 

FIGS.  404  and  405. — DEVONIAN  TBILOBITES  :  404.  Dalmania  punctata,  Europe.     405.  Phacops  Jatifrons, 

Europe. 

Insects. — The  earliest  insects  yet  discovered  are  found  in  the  De- 
vonian of  Nova  Scotia.  It  is  natural  that  insects  should  appear  along 
with  forest  vegetation,  and  indeed  the  insects  and  the  plants  are  found 
in  the  same  strata. 

The  Devonian  insects  belong  to  the  Neuroptera  (nerve-wing),  like 
the  dragon-fly  and  ephemera,  yet  a  chirping  organ  has  been  detected 
which  allies  them  with  the  crickets,  grasshoppers  (Orthoptera),  etc. 
They  seem,  therefore,  to  be  a  connect- 
ing link  between  JVeuropters  and  Orthop- 
ters.  An  organ  adapted  to  produce 
definite  kinds  of  sound  to  attract  their 
mates,  of  course,  implies  an  organ 
adapted  to  appreciate  sound.  Evident- 
ly, therefore,  the  ear  was  already  some- 
406.— Wing~of  Piatephemera  antiqua.  what  advanced  in  organization  in  these 

Devonian,  America  (after  Dawson).  •  , 

Fishes. — But  the  grand  characteristic  of  the  Devonian  age  is  the 
appearance  and  culmination  of  the  class  of  fishes.  This  is  a  great  step 
in  advance ;  for  we  have  here  the  introduction,  not  only  of  a  new  class, 
but  a  new  department  (Vertebrata),  and  the  highest  of  the  animal  king- 
dom. These  earliest  fishes,  as  might  be  expected,  however,  were  far 
different  from  typical  fishes  of  the  present  day.  They  belonged  wholly 


ANIMALS. 


323 


to  the  two  orders  Ganoids  (gar-fish,  sturgeons,  and  mud-fishes)  and 
Placoids  (sharks,  skates,  and  rays),  and  to  families  of  these  orders  which 
are  now  either  wholly  or  nearly  extinct.  Appearing  first  in  Uppermost 
Silurian  and  Lower  Devonian,  few  in  number  and  small  in  size,  and  of 
strangely-uncouth  forms  in  Cephalaspis  (Fig.  408)  and  Pteraspis  (Fig. 
407),  the  earliest-known  genera,  this  class  soon  increased  until  the 
Devonian  seas  swarmed  with  them.  Probably  never  in  the  history  of 
the  earth  have  fishes  existed  in  greater  numbers,  variety,  and  size ;  and 
certainly  never  have  they  been  more  thoroughly  armed  for  offense  and 
defense.  The  Onychodus  (agate-toothed — Fig.  417),  in  the  Lower  De- 
vonian of  the  United  States,  had  jaws  eighteen  inches  long,  and  teeth 
two  inches  or  more  long.  The  animal  itself  is  supposed  to  have  been 
twelve  to  fifteen  feet  in  length.  The  Dinichthys  of  Ohio  had  jaws 
twenty-two  inches  long,  and  the  animal  was  eighteen  feet  long.  The 
Asterolepis  (star-scale),  described  by  Hugh  Miller,  was  still  more  gigan- 
tic, being  probably  twenty  to  thirty  feet  in  length.  The  teeth  of  many 
of  the  Devonian  Ganoids  were  decidedly  reptilian  in  character,  i.  e., 
long,  conical,  and  fluted  at  the  base,  as  in  many  reptiles  both  living  and 
extinct.  The  following  figures  represent  some  of  the  more  characteris- 
tic Devonian  fishes  (see  also  on  pages  324  and  325).  Of  Placoids,  on 
account  of  their  cartilaginous  skeleton  and  absence  of  scales,  only  the 
teeth  and  spines  are  found.  In  some  of  the  species  these  spines  were 
eighteen  inches  in  length. 

Of  the  fishes  above  named,  some  have  been  so  recently  discovered, 
and  so  remarkable  in  character,  that  they  seem  to  deserve  more  than  a 


FIG.  408. 

PIGS.  407  and  403.— DEVONIAN  FISHES— Placoderms :  407.  Pteraspis.  restored  by  Powrie  and  Lankaster 
(after  Dawson).    408.  Cephalaspis  Lyelli  (after  Nicholson). 


324 


PALAEOZOIC  SYSTEM  OF  ROCKS. 


bare  mention.  This  is  true  especially  of  the  Onychodus  and  the  Din- 
ichthys,  recently  discovered  in  the  Devonian  of  Ohio,  and  described  by 
Newberry. 

The  Onychodus  sigmoides  was  a  ganoid  fish,  twelve  to  fifteen  feet 


Fia.  411. 


FIG.  412. 

FIGS.  409-412.— DEVONIAN  FISHKS— Placoderms :  409.  Pterichthys  cornutus  (after  Nicholson).  410. 
Coecosteus  decipiens  (after  Owen),  tepidoganoide :  411.  Holoptychius  nobilissimus  (after  Nichol- 
son). 412.  Osteolepis  (after  Nicholson). 


ANIMALS. 


325 


in  length,  with  lower  jaw  eighteen  inches  long,  set  with  sharp,  conical 
teeth,  about  three-fourths  of  an  inch  long,  in  the  usual  position.  In  ad- 
dition to  these,  just  at  the  chin-suture,  were  set  a  vertical  row  of  pecul- 


Fio.  416. 

FIGS.  413-416.— DEVONIAN  FISHES— Lepidoganoids :  413.  Glyptolemus  Kinairdii  (after  Nicholson).  414. 
Diplacanthus  pracilis  (after  Nicholson).  Placoids :  415.  Ctenacanthus  vetustus,  Spine  reduced  (after 
Newberry).  416.  Machseracanthus  major,  Spine  reduced  (after  Newbeny). 

iarly-shaped  teeth,  at  least  two  inches  long,  pointing  forward  (Fig. 
417  c.)  The  body  was  covered  with  circular  imbricated  scales  an  inch  in 
diameter  (Fig.  417  a). 

The  Dinichthys  is  the  hugest  of  Devonian  fishes  yet  found  in 
America,  and  second  only  to  the  Asterolepis  of  the  European  Devonian. 
According  to  Newberry,  the  body  of  this  fish  was  fifteen  to  eighteen 
feet  long  and  three  feet  thick.  The  jawbones,  both  upper  and  lower, 
are  bent,  the  one  downward,  the  other  upward,  at  the  extreme  end,  and 
extended  to  form  two  strong,  sharp  front  teeth,  above  and  below,  while 
behind  these  the  upper  margin  of  the  jaw  is  compressed  into  a  sort  of 


326  PALEOZOIC  SYSTEM  OF  ROCKS. 

knife-edged  enameled  bone,  acting  together  like  shear-blades,  or  in  one 
species  sharply  dentate.  The  diagram  Fig.  418  (in  which,  however,  the 
bones  are  not  in  natural  position)  illustrates  this  structure.  Newberry 


FIG.  417.— Onychodus  sigmoides  (after  Newbcny):  a,  Scale,  natural  size;   6,  a  Tooth,  natural  size;  c,  a 
Kow  of  Front  Teeth,  reduced. 

has  drawn  attention  to  the  -remarkable  resemblance  of  this  jaw-structure 
to  that  of  the  Devonian  Coccosteus  and  the  living  Lepidosiren,  the  most 
reptilian  of  all  known  living  fishes.  This  resemblance  is  shown  in  the 
accompanying  figures  (419  and  420). 


FIG.  418.-JawsofDinichthysTorrelli,  x  5'5  (after  Newberry). 


FIG.  420.— Jaws  of  Lepidosiren  (side-view, 
after  Newberry). 


FIG.  419.— Jaws  of  Dinichthys  (side-view, 
after  Newberry). 


Like  the  Coccosteus  (Fig.  410),  not  only  the  head  but  also  the  whole 
fore-part  of  the  body  of  the  Dinichthys,  both  above  and  below,  was  cov- 
ered with  large  protecting  plates.  The  want  of  scales  in  the  hinder 
parts  and  the  cartilaginous  condition  account  for  the  fact  that  these 
parts  have  not  yet  been  found. 

Among  other  remarkable  fishes  found  in  the  Devonian  of  Ohio  may 
be  mentioned  Macropetalichthys  (Fig.  421),  several  species  of  Coccosteus, 
and  several  of  Acanthaspis — a  genus  allied  to  the  Cephalaspis  (Fig. 


ANIMALS. 


327 


408).     In  the  Devonian  of  New  York,  also,  a  number  of  species  have 
been  found. 

Ganoids  derive  their  name  from  the  thick,  bony,  enameled  scales 
which  cover  the  body,  forming  an  impenetrable  coat-of-mail.  Now,  in 
the  Devonian  Ganoids,  as  seen  in  the  figures,  these  scales  were  some- 
times large  and  imbricated  (Fig.  411),  sometimes  rhomboidal,  arranged 
in  oblique  rows  and  nicely  jointed,  as  in  gar-fishes  (Lepidosteus  and 
Polypterus)  of  the  present  day  (Figs.  412-414),  and  sometimes  large, 
immovably  soldered  polygonal  plates  (Figs.  409,  410).  Sometimes 


FIG.  421.— Skull  of  Macropetalichthys 
Sullivanti,  reduced  in  size. 


FIG.  422.— a,  Head  and  fore  limb  of  a 
Ceratodus ;  6,  Hind  limb  of  same 
(after  Gunther). 


the  plates  covered  only  the  head  (Cephalaspis),  sometimes  the  head 
and  forward  portion  of  the  body,  and  left  the  tail  free  for  locomo- 
tion (Coccosteus) ;  sometimes  the  whole  body  seems  inclosed  almost 
immovably  in  such  plates,  and  the  locomotion  was  effected  in  great 
part  by  arm-like  fins  (Pterichthys). 

Most  of  the  largest  Devonian  fishes,  as  the  huge  Asterolepis  and 
the  Dinichthys,  belonged  to  the  family  of  Plate-covered  Ganoids.  It 
is  to  this  bony  coat-of-mail  that  we  are  indebted  for  the  fine  preserva- 
tion of  Devonian  Ganoids. 

Affinities  of  Devonian  Fishes. — Devonian  Ganoids  may  be  con- 
veniently divided  into  two  sub-orders,  viz.,  Lepido-ganoids  (Scale  Ga- 
noids), or  Ganoids  proper  (Figs.  411-414),  and  Placo-ganoids  (Plate 
Ganoids),  or  Placoderms  (Figs.  407-410).  The  Placoderms  are  char- 
acteristic of  the  Devonian;  the  Lepido-ganoids  continue  in  diminish- 
ing numbers  even  to  the  present  time.  The  Placoderms  have  no 
living  near  congeners,  although  the  Dinichthys,  as  just  explained, 
has  some  affinities  with  the  Lepidosirens.  The  nearest  living  allies 
of  the  Lepido-ganoids  are  the  Polypterus  of  the  Nile,  the  Lepidos- 
teus, or  Gar-fish,  of  North  American  rivers,  the  Amia,  or  mud-fish,  of 
the  same  waters,  the  Lepidosiren  of  the  African  and  South  American 


328  PALEOZOIC  SYSTEM   OF  ROCKS. 

rivers,  and  the  recently-discovered  Ceratodus  of  Australian  rivers,1  a 
genus  which  ranged  in  time  from  the  Triassic  until  now. 

The  Polypterus  and  the  Ceratodus,  especially  the  latter,  have  one 
very  striking  reptilian  feature,  viz.,  the  paired-fins  have  a  scaled  lobe, 
supported  by  a  many-jointed  cartilaginous  axis,  running  down  the 
centre,  and  from  which  the  rays  come  off  on  each  side  (Fig.  422).  The 


FIG.  423.-D'ental  Plate  of  Cestracion  Phillippi. 

paired-fins  in  these  bear  the  same  relation  to  the  ordinary  paired-fin  of 
fishes  which  the  vertebrated  tail-fin  does  to  the  ordinary  tail-fin  (see 
next  page).  It  is  a  true  scelidate,  or  legged  fin,  and  is  connected, 
through  the  imperfect  limb  of  the  Lepidosiren,  with  true  limbs  of  am- 
phibians. Now,  many  of  the  Devonian  fishes  (Crossopterygians  of 
Huxley)  (Figs.  411  and  413)  have  this  style  of  fin  in  a  marked  degree. 

The  living  Placoid,  which  most  resembles  the  Devonian  Placoids,  is 
the  Cestracion  Phillipsi  of  Australia  (Fig.  429).  Instead  of  lancet- 
shaped  teeth,  which  characterize  most  modern  sharks,  the  jaws  of  the 
Cestracion  are  covered  with  a  broad  pavement  of  rounded  plates,  much 
like  a  pavement  of  cobble-stones  (Fig.  423).  The  family  of  pavement- 
toothed  sharks  are  called  Cestracionts  from  this  living  representative. 
The  Devonian  Placoids  were  all,  or  nearly  all,  Cestracionts. 

General  Characteristics  of  Devonian  Fishes. — Leaving  out  some 
small  aberrant  orders,  fishes  may  be  divided  into  three  orders,  viz., 
Teleosts,  Ganoids,  and  Placoids.  The  Teleosts  (perfect  bone)  comprise 
all  the  ordinary  typical  fishes.  By  far  the  larger  number  of  living  fishes 
belong  to  this  order.  The  Ganoids  are  nearly  extinct,  but  are  still 
represented  by  the  Polypterus,  the  Lepidosteus,  the  Amia,  and  the  Stur- 
geon (Accipenser)  ;  and  it  is  probable  that  we  should  include  also  the  Dip- 
noi :  i.  e.,  Ceratodus,  of  the  Australian  rivers,  and  Lepidosiren,  of  African 

1  These  last  two  genera  are  by  many  zoologists  put  by  themselves  into  a  distinct  order 
of  fishes,  the  Dipnoi ;  but  they  are  undoubtedly  very  closely  allied  to  the  early  Ganoids. 


ANIMALS. 


FTG.  427. 


FIGS.  424-429. — NEAREST  LIVING  ALLIES  OF  DEVONIAN  FISHES  :  424.  Ceratodus  Fosterii,  x  fa  (after 
Gunther).  425.  Polypterus.  426.  Lepidosiren.  427.  Lepidosteus  (Gar-Fish).  428.  Amia  (American 
Mud-fish).  429.  Cestracion  Phillippi  (a  Living  Cestraciont  from  Australia). 


330  PALAEOZOIC  SYSTEM  OF  ROCKS. 

and  South  American  rivers.     The  Placoids  (sharks  and  skates,  etc.) 
are  still  abundant,  but  far  less  so  than  the  Teleosts. 

Now,  as  already  said :  1.  The  Devonian  fishes  were  all  Ganoids  and 
Placoids,  especially  the  former.  There  were  no  ordinary  typical  fishes 
(Teleosts)  at  all  at  that  time.  2.  The  Ganoids  of  the  present  day  have, 
some  of  them,  bony  skeletons  (Lepidosteus),  and  some  cartilaginous  skel- 
etons (Sturgeon)  ;  the  Devonian  Ganoids  all  had  more  or  less  cartilagi- 
nous skeletons.  Therefore,  since  all  Placoids  have  cartilaginous  skel- 
etons, all  the  fishes  of  these  early  times  had  cartilaginous  skeletons. 
3.  Of  Ganoids  of  the  present  day,  some  have  the  mouth  at  the  end  of  the 
snout  (gar-pike),  some  beneath  or  on  the  ventral  surface  (sturgeons). 


FIG.  430. — a,  Hoinocercal ;  6,  Heterocercal. 

The  same  was  true  in  Devonian  times.  The  Lepido-ganoids  had  ter- 
minal mouth ;  the  Placoderms,  ventral  mouth  ;  and,  since  Placoids  all 
have  ventral  mouth,  all  the  Devonian  fishes,  except  the  Lepido-ganoids, 
had  the  mouth  on  the  ventral  surface.  4.  There  are  two  types  of  fish 
tail-fins,  differing  both  in  shape  and  structure.  These  are  the  homo- 
cereal  (even-lobed),  found  in  Teleosts  (Fig.  430  a) ;  and  the  heterocercal 
(uneven-lobed),  found  in  Placoids  (Fig.  4305).  In  the  homocercal,  or 
even-lobed,  the  vertebral  column  terminates  abruptly  in  one  or  several 
large  flat  bones,  from  which  diverge  the  fin-rays  (Fig.  431  a).  In  the 
heterocercal,  or  uneven-lobed,  the  vertical  column  runs  to  the  extreme 
point  usually  of  the  upper  lobe  (Fig.  431  £).  Such  a  tail-fin,  therefore, 


t 

<z 

FIG.  431.— a,  Homocercal  (Sword-fish) ;  b,  Heterocercal  (Sturgeon). 

is  said  to  be  vertebrated ;  and  this  is  the  better  name  for  this  style  of 
tail,  as  the  structure  is  more  important  than  shape,  and  in  some  cases  a 
vertebrated  tail  may  be  nearly  or  quite  symmetrical,  as  in  Polypterus 
(Fig.  425),  and  Glyptolemus  (Fig.  413).  Now,  while  the  tails  of  living 
Ganoids  are  some  decidedly  vertebrated,  and  some  only  slightly  so, 


ANIMALS.  331 

those  of  Devonian  Ganoids  are  all  decidedly  vertebrated.  And  since 
Placoids  are  all  vertebrated-tailed,  all  Devonian  fishes  are  vertebrated- 
tailed. 

Rank  of  Devonian  Fishes. — We  have  called  Teleosts  typical  fishes. 
In  Ganoids  and  Placoids,  especially  the  former,  and  still  more  especially 
in  the  Devonian  Ganoids,  combined  with  their  distinctive  fish-charac- 
ters, there  are  other  characters  which  ally  them  with  reptiles,  and  also 
still  others  which  may  be  termed  embryonic.  The  most  important  rep- 
tilian characters  of  Ganoids,  especially  Devonian  Ganoids,  are :  1.  An 
external  armor  of  thick  bony  plates  or  scales.  2.  Large,  conical  teeth, 
with  channeled  base  (Fig.  432  a), 
and  labyrinthine  internal  structure,  as 
shown  in  section  (Fig.  432  b).  Some- 
times this  structure  is  more  complex 
than  here  represented.  3.  A  some- 
what cellular  Swim-bladder,  in  SOme  FIG.  432.— Structure  of  a  Ganoid  Tooth  (after 

cases  freely  supplied  with  blood,  open- 
ing by  a  tube  into  the  pharynx,  and  therefore  showing  much  anal- 
ogy to,  and  in  some  cases  (Ceratodus)  acting  as,  an  imperfect  lung. 
We  do  not  know  that  this  was  true  of  the  Devonian  Ganoids,  but  it  is 
true  of  their  nearest  living  allies,  viz.,  Polypterus,  Lepidosteus,  Amia, 
and  Ceratodus.  4.  In  many  cases,  paired  fins  which  were  jointed. 

Combined  with  these  decidedly  reptilian  characters  are  others 
which  are  as  decidedly  embryonic.  The  most  conspicuous  of  these 
are :  1.  The  cartilaginous  condition  of  the  skeleton,  and  even  the  reten- 
tion of  the  embryonic  fibrous  chorda  dorsalis,  imperfectly  articulated 
into  a  vertebrate  column  ;  and,  2.  In  the  Placoderms,  the  ventral  posi- 
tion of  the  mouth.  The  vertebrated  tail-fin  is  regarded  by  some  as  em- 
bryonic, and  by  others  as  reptilian. 

In  Placoids  there  is  a  similar  combination  of  reptilian  and  embry- 
onic characters,  except  in  this  case  the  embryonic  seem  to  predominate. 
These  are,  as  before — 1.  The  cartilaginous  skeleton  ;  2.  The  inferior  posi- 
tion of  the  mouth.  But  also,  in  addition,  3.  The  leathery  or  imperfectly 
rayed  fins ;  4.  The  want  of  an  opercle  or  gill-cover,  growing  backward 
over,  and  thus  covering  the  gill-slits  ;  5.  Perhaps  the  ligamentous  in- 
stead of  bony  attachment  of  the  teeth. 

On  the  other  hand,  the  Placoids  of  the  present  day  at  least  possess 
very  high  reptilian  characters  in  their  reproduction.  In  all  Placoids  their 
impregnation  is  internal,  and  instead  of  laying  great  numbers  of  unim- 
pregnated  ovules,  like  most  Teleosts,  they  either  lay  few  large,  well- 
covered  eggs  like  reptiles  and  birds  (skates  and  some  sharks),  or  else 
their  eggs  hatch  within  and  they  bring  forth  young  alive  (ovo-vivip- 
arous")  like  some  reptiles;  or  in  some  cases  there  is  even  an  attach- 
ment between  the  yolk-sac  of  the  internally  hatched  young  and  the 


PALAEOZOIC  SYSTEM  OF  ROCKS. 


oviduct  of  the  mother,  somewhat  similar  to  that  of  the  placenta  to  the 
uterus  of  the  mammal.  The  young  of  Placoids  also  at  first  have  a  kind 
of  external  branchiae  like  those  of  amphibian  reptiles. 

The  following  schedule  shows  the  combination  of  characters  enu- 
merated. It  is  seen  that  in  Ganoids  the  reptilian  characters,  in  Placoids 
the  embryonic  characters,  predominate.  But,  on  the  other  hand,  the 


GANOIDS. 

PLACOIDS. 

(armor 
teeth 

reproduction 
tail-fin 

I  Reptilian 

Reptilian     - 

swim-bladder 

skeleton 

j 

paired-fins 

mouth 

tail  fin 

fins 

V  Embryonic 

Embryonic 

{skeleton 
mouth 

gills 
teeth 

J 

reptilian  characters  of  Placoids  are  more  decided  and  higher.  The 
Lepido-ganoids  of  Devonian  and  Carboniferous  times  were  far  more 
reptilian  than  existing  Ganoids ;  hence  these  have  been  appropriated 
called  Sauroid  fishes. 

Bearing  of  these  Facts  on  the  Question  of  Evolution. — On  account 
of  this  combination  of  connecting  and  embryonic  characters — of  char- 
acters which  seem  higher  and  others  which  seem  lower  than  those  of 
typical  fishes — there  has  been  much  dispute  as  to  the  rank  of  Ganoids 
and  Placoids,  and  especially  of  Devonian  fishes,  and  therefore  as  to 
their  bearing  on  the  question  of  evolution.  The  dispute,  however,  has 
been  mostly  the  result  of  a  misconception  of  the  true  nature  of  evolu- 
tion. The  most  fundamental  law  of  evolution  is  differentiation;  i.  e., 
is  a  separation  of  one  generalized  form  into  several  specialized  forms — 
a  separation  of  one  stem  into  several  branches.  The  Devonian  fishes 
are  an  admirable  illustration  of  this  law.  The  first  introduced  fishes 
were  not  typical  fishes,  but  Sauroids,  i.  e.,  fishes  which  combined  with 
their  distinctive  fish-characters  others  which  allied  them  with  reptiles. 
They  were  the  representatives  and  progenitors  of  both  classes ;  from 
this  common  stem  diverged  two  branches,  viz.,  typical  fishes  on  the  one 
hand,  and  reptiles  on  the  other.  This  is  but  one  example  of  a  very 
general  law,  which  may  be  formulated  thus :  The  first  introduced  of 
any  class  or  order  were  not  typical  representatives  of  that  class  or 
order,  but  connecting  links  with  other  classes  or  orders,  the  complete 
separation  of  the  two  or  more  classes  or  orders  represented  being  the 
result  of  subsequent  evolution.  Such  connecting  links  are  variously 
called  connecting  types,  synthetic  types,  comprehensive  types,  com- 
bining types,  generalized  types,  etc.  We  shall  find  many  examples  of 
such  in  the  course  of  the  history  of  the  organic  kingdom. 

Suddenness  of  Appearance. — But  it  is  impossible  to  overlook  the 


CARBONIFEROUS  SYSTEM.  333 

comparative  suddenness  of  the  appearance  of  a  new  class — fishes — and 
a  new  department — vertebrates — of  the  animal  kingdom.  Observe  that 
at  the  horizon  of  appearance  in  the  uppermost  Silurian  there  is  no  ap- 
parent break  in  the  strata,  and  therefore  no  evidence  of  lost  record : 
and  yet  the  advance  is  immense.  It  is  impossible  to  account  for  this 
unless  we  admit  paroxysms  of  more  rapid  movement  of  evolution — 
unless  we  admit  that,  when  conditions  are  favorable  and  the  time  is 
ripe  for  a  particular  change,  it  takes  place  with  exceptional  rapidity, 
perhaps  in  a  few  generations. 

Reptiles  have  not  yet  been  found  in  the  Devonian ;  Fishes  there- 
fore were  the  highest  and  most  powerful  animals  then  living.  They 
were  the  rulers  of  the  Devonian  seas.  The  previous  rulers,  therefore, 
viz.,  Orthoceratites  and  Trilobites,  according  to  a  necessary  law,  in  the 
struggle  for  life,  diminish  in  size  and  number,  and  seek  safety  in  a  sub- 
ordinate position. 

SECTION  3. — CARBONIFEROUS  SYSTEM. — AGE  OF  ACKOGENS  AND 
AMPHIBIANS. 

Retrospect. — Before  taking  up  in  detail  this  important  and  interest- 
ing age,  it  will  be  instructive  to  glance  back  over  the  ground  traversed, 
and  draw  some  conclusions. 

If  we  compare,  in  physical  geography,  the  American  with  the  Eu- 
ropean Continent,  we  find  the  one  marked  by  simplicity  and  the  other 
by  complexity  of  structure.  This  is  true  not  only  of  the  map-outline, 
but  also  of  the  profile-outline,  or  orographic  structure.  Now,  as  history 
furnishes  the  key  to  social  and  political  structure,  so  geology  furnishes 
the  key  to  physical  structure.  The  American  Continent — at  least  in  its 
eastern  part — has  developed  steadily  from  the  Laurentian  nucleus  south- 
ward and  eastward,  and  probably  northward.  We  have  already  seen 
how  the  Silurian  area  was  added  to  the  Laurentian,  and  the  Devonian 
to  the  Silurian.  It  shall  be  our  pleasure,  hereafter,  to  show  the  con- 
tinuance of  this  steady  development  throughout  the  whole  geological 
history.  For  our  knowledge  on  this  interesting  subject  we  are  indebted 
almost  wholly  to  Prof.  Dana. 

In  the  case  of  America,  the  continent  thus  sketched  in  outline  in 
the  earliest  times  has  been  steadily  worked  out  in  detail  throughout 
all  subsequent  time ;  with  some  oscillations,  true,  determining  uncon- 
formability  of  strata,  rapid  changes  of  physical  geography  and  climate, 
and  therefore  of  species,  thus  marking  the  great  divisions  of  time,  but 
on  the  whole  without  change  of  plan  or  wavering  of  purpose ;  in  the 
case  of  Europe,  on  the  contrary,  geological  history  consists  of  a  series 
of  oscillations  so  great  that  it  amounts  to  a  successive  making  and 
unmaking  of  the  continent. 


334  PALAEOZOIC  SYSTEM  OP  ROCKS. 

Hence,  nearly  all  geological  problems  are  expressed  in  simpler  terms, 
and  are  more  easily  solved  here  than  there.  Hence,  also,  while  in  Eu- 
rope the  ages  and  periods  are  separated  by  unconformability  of  the 
rock-system,  as  well  as  change  in  the  life-system,  in  America  they  are 
separated  mainly  by  change  in  the  life-system  only. 

Subdivisions  of  the  Carboniferous  System  and  Age. — The  Carbonifer- 
ous age  is  subdivided  into  three  periods,  viz.  :  1.  Sub-Carboniferous; 
2.  Coal-measures,  or  Carboniferous  proper;  3.  Permian. 

The  sub-Carboniferous  was  the  period  of  preparation ;  the  Coal- 
measures  the  period  of  culmination  /  the  Permian  the  period  of  decline 
and  transition  to  the  Mesozoic.  The  whole  thickness  of  the  carbon- 
iferous strata  in  Nova  Scotia  is  14,570  feet ;  in  South  Wales  it  is  14,000 
feet,  and  in  Pennsylvania  9,000  feet. 

The  sub-Carboniferous  consists  mainly  of  marine  formations ;  the 
Coal-measures  mainly  of  fresh-water  formation — the  former  mainly  of 
limestone,  the  latter  mainly  of  sands  and  clays ;  the  fossils  of  the  for- 
mer are,  therefore,  mainly  marine  animals,  of  the  latter  mainly  fresh- 
water and  land  animals  and  plants,  though  marine  animals  are  also 
found.  In  both  Europe  and  America  the  coal-basins  consisting  of  the 
latter  are  underlaid  by  the  former,  which,  moreover,  outcrop  all  around, 
forming  a  penumbral  margin  to  the  dark  areas  representing  coal-basins 
on  geological  maps  (see  map,  page  278).  Between  these  two,  or,  rather, 
forming  the  lowest  member  of  the  Coal-measures,  there  is,  in  many 
places,  a  thick,  coarse  sandstone,  called  the  millstone  grit. 

After  this  general  contrast,  we  will  now  concentrate  nearly  our 
whole  attention  upon  the  Carboniferous  period  proper ;  because  in  this 
middle  period  culminated  all  the  more  striking  characteristics  of  the 
age.  In  speaking  of  the  life-system,  however,  we  will  draw  from  both 
sub-Carboniferous  and  Carboniferous  indifferently.  The  Permian  we 
shall  treat  only  as  a  transition  to  the  next  era. 

Carboniferous  Proper — Rock-System  or  Coal- Measures. 

The  Name. — The  Carboniferous  period  is  but  one  of  the  three  peri- 
ods of  this  age.  The  Carboniferous  age  is,  again,  but  one  of  the  three 
ages  of  the  Palaeozoic  era,  while  the  Palaeozoic  era  is  itself  but  one  of 
the  four  great  eras,  exclusive  of  the  present,  of  the  whole  recorded  his- 
tory of  the  earth.  The  Carboniferous  period,  therefore,  is  probably  not 
more  than  one-thirtieth  part  of  that  recorded  history.  Yet,  during  that 
period  were  accumulated,  and  in  the  strata  of  that  period  (Coal-meas- 
ures) are  still  inclosed,  at  least  nine-tenths  of  all  the  worked  coal,  and 
probably  nearly  nine-tenths  of  all  the  workable  coal  in  the  world.  It  is 
essentially  the  coal-bearing  period.  When  we  remember  that  every 
geological  period  has  its  characteristic  fossils,  by  means  of  which  the 
formation  may  be  at  once  recognized  by  the  experienced  eye,  it  is  easy 


ROCK-SYSTEM   OR   COAL-MEASURES. 


335 


to  see  the  importance  of  this  simple  fact  as  a  guide  to  the  prospector. 
It  has  been  estimated  that  the  money,  time,  and  energy,  uselessly  ex- 
pended in  the  State  of  New  York  in  explorations  for  coal,  where  any 
geologist  might  be  sure  there  was  no  coal,  would  suffice  to  make  a  com- 
plete geological  survey  of  the  State  several  times  over !  The  same  is 
true  of  Great  Britain  and  many  other  countries. 

Thickness  of  Strata. — Although  constituting  so  small  a  portion  of 
the  whole  stratified  crust  of  the  earth,  the  coal-measures  are  in  some 
places  of  enormous  thickness.  In  Nova  Scotia  they  are  13,000  feet ;  in 
South  Wales,  12,000  feet ;  in  Pennsylvania,  4,000  feet ;  in  West  Vir- 
ginia, over  4,500  feet. 

Mode  of  Occurrence  Of  Coal. — Such  being  the  thickness  of  the  coal- 
measures,  it  is  evident  that  but  a  small  proportion  consists  of  coal.  The 
coal-measures  consist,  in  fact,  of  thick  strata  of  sandstone,  shales,  and 
limestone,  like  other  formations ;  but  in  addition  to  these  are  inter- 
stratified  thin  seams  of  coal  and  beds  of  iron-ore.  Even  in  the  richest 
coal-measures,  the  proportion  of  coal  to  rock  is  not  more  than  as  1  to  50, 
and  the  proportion  of  iron  is  still  much  smaller.  In  some  coal-fields,  as, 
for  example,  in  the  Appalachian,  mechanical  sediments, 
shales,  and  sand-stones,  predominate  ;  in  others,  as  in  the 
Western  coal-fields,  organic  sediments  or  limestone  pre- 
dominate. 

The  five  kinds  of  strata  mentioned  are  repeated  in 
the  same  coal-basin  very  many  times — perhaps  100  or 
more,  as  in  the  accompanying  section  ;  but,  in  comparing 
one  coal-field  with  another,  or  in  the  same  coal-field,  in 
comparing  one  portion  of  the  series  with  another,  there 
is  no  regular  order  of  succession  discoverable.  Except 
that  immediately  in  contact  with  the  seam  beneath,  there 
is  nearly  always  a  thin  seam  of.  fine  fire-clay.  This  con- 
stant attendant  of  a  coal-seam  is  called  the  under-day. 
Again,  immediately  above,  and  therefore  forming  the 
roof  of  the  opened  seam,  there  is  frequently,  though  not 
so  constantly,  a  shale  which,  being  impregnated  with  car- 
bonaceous matter,  is  called  the  black  shale  or  black  slate. 
These  accompaniments  are,  however,  usually  too  thin  to 
appear  on  sections. 

In  different  portions,  however,  of  the  same  coal-field, 
at  the  same  geological  horizon,  we  are  apt,  to  find  the  Fl 
same  order.  This  is  the  necessary  result  of  the  continu- 
ity of  the  strata  over  the  whole  basin.  If  we  represent 
coal-basins,  with  their  five  different  kinds  of  strata,  by 
reams  of  variously-colored  paper,  then,  while  the  order 
of  succession  may  be  different  in  the  different  reams,  and 


PALAEOZOIC   SYSTEM  OF   ROCKS. 


in  the  upper  or  lower  portion  of  the  same  ream,  yet  at  the  same  level 
we  find  the  same  order  in  every  portion  of  the  same  ream.  This  is  a 
test  of  a  field  even  when  separated  by  denudation  into  several  basins. 
It  is  also  a  mode  of  identifying  individual  coal-seams  ;  for,  if  the  strata 
be  continuous,  then  the  seam  will  have  the  same  accompanying  strata 
above  and  below.  The  great  Pittsburg  seam  has  been  thus  identified, 
with  great  probability,  over  an  area  of  14,000  square  miles,  and,  allow- 
ing for  removal  by  denudation,  over  an  original  area  of  34,000  square 
miles.  Rodgers  thinks  the  original  area  may  have  been  90,000  square 
miles.1  This  rule  for  the  identification  of  coal-seams  of  known  value  is 
often  of  practical  importance;  but  it  must  be  remembered  that  the 
strata  of  coal-measures,  both  the  seams  and  the  accompanying  shale 
and  sandstones,  like  all  other  strata,  thin  out  on  their  edges  (p.  173). 
Nevertheless,  there  is  a  most  extraordinary  continuity  in  the  strata  of 
the  coal-measures. 

Plication  and  Denudation. — Coal-bearing  strata,  like  all  other  strata, 


FIG.  434. — Panther  Creek  and  Summit  Hill  Traverse  (after  Dadow). 

were,  of  course,  originally  horizontal  (p.  173)   and  continuous,  but, 
like  other  strata,  they  are  now  found  sometimes  horizontal  and  some- 


Fio.  435.— Nesquehoning  Basins  (after  Dadow). 


times  dipping  at  all  angles,  and  folded  in  the  most  complex  manner.    In 
the  Appalachian  region,  especially  in  the  anthracite  region  of  Northern 


FIG.  436.— Illinois  Coal-Field  (after  Dadow). 

Pennsylvania,  the  strata  are  very  much  disturbed,  and  the  coal-seams  in- 

terstratified  with  them  are  often  nearly  perpendicular  (Figs.  435  and  437), 

1  Phillips,  "  Geology,"  p.  217. 


COAL-MEASURES. 


337 


while  in  Indiana  and  Iowa  the  coal-strata  are  nearly  or  quite  horizontal 
(Fig.  436).     But,  whether  horizontal,  or  gently  folded,  or  strongly  pli- 


FIG.  437.— Section  near  Nesquehoning  (after  Taylor). 

cated,  in  all  cases  denudation  has  carried  away  much  of  the  upper  por- 
tions, leaving  them  in  isolated  patches  as  mountains  or  basins,  as  shown 
in  the  map  of  Northern  Pennsylvania  (Fig.  439)  and  in  the  section 
(Fig.  438). 


FIG.  433.— Section  of  Appalachian  Coal-Field,  Pennsylvania,  showing  Effects  of  Erosion  on  Gently-Undu- 
lating Strata  (after  Lesley). 

By  means  of  the  rule  for  identifying  seams  given  above,  it  is  often 
easy  to  trace  the  same  seam  from  one  basin  to  another,  or  from  one 
mountain-side  to  another,  with  great  certainty. 


FIG.  439.— Map  of  Anthracite  Region  of  Pennsylvania  (after  Lesley). 

Faults. — It  is  plain,  from  what  has  been  said  above,  that  there  is 
an  essential  difference  between  a  coal-seam  and  a  metalliferous  vein. 
22 


338  PALAEOZOIC  SYSTEM  OF  ROCKS. 

Coal-seams  are  conformable  with  the  strata,  and  are  therefore  worked 
wholly  between  the  strata.  This  -would  be  a  comparatively  easy  matter 
if  it  were  not  for  slips  or  faults  which  often  occur,  and  sometimes  make 
the  working  unprofitable.  In  case  of  a  fault,  it  is  important  to  remem- 
ber the  rule  already  given  on  page  225,  viz.,  that  most  commonly  the 
strata  on  the  foot-wall  side  of  the  fissure  goes  upward.  In  the  following 


FIG.  440.— Section  across  Yarrow  Colliery,  showing  the  Law  of  Faults  (after  De  la  Beche). 

section  of  Yarrow  colliery  it  will  be  seen  that  all  the  slips  follow  this 
law. 

Thickness  of  Seams. — Coal-seams  vary  in  thickness  from  a  fraction 
of  an  inch  to  forty  or  fifty  feet.  A  workable  seam  must  be  at  least 
three  feet  thick.  A  pure,  simple  seam  is  seldom  more  than  eight  or 
ten  feet.  Mammoth  seams,  such  as  occur  in  the  anthracite  region  of 
Pennsylvania,  and  in  Southern  France,  are  produced  by  the  running 
together  of  several  seams  by  the  thinning  out  of  the  interstratified 
shales  and  sandstones.  They  are,  therefore,  almost  always  compound 
seams,  i.  e.,  separated  by  thin  partings  of  clay — too  thin  to  form  a  good 
roof  or  floor,  and  therefore  all  worked  together. 

Number  and  Aggregate  Thickness. — In  a  single  coal-field,  we  have 
said,  the  strata,  including  the  coal-seams,  are  repeated  many  times. 
In  the  South  Joggins's  section,  Nova  Scotia,  there  are  eighty-one  coal- 
seams,  though  most  of  these  are  not  workable.  In  North  England  there 
are  twenty  to  thirty  seams.  In  South  Wales  there  are  more  than  100 
seams,  seventy  of  which  are  worked.  In  South  Lancashire  there  are 
seventy -five  seams  over  one  foot  thick ;  in  Belgium  100  seams,  and  in 
Westphalia  117  seams.  The  aggregate  thickness  of  all  the  seams  in 
Lancashire  is  150  feet ;  in  Pottsville,  Pennsylvania,  113  feet ;  in  West- 
ern coal-fields,  seventy  feet. 

The  thickest  and  purest  are  usually  near  the  middle  of  the  series. 
Evidently  the  conditions  favorable  for  the  formation  and  preservation 
of  coal  commenced  gradually,  even  back  in  the  Devonian,  reached  their 
culmination  in  the  middle  Coal-measures,  and  gradually  passed  away. 
This  geological  day  had  its  morning,  its  high  noon,  and  its  evening. 

Coal  Areas  Of  the  United  States. — In  no  other  country  are  the 
coal-fields  so  extensive  as  in  the  United  States.  The  principal  coal- 
fields are  shown  on  map  of  Eastern  United  States,  on  page  278. 

1.  Appalachian  Coal-Field. — This,  the  greatest  coal-field  in  the 
world,  commences  in  Northern  Pennsylvania,  covers  the  whole  of  West- 


COAL-MEASURES.  339 

ern  Pennsylvania  and  Eastern  Ohio,  a  large  portion  of  West  Virginia 
and  Eastern  Kentucky,  then  passes  southward  through  East  Tennessee, 
touches  the  northwest  corner  of  Georgia,  and  ends  in  Middle  Alabama. 
In  general  terms,  it  occupied  the  western  slope  of  the  Appalachian 
from  the  confines  of  New  York  to  Middle  Alabama.  Its  area  is  at  least 
60,000  square  miles. 

2.  Central  Goal-Field. — This  covers  the  larger  portion  of  Illinois, 
the  southwest  portion  of  Indiana,  and  the  western  portion  of  Kentucky. 
Its  area  is  about  47,000  square  miles. 

3.  Western  Coal-field. — This  covers  the  southern  portion  of  Iowa, 
the  northern  and  western  portion  of  Missouri,  the  eastern  portion  of 
Kansas,  and  then  passes  southward  through  Arkansas  into  Texas.     Its 
area  is  estimated  at  78,000  square  miles.     These  two  coal-fields  are 
seen  to  be  connected  by  sub-Carboniferous.     They  are  probably  one  im- 
mense field  separated  by  erosion. 

4.  Michigan  Coal-field. — In  the  very  centre  of  the  State  of  Mich- 
igan there  is  another  coal-field  occupying  an  area  of  6,700  square  miles. 

5.  Rhode  Island  Coal-Field. — A  small  patch  of  500  square  miles' 
area  is  found  in  Rhode  Island,  extending  a  little  into  Massachusetts. 

6.  Nova  Scotia  and  New  Brunswick. — This  is  a  large  area  on  both 
sides  of  the  bay  of  Fundy.     It  is  estimated  at  18,000  square  miles. 

The  following  table  gives  approximately  the  areas  of  American 
coal-fields  of  the  Carboniferous  age : 

Appalachian 60)000 

Central 47,000 

Western 78,000 

Michigan 6^00 

Rhode  Island 500 


192,200 
Nova  Scotia 18)000 

210,200 

Of  the  190,000  square  miles  coal-area  of  the  United  States,  120,000 
square  miles  is  estimated  as  workable. 

Extra-Carboniferous  Coal.— All  the  fields  mentioned  above  belong 
to  the  Carboniferous  age.  But,  besides  these,  the  United  States  is  very 
rich  in  coal  of  other  periods.  Probably  20,000  to  25,000  square  miles 
might  be  added  from  strata  of  later  times,  making  in  all  150,000  square 
miles  of  workable  coal.  But  of  these  latter  fields  we  will  speak  in 
their  proper  places. 

Coal-Areas  of  Different  Countries  compared.— The  following  table, 
taken  principally  from  Dana,  exhibits  the  comparative  coal-areas  of 
the  principal  coal-producing  countries  of  the  world : 


340 


PALAEOZOIC  SYSTEM  OF  ROCKS. 


United  States 120,000   to  150,000  square  miles. 

British  America 18,000  " 

Great  Britain 12,000  " 

Spain 4,000  " 

France 2,000  " 

Germany 1,800  " 

Belgium ' 518  " 

Europe,  estimated 100,000  " 

Relative  Production  of  Coal. — But  if  the  extent  of  coal-area  repre- 
sents approximately  the  amount  of  wealth  of  this  kind  present  in  the 
strata,  the  production  of  coal  represents  how  much  of  this  wealth  is 
active  capital;  it  represents  the  development  of  those  industries  de- 
pendent on  coal.  In  this  respect  Great  Britain  is  far  in  advance  of  all 
other  countries,  as  seen  by  the  following  table,  compiled  from  the  best 
sources  at  hand : 


ANNUAL  COAL-PRODUCTION  IN 
MILLIONS  OF  TONS. 

1845. 

1864. 

1872. 

1874. 

1875. 

Great  Britain 

31   5 

90 

123 

125 

132 

United  States 

4  5 

22 

50 

Germany 

46 

Belgium  

4  9 

10 

15 

*  * 

France  

4  1 

10 

17 

Inspection  of  the  table  shows  that  in  the  principal  coal-producing 
countries  there  is  a  rapid  increase  of  production.  It  is  believed  that,  if 
the  same  rate  of  increase  continues,  the  annual  production  of  Great 
Britain  will  be  in  thirty  years  250,000,000  tons,  and  the  whole  work- 
able coal  will  be  exhausted  in  110  years.1  As  might  be  expected, 
therefore,  British  statesmen  and  scientists  are  casting  about  with  much 
anxiety  for  means  by  which  to  promote  the  more  economic  use  of  coal. 
Fortunately,  our  own  country  is  supplied  with  almost  inexhaustible 
stores  of  this  source  of  industrial  prosperity. 

Origin  of  Coal,  and  of  its  Varieties. 

That  coal  is  of  vegetable  origin  is  now  no  longer  doubtful.  We 
will  only  briefly  enumerate  the  evidences  on  which  is  based  the  present 
scientific  unanimity  on  this  subject : 

1.  The  remains  of  an  extinct  vegetation  are  found  in  abundance  in 
immediate  connection  with  coal-seams ;  stumps  and  roots  in  the  under- 
clay,  and  leaves  and  stems  in  the  black  slate  in  contact  with  the  seam 
and  even  imbedded  in  the  seam  itself.  2.  These  vegetable  remains  are 
not  only  associated  with  the  coal-seam,  but  have  often  themselves  be- 
come coal,  though  still  retaining  their  original  form  and  structure. 
1  Armstrong,  Nature,  vol.  vii.,  p.  291. 


ORIGIN  OF  COAL   AND   ITS  VARIETIES. 


341 


3.  Not  only  these  easily-recognizable  imbedded  vegetable  fragments, 
but  the  imbedding  substance  also,  the  whole  coal-seam,  even  the  most 
structureless  portions,  and  the  hardest  varieties,  such  as  anthracite, 
when  carefully  prepared  in  a  suitable  manner  and  examined  with  the 
microscope,  show  vegetable  structure.  Even  the  ashes  of  coal,  carefully 
examined,  show  vegetable  cells  with  characteristic  markings.  The  fol- 
lowing figures  show  the  results  of  such  examination.  4.  A  perfect  grada- 


FIG.  441. — Section  of  Anthracite  :  a,  natural  size ;  b 
and  c,  magnified  (after  Bailey). 


FIG.  442.— Vegetable  Structure  in  Coal 
(after  Dawson). 


tion  may  be  traced  from  wood  or  peat,  on  the  one  hand,  through  brown 
coal,  lignite,  bituminous  coal,  to  the  most  structureless  anthracite  and 
graphite,  on  the  other,  showing  that  these  are  all  different  terms  of  the 
same  series.  In  chemical  composition,  too,  the  same  unbroken  series 
may  be  traced.  5.  Lastly,  the  best  and  most  structureless  peat,  by  hy- 
draulic pressure,  may  be  made  into  a  substance  having  many  of  the 
qualities  and  uses  of  coal. 

We  may,  with  perhaps  less  confidence,  go  farther,  and  say  that  all 
the  carbon  and  hydrocarbon  known  to  us  are  probably  of  organic  origin. 
Carbon  probably  existed  at  first  only  as  carbonic  acid,  and  has  been  re- 
duced from  that  condition  only  bv  organic  agency. 

Varieties  of  Coal. — The  varieties  of  coal  depend  upon  the  purity, 
upon  the  degree  of  bituminization,  and  upon  the  proportion  of  fixed 
and  volatile  matter. 

Varieties  depending  upon  Purity. — Coal  consists  partly  of  organic 
or  combustible  and  partly  of  inorganic  or  incombustible  matter.  On 
burning  coal,  the  organic,  combustible  matter  is  consumed,  and  passes 
away  in  the  form  of  gas,  while  the  inprganic,  incombustible  is  left  as 
ash.  Now,  the  relative  proportion  of  these  may  vary  to  any  extent. 
We  may  have  a  coal  of  only  one  or  two  per  cent.  ash.  We  may  have  a 


342  PALEOZOIC  SYSTEM   OF  ROCKS. 

coal  of  five,  ten,  fifteen,  twenty  per  cent,  ash ;  the  coal  is  now  becoming 
poor.  We  may  have  a  coal  of  thirty  or  forty  per  cent,  ash ;  this  is  called 
bony  coal,  or  shaly  coal;  it  is  the  valueless  refuse  of  the  mines.  We  may 
have  a  coal  of  fifty  or  sixty  per  cent,  ash ;  but  it  now  loses  the  name  as 
well  as  the  ready  combustibility  of  coal,  and  is  called  coaly  shale.  Fi- 
nally, we  may  have  a  coal  of  seventy,  eighty,  ninety,  ninety-five  per 
cent,  ash;  and  thus  it  passes,  by  insensible ' degrees,  through  black 
shale  into  perfect  shale.  This  passage  is  often  observed  in  the  roof 
of  a  coal-seam. 

Now,  all  vegetable  tissue  contains  incombustible  matter,  which,  on 
burning,  is  left  as  ash.  The  amount  of  ash  in  vegetable  matter  is  on  an 
average  about  one  to  two  per  cent.  But  as,  in  the  process  of  change 
from  wood  to  coal,  much  of  the  organic  matter  is  lost  (p.  343,  et  seq.),  and 
the  relative  amount  of  ash  is  thereby  increased,  we  may  say  that,  if  a 
coal  contains  five  per  cent,  or  less  of  ash,  it  is  absolutely  pure — i.  e.,  its 
ash  comes  wholly  from  the  plants  of  which  it  is  composed;  but  if  a  coal 
contains  more  than  ten  per  cent,  ash,  it  is  probably  impure — i.  e.,  mixed 
with  mud  at  the  time  of  its  accumulation. 

Varieties  of  Coal  depending  on  the  Degree  of  Bituminization. — 

The  previously-mentioned  varieties  consist  of  pure  and  impure  coals; 
these  consist  of  perfect  and  imperfect  coals.  We  find  the  vegetable 
matter,  accumulated  in  different  geological  periods,  in  different  stages 
of  that  peculiar  change  called  bituminization.  Brown  coal  and  lignite 
are  examples  of  such  imperfect  coal.  They  are  always  comparatively 
modern. 

Varieties  depending  upon  the  Proportion  of  Fixed  and  Volatile 

Matter.  —  Coal,  even  when  pure  and  perfectly  bituminized,  consists 
still  of  many  varieties,  having  different  uses,  depending  upon  the  pro- 
portion of  fixed  and  volatile  matters.  These  are  the  true  varieties  of 
coal. 

In  pure  and  perfect  coal,  then,  the  combustible  matter  is  part  fixed 
and  part  volatile.  These  may  be  easily  separated  by  heating  to  red- 
ness in  a  retort.  By  this  means  the  volatile  matter  is  all  driven  off, 
and  may  be  collected  as  tar,  oil,  etc.,  in  condensers,  and  as  permanent 
gases  in  gasometers  ;  and  the  fixed  matter  is  left  in  the  retort  as  coke. 
Now,  the  proportion  of  these  may  vary  greatly  in  different  coals,  and 
affect  the  uses  to  which  the  coal  is  applied.  For  example,  when  the 
coal  consists  wholly  of  fixed  carbon,  it  is  called  graphite.  This  is  not 
usually  considered  a  variety  of  coal,  because  it  is  not  readily  combusti- 
ble ;  but  it  is  evidently  only  the  last  term  of  the  coal  series.  Its  soft, 
greasy  feel,  and  its  uses  for  pencils,  as  a  friction -powder,  and  as  a 
material  for  crucibles,  are  well  known. 

When  the  combustible  matter  of  the  coal  contains  ninety  to  ninety- 
five  per  cent,  fixed  carbon,  it  is  called  anthracite.  This  is  a  hard,  brill- 


ORIGIN   OF   COAL  AND   ITS  VARIETIES.  343 

iant  variety,  with  conchoidal  fracture  and  high  specific  gravity.  It  burns 
with  almost  no  flame  and  produces  much  heat.  It  is  an  admirable  coal 
for  all  household  purposes,  and,  with  hot  blast,  may  be  used  in  iron- 
smelting  furnaces. 

If  the  combustible  matter  contains  eighty  to  eighty-five  per  cent, 
fixed  carbon,  and  fifteen  to  twenty  per  cent,  volatile  matter,  it  becomes 
semi-anthracite  or  semi-bituminous  coal,  of  various  grades.  These  are 
free-burning,  rapid-burning  coals,  producing  long  flame  and  high  tem- 
perature, because  they  do  not  cake  and  clog.  They  are  admirably 
adapted  for  many  purposes,  but  especially  for  the  rapid  production 
of  steam,  and  therefore  for  locomotive-engines,  and  hence  are  often 
called  steam-coals. 

If  the  volatile  combustible  matter  rises  to  the  proportion  of  thirty 
to  forty  per  cent.,  it  becomes  full  bituminous  coals,  which  always  burn 
with  a  strong,  bright  flame,  and  often  cake  and  form  clinkers.  This  is 
perhaps  the  commonest  form  of  coal,  and  may  be  regarded  as  typical 
coal. 

If  the  volatile  matter  approaches  or  exceeds  fifty  per  cent.,  then  it 
forms  highly-bituminous  or  fat  or  fusing  coals.  This  variety  is  espe- 
cially adapted  to  the  manufacture  of  gas  and  of  coke. 

Besides  these  there  are  several  varieties  depending  on  physical 
character.  Thus  cannel  or  parrot  coal  is  a  dense,  dry,  structureless, 
lustreless,  highly-bituminous  variety,  which  breaks  with  a  conchoidal 
fracture.  There  may  be  also  some  varieties  depending  upon  the  kind 
of  plants  of  which  coal  was  made,  but  of  this  we  have  no  certain  evi- 
dence. 

Origin  of  these  Varieties. — There  can  be  little  doubt  that  these, 
the  true  varieties,  are  produced  by  slight  differences  in  the  nature  and 
degree  of  chemical  change  in  the  process  of  bituminization. 

It  will  be  seen  by  the  following  table,  giving  approximate  formulae, 
that  vegetable  matter  and  coal  of  various  grades  have  the  same  general 
composition,  except  that  in  the  latter  case  some  of  the  carbon  and 
much  of  the  hydrogen  and  oxygen  have  passed  away  in  the  process  of 
change : 

Vegetable  matter,  cellulose C36H6003o 

Bituminous  coal C26H10Oj 

Anthracite    "    C40H80 

Graphite        "   C  pure 

The  excess  of  the  hydrogen  and  oxygen  lost  is  still  better  shown  in 
the  following  table,  in  which  the  constituents  are  given  in  proportion- 
ate weights  instead  of  equivalents,  and  the  carbon  reduced  to  a  con- 
stant quantity : 


344 


PALEOZOIC  SYSTEM   OF  ROCKS. 


KINDS  OF  VEGETABLE  MATTER  AND  COALS. 

Carbon. 

Hydrogen. 

Oxygen. 

Cellulose                         ...             

100.00 

16.68 

133  33 

Wood         

100.00 

12.18 

83  0*7 

Peat  

100.00 

9.83 

55  67 

Lignite 

10000 

8  37 

49  42 

10000 

6  12 

21  23 

100.00 

2  84 

1  74 

Graphite         "                   .  .           

100.00 

0  00 

000 

Cellulose Cs6H6003o 

Decayed C35H56028 

More  decayed.  .Cs^-Aie 
Final  result. . .  .CS1 


Now,  there  are  two  modes  of  decomposition  to  which  vegetable 
matter  may  be  subjected,  viz. :  1.  In  contact  with  air;  and,  2.  Out  of 
contact  with  air.  The  first  is  partly  decomposition,  and  partly  oxida- 
tion by  the  air  (eremacausis) ;  the  second  is  wholly  decomposition. 

In  Contact  With.  Air. — Under  these  conditions,  the  carbon  of  the 
vegetable  matter  unites  with  the  oxygen  of  the  vegetable  matter,  form- 
ing carbonic  acid  (CO2);  and  the  hydrogen  of  the  vegetable  matter 
unites  with  the  oxygen  of  the  air,  forming  water  (H2O).  Further,  it 
is  evident  that  for  every  equivalent  of  carbon  thus  lost  there  are  two 
equivalents  of  oxygen  and  four  equivalents  of  hydrogen  lost,  so  as 
always  to  maintain  the  same  relative  propor- 
tion of  H  and  O,  viz.,  the  proportion  forming 
water  (HaO).  The  final  result  of  this  process 
is  pure  carbon.  It  is  very  improbable,  how- 
ever, that  anthracite  or  graphite  is  formed  in 
this  way ;  for  vegetable  matter,  by  aerial  de- 
cay, falls  to  powder,  it  is  very  probable,  however — nay,  almost  cer- 
tain— that  a  peculiar  substance,  pulverulent  and  retaining  vegetable 
structure  in  a  remarkable  degree,  called  mineral  charcoal,  found  very 
commonly  in  some  stratified  coals,  has  been  formed  by  partial  aerial 
decay,  somewhat  as  represented  in  the  table.  Mineral  charcoal  has  a 
high  percentage  of  carbon,  with  very  little  hydrogen  and  oxygen. 

Out  of  Contact  with  Air. — When  vegetable  matter  is  buried  in 
mud  or  submerged  in  water,  its  elements  react  on  each  other.  Some  of 
the  carbon  unites  with  some  of  the  oxygen,  forming  carbonic  acid  (COa) ; 
some  of  the  carbon  unites  with  some  of  the  hydrogen,  forming  carbu- 
reted hydrogen,  or  marsh-gas  (CHJ  ;  and  some  of  the  hydrogen  unites 
with  some  of  the  oxygen,  forming  water  (HaO).  These  products  are 
probably  formed  in  all  cases  of  vegetable  decomposition  under  these 
conditions.  If,  for  example,  we  stir  up  the  mud  at  the  bottom  of  stag- 
nant pools  where  weeds  are  growing,  the  bubbles  which  rise  always 
consist  of  a  mixture  of  C02  and  CH4.  In  every  coal-mine  these  same 
gases  are  constantly  given  off;  the  one  being  the  deadly  choke-damp 
and  the  other  the  terrible  fire-damp  of  the  miners.  Now,  by  varying 
the  relative  amounts  of  these  products,  it  is  easy  to  see  how  all  the 


ORIGIN   OF   COAL  AND   ITS  VARIETIES.  345 

principal  varieties  of  bituminous  coal  maybe  formed.  I  have  given 
below  the  approximate  composition  of  typical  varieties  of  bituminous 
coal,  and  of  graphite,  and  constructed  formulae  expressing  the  chemical 
change  by  which  they  are  formed  : 


Vegetable  matter— cellulose C36H6o03o l 

(  9C02 ) 
Subtract  -J  3CH4  V CiaH31029 

(  11H20  ) 
And  there  remain C24H260  =  cannel. 


Again,  vegetable  matter 

(    7C02 ) 
Subtract  4    3CH4  J. C10H40028 

(  14H20 

And  there  remain . 


Again,  vegetable  matter C38H6o03o 

(  10C02  ) 
Subtract  J.  10CH4  >• CaoHsoOso 

(  10H20  ) 
And  there  remains Ci6  =  graphite. 


The  composition  of  vegetable  matter  varies  considerably.  The  com- 
position of  the  varieties  of  coal  is  differently  given  by  different  au- 
thorities. Different  reactions  from  those  above  given  might  be  con- 
trived which  would  give  as  good  results.  These  reactions,  therefore, 
are  not  given  as  certainly  the  actual  reactions  which  take  place.  They 
are  only  intended  to  show  the  general  character  of  the  changes  which 
take  place  in  the  formation  of  coal. 

Metamorphic  Coal. — It  is  probable  that  bituminous  coal  is  the  nor- 
mal coal  formed  by  the  above  process,  and  that  the  extreme  forms,  an- 
thracite and  graphite,  are  the  result  of  an  after-change  produced  by 
heat.  But  some  geologists  go  further:  they  believe  that  anthracite 
has  been  changed  by  intense  heat  sufficient  to  vaporize  the  volatile 
matters,  which  then  condense  in  fissures  above,  as  bitumen,  petroleum, 
etc. ;  that,  as  in  art,  when  bituminous  coal  is  subjected  to  heat  out  of 
contact  with  air,  the  fixed  carbon  is  left  as  coke,  the  tarry  and  liquid 
matters  are  condensed  in  purifiers,  and  the  permanent  gases  collected  in 
gasometers :  so  in  Nature,  when  beds  of  bituminous  coal  are  subjected 
to  intense  heat  in  the  interior  of  the  earth,  the  fixed  carbon  is  left  as 

1  The  composition  of  wood— timber— -is  usually  given  as  about  Ci2H1808.  I  have 
taken  the  formula  of  cellulose  instead,  viz.,  C6H1005;  or,  taking  six  equivalents  for  con- 
venience of  calculation,  C36H6o03o.  I  believe  this  to  be  much  nearer  the  composition  of 
vegetable  matter  of  the  Coal  period  than  is  the  formula  of  hard  wood  like  oak  or  beech. 
All  the  results  may  be  worked  out,  however,  with  equal  ease  in  either  formula  for  vege- 
table matter. 


34:6  PALAEOZOIC  SYSTEM  OF  ROCKS. 

anthracite,  the  tarry  and  liquid  matters  collected  in  fissures,  as  bitumen 
and  petroleum,  while  the  gases  escape  in  burning  springs.  The  process 
is  of  course  slow  and  under  heavy  pressure,  and  therefore  the  residuum 
is  not  spongy  like  coke.  According  to  this  view,  anthracite  and  bitu- 
men are  necessary  correlatives. 

There  can  be  no  doubt  that  the  graphitic  and  anthracitic  varieties 
of  coal  are  always  associated  with  folding  and  metamorphism  of  the 
strata  :  1.  In  the  universally-folded  and  metamorphic  Laurentian  rocks 
only  graphite  is  found.  2.  In  Pennsylvania,  in  the  strongly-folded  and 
highly-metamorphic  eastern  portion  of  the  field,  the  coal  is  anthracite ; 
while,  as  we  go  westward,  and  the  rocks  are  less  and  less  metamorphic, 
the  coal  is  more  and  more  bituminous,  until,  when  the  rocks  are  hori- 
zontal and  unchanged,  the  coal  is  always  highly  bituminous.  The  same 
has  been  observed  in  Wales :  anthracite  is  always  found  in  metamorphic 
regions,  and  the  coal  is  more  and  more  bituminous  as  the  rocks  are  less 
and  less  metamorphic.  3.  Again,  the  anthracitic  condition  of  coal  may 
be  sometimes  traced  to  the  local  effect  of  trap  or  volcanic  overflows. 
In  a  word,  anthracite  is  metamorphic  coal ;  and,  according  to  this  view, 
the  same  heat  which  changed  the  rocks  has  distilled  away  the  volatile 
matters,  which  may  condense  above,  as  bitumen  or  petroleum. 

We  have  given  above  the  common  view.  It  is  partly  true  and  part- 
ly erroneous.  The  true  view  seems  to  be  as  follows : 

Anthracite  may,  indeed,  be  regarded  as  metamorphic  coal,  but  it  is 
not  probable  that  bitumen  is  its  necessary  correlative ;  it  is  not  prob- 
able that  the  heat  of  metamorphism  is  sufficient  to  produce  destructive 
distillation.  We  have  already  shown  (p.  215)  that  a  moderate  heat  of 
300°  to  400°  Fahr.  in  the  presence  of  water  is  sufficient  to  produce 
metamorphism.  Such  a  degree  of  heat  would,  doubtless,  hasten  the 
process  explained  on  page  215.  The  folding  and  erosion  of  the  rocks, 
and  the  consequent  exposure  of  the  edges  of  the  seams,  would  still 
further  hasten  the  process,  and  bring  about  anthracitism  by  facilitating 
the  escape  of  the  products  of  decomposition.  In  all  coal-mines  CO2, 
CH4,  and  H,,O,  are  eliminated  now ;  only  continue  this  process  long 
enough,  and  anthracite  and,  finally,  graphite  is  the  result.  We  must 
conclude,  then,  that  high  heat  is  not  necessary  to  produce  anthracitism  ; 
for,  if  it  is  unnecessary  for  metamorphism  of  rocks,  much  less  is  it  neces- 
sary for  metamorphism  of  coal. 

Plants  of  the  Coal — their  Structure  and  Affinities. 

The  flora  of  the  coal-measures  is  the  most  abundant  and  perfect  of 
all  extinct  florae.  Of  about  2,500  to  3,000  known  fossil  species  of  plants 
nearly  700,  or  about  one-fourth,  are  from  the  coal-measures.  This  flora 
is  peculiarly  interesting  to  the  geologist,  not  only  on  account  of  its  rela- 
tive abundance,  but  also  and  chiefly  because  being  the  first  diversified 


PLANTS   OF  THE   COAL.  347 

and  somewhat  highly-organized  flora,  it  is  natural  to  suppose  that  the 
great  classes  and  orders  of  the  vegetable  kingdom  commenced  to  diverge 
here.  We  will,  therefore,  discuss  the  affinities  of  these  plants  some- 
what fully. 

Where  found. — The  plants  of  the  Coal  are  found  principally :  1.  In 
the  form  of  stools  and  roots  in  their  original  position  in  the  under-day; 
2.  Of  leaves,  and  branches,  and  flattened  trunks,  on  the  upper  surface 
of  the  coal-seam,  and  in  the  overlying  black  shale;  3.  And,  finally,  in 
the  form  of  logs,  apparently  drift-timber,  in  the  sandstones  above  the 
coal-seam.  The  black  shale  overlying  the  seam  is  often  full  of  leaves 
and  fronds  of  ferns,  and  of  the  flattened  trunks  of  other  families,  in  the 
most  beautiful  state  of  preservation,  so  that  even  the  finest  venation  of 
the  leaves  is  perfectly  distinct.  In  some  cases  where  the  shale  is  light- 
colored,  so  as  to  contrast  strongly  with  the  jet-black  leaves,  the  effect 
on  first  opening  a  seam  is  very  striking,  and  has  been  compared  to  the 
frescoes  on  the  ceilings  of  Italian  palaces. 

Principal  Orders. — Leaving  out  some  plants  of  doubtful  affinity, 
the  plants  of  the  Coal  may  be  referred  to  five  orders  or  families,  viz., 
Conifers,  Ferns,  Lepidodendrids,  Sigillarids,  and  Calamites.  It  is 
usual  to  refer  these  last  three  to  the  two  orders  Lycopods  and  Equisetae; 
but  they  are  so  peculiar,  and  their  affinities  still  so  doubtful,  that  we 
have  preferred  to  treat  them  as  distinct  orders. 

All  these,  as  already  seen,  commenced  in  the  Devonian,  as  did  also 
the  preservations  of  their  tissues  as  coal ;  but  both  the  vegetation  and 
the  conditions  necessary  for  their  preservation  culminated  in  the  Coal 
period,  and  therefore  we  have  put  off  their  discussion  until  now.  Con- 
trary to  our  usual  custom,  we  will  commence  with  the  highest,  viz. : 

1.  Conifers. — These  are  found  mostly  in  the  form  of  stumps,  and  logs, 
and  fruit,  and  leaves.  The  logs  are  found  mostly  in  the  sandstones, 
and  therefore  have  been  supposed,  and  apparently  with  good  reason,  to 
be  the  remains  of  drift-logs  of  the  highland  trees  of  the  times,  carried 
down  by  rapid  currents.  They  are  known  to  be  conifers  by  the  exoge- 
nous structure  of  the  trunk,  together  with  the  discigerous  tissue  of  the 
wood  (Fig.  373  p.  316),  and  in  some  cases  by  the  foliage  (Fig.  446)  and 
by  the  fruit.  Several  genera  have  been  described,  but  they  all  differ 
greatly  from  the  ordinary  conifers  of  temperate  climates.  Their  nearest 
living  congeners  seem  to  be  among  the  tropical  family  Araucarice  (Nor- 
folk Island  pine),  or  among  the  broad-leaved  conifers  like  the  Salis- 
buria  of  China  (Fig.  444),  and  the  curious  Welwitschia  of  South  Africa 
(Fig.  443).  This  last  anomalous  conifer,  with  a  trunk  three  or  four 
feet  in  diameter,  and  only  one  foot  high,  bears  but  two  strap-shaped 
leaves  (the  original  cotyledons)  of  great  size  (two  or  three  feet  wide 
and  six  feet  long),  which  lasts  during  its  whole  life  of  100  years 
(Maout  and  Decaisne). 


348 


PALEOZOIC   SYSTEM   OF   ROCKS. 


The  Cordaites  is  now  usually  referred  to  the  Conifers,1  though  so 
anomalous  in  its  foliage.  It  formed  a  straight  trunk,  sometimes  sixty 
to  seventy  feet  long,  clothed  atop  with  long,  strap-shaped  leaves  like 
the  Dracaena.  Fig.  447  is  a  restoration  by  Dawson. 

Nut-like  fruits,  called  Trigonocarpus,  Cardiocarpus,  Rhabdocarpus, 
etc.,  found  in  great  numbers  in  the  Coal-measures,  are  referred  to  Coni- 


FIG.  444  c. 


FIGS.  443-445.— BBO AD-LEAVED  CONIFERS.  LIVING  CONGENERS  or  SOME  COAL-PLANTS:  443.  Wehvitschia 
(the  whole  plant).  444.  Salisburia  (Ginko) :  a,  a  branch ;  6,  section  of  fruit ;  c,  a  leaf,  natural  size. 
445.  Phyllocladus,  a  branch. 

fers.     Trigonocarpus  is  very  similar  in  structure  to  the  nuts  of  the 
Salisburia,  the  Torreya  or  California  nutmeg,  and  the  yew,  or  possibly 

1  Some  place  them  among  Cycads. 


PLANTS   OF   THE   COAL. 


349 


to  those  of  Cycads.     Cardiocarpus  is  strikingly  similar  to  the  winged 
nut  of  the  Welwitschia.     It  is  believed  to  be  the  fruit  of  Cordaites. 


FiQ.,446.— Araucarites  gracilis,  reduced 
(after  Dawson). 


FIG.  44T.-Cordaites  (restored  by 
Dawson). 


The  anomalous  forms  called  Antholithes  are  supposed  by  Newberry 
to  be  the  fruit  of  allies  of  Cordaites. 

Affinities  of  Carboniferous  Conifers. — The  affinities  of  the  early 
Conifers  are  very  obscure,  but  there  is  little  doubt  that  they  were  all  re- 
markable generalized  types.  They  seem  to  be  allied,  on  the  one  hand, 
through  the  Araucariae  and  the  Lepidodendrons,  with  the  Club-mosses; 
and  on  the  other,  through  the  broad-leaved  yews,  such  as  Salisburia, 
Phyllocladus,  etc.,  with  the  Ferns.  The  leaf  of  a  Salisburia  (Fig.  444) 
is  dichotomously  veined  like  a  fern.  A  leafy  branch  of  a  Phyllocladus 
(Fig.  445)  is  strikingly  like  that  of  a  Coal-fern,  Cyclopteris  (Noegge- 
rathia).  Some  of  the  Conifers  of  this  period  differ  from  all  living  Coni- 
fers, in  having  a  large  pith  (Fig.  461),  and  a  somewhat  loose  tissue, 
which  may  be  regarded  as  an  embryonic  character. 

In  conclusion,  the  Conifers  of  the  Coal  are  undoubted  Conifers,  but 
have  a  decided  alliance  with  the  vascular  Cryptogams,  viz.,  with  Lyco- 
pods,  especially  the  gigantic  Lycopods  of  the  Coal,  and  with  Ferns. 

2.  Ferns. — Ferns  are  the  most  abundant  plants  of  the  Coal  period, 
both  as  to  individuals  and  as  to  variety  of  species.  About  one-third  to 
one-half  of  all  the  known  species  of  coal-plants,  both  in  this  country 
and  in  Europe,  belong  to  this  order.  They  represent  both  ordinary 
forms,  i.  e.,  those  with  creeping  stems,  and  Tree-ferns,  like  those  now 
growing  only  in  warm  latitudes  (Fig.  463).  They  are  known  to  be 


350 


PALEOZOIC  SYSTEM  OF  ROCKS. 


ferns  by  their  large  complex  fronds  (Fig.  464),  sometimes  six  to  eight 
feet  long;  by  the  dichotomous  venation  of  their  leaves  (Fig.  467  a); 


FIGS.  448-460. — FRUITS  OP  COAL-PLANTS,  PBOBABLT  CONIFERS:  44S-450  Triffonooarpon  (after  New- 
berry).  451-458.  Cardiocarpon  (after  Newberry  and  Dawson).  459,  460.  Ehabdocarpon  (after 
Newberry). 

and  by  the  position  of  their  organs  of  fructification  (spore-cases)  on  the 


PLANTS   OF  THE   COAL. 


351 


under  surfaces  of  the  leaves  (Figs.  468  and  469).  In  some  localities 
these  spore-cases  are  so  abundant  that  the  coal  seems  to  be  almost 
wholly  made  up  of  them.  The  trunks  of  Tree-ferns  are  known  by  the 


FIG.  461.— Trunk  of  a  Conifer:         FIG.  462.— Section  of  same:  5,  woody  wedges;  c,  pith  and  pith- 
ed,  bark  :  &,  wood ;  c,  me-  rays, 
dullary  sheath  ;  d,  pith. 

large,  ragged,  ovoid  marks  left  by  the  falling  of  the  fronds  (leaf-scars 
— Figs.  478  and  479),  and  by  the  peculiar  arrangement  of  the  vascular 
tissue  in  the  cellular  in  the  cross-section.  Some  coal  Tree-ferns  had 
their  large  fronds  in  two  vertical  ranks  (Megaphyton — Fig.  464). 


. — Living  Tree-Fern. 


F,Q.  464.— Megaphyton.  a  Coal -Fern 
restored  (after  Dawson). 


The  Ferns  of  the  Coal  are,  therefore,  unmistakably  Ferns,  yet  bota- 
nists recognize  some  features  which  connect  them  with  other  classes. 
Caruthers  thinks  that  he  finds  in  the  internal  structure  of  the  stems  of 


352 


PALEOZOIC  SYSTEM  OF  ROCKS. 


FIG.  466. 


FIGS.  465-46S.— COAL-FERNS  :  465.  Callipteris  Sullivanti  (after  Lesqnereux).  466.  Pecopteris  Strongii  (after 
Lesquereux).  467.  Alethopteris  Massilonis  (after  Lesquereux) ;  a,  same  enlarged  to  show  dichotornous 
venation.  468.  Neuropteris  flexuosa,  showing  spore-cases  (after  Brongniart). 


PLANTS   OF   THE   COAL. 


353 


o-  471. 


FIG.  472. 


FIG.  473.  FIG.  474. 


-        .  .        .  .        .  .        . 

FIGS.  469-474.—  COAL-FERNS  :  469.  Anemopteris  oblongata  (after  Bron^niart).  470.  Ortontopteris  Wor- 
theni  (after  Lesquereux).  471.  Ilymenophyllitis  alatus  (after  Lesquereux).  472.  Neuropteris 
flexuosa  (after  Lesquereux).  473.  Neuropteris  hirsuta  (.after  Lesquereux).  474.  Pecopteris  lonchitica 
(after  Lesquereux). 

23 


354 


PALAEOZOIC   SYSTEM   OF  ROCKS. 


FIG.  476. 


FIGS.  475-479.— COAL-FERNS  :  475.  Odontopteris  gracillitna  (after  Lesqnereux).  476.  Ilymenophvllitis 
splendens  (after  Lesquereux).  477.  Leaf-Scars  of  Palaxmteris,  x  J  i  after  Dawson).  478.  Leaf-Star 
of  Megaphyton,  x  |  (after  Dawson).  479.  Caulopteris  priineva,  showing  Leaf-Scars. 

Tree-ferns  of  the  Coal  two  types  which  are  the  foreshadowings  of  the 
Monocotyls  on  the  one  hand,  and  the  Dicotyls  on  the  other ;  and  that 
therefore  they  are  probably  the  progenitors,  not  only  of  the  Tree-ferns 
of  the  present  day,  but  also  of  the  Palms  and  the  foliferous  Exogens.1 
The  next  three  orders  we  will  discuss  more  fully  for  two  reasons : 
First,  the  Conifers  were  probably  mostly  highland  plants,  and  only 
found  their  way  into  the  coal-swamps  by  accident,  being  in  fact 
brought  down  by  freshets.  The  Ferns  formed  the  thick  underbrush  of 
the  coal-swamps.  Neither  of  these  contributed  a  very  large  share  to 
the  material  of  the  coal-seams.  The  great  trees  of  the  coal-swamps, 
1  Nature,  vol.  vi.,  p.  480,  and  Scott,  American  Journal,  vol.  ix.,  p.  65. 


PLANTS   OF   THE   COAL. 


355 


and  which  formed  the  larger  part  of  the  material  preserved  as  coal, 
were  Lepidodendrids,  Sigillarids,  and  Calamites. 

Again,  the  Conifers  and  Ferns  were  unmistakably  Conifers  and  Ferns, 
though  certainly  with  characters  connecting  with  other  orders  and 
classes ;  but  the  three  orders  now  about  to  be  discussed  combine  so 
completely  the  characters  of  widely-separated  classes  that  there  is  still 
some  doubt  as  to  their  real  place.  For  that  very  reason,  however,  they 
are  peculiarly  interesting  to  the  evolutionist. 

3.  Lepidodendrids. — These  are  so  called  from  the  typical  genus 
Lepidodendron.  We  will  describe  only  this  genus. 

Lepidodendrons  are  found  most  commonly  in  flattened  masses  rep- 
resenting portions  of  the  trunk  or  branches, 
very  regularly  marked  in  rhomboidal  pattern, 
and  much  resembling  the  impression  of  the 
scaly  surface  of  a  Ganoid  fish.  The  name  Le- 
pidodendron  (scale-tree)  is  derived  from  this 
fact  (Figs.  481-483).  These  marks  are  the 
scars  of  the  regularly-arracged  and  crowded 
leaves.  All  portions  of  the  plant,  however, 
viz.,  the  roots,  the  trunk,  the  branches,  the 
leaves,  and  the  fruit,  have  been  found  in  abun- 
dance. From  these  the  general  appearance  of 
the  tree  has  been  approximately  reconstructed. 
Imagine,  then,  a  tree  two  to  four  feet  in  di- 
ameter at  base,  forty  to  sixty  feet  high,  with 
wide-spreading  roots,  well  adapted  for  support 
on  a  swampy  soil ;  the  surface  of  the  trunk  and 
branches  regularly  marked  in  rhomboidal  pat- 
tern, representing  the  phyllotaxis ;  the  trunk 
dividing  and  subdividing,  but  not  profusely, 
into  branches,  which  are  thickly  clothed  with 
scale-like,  or  spine-like,  or  needle-like  leaves  (Figs.  484  and  486),  and 
terminated  by  a  club-shaped  extremity  like  the  terminal  cones  of  some 
conifers,  or  still  more  like  the  club-shaped  extremities  of  club-mosses 
(Figs.  485,  487,  488) :  and  we  will  have  a  tolerably  correct  idea  of  the 
Lepidodendron. 

The  general  appearance  of  the  tree  is  that  of  an  Araucarian  conifer, 
or  of  a  gigantic  club-moss.  The  fruit,  however,  turns  the  scale  of  affin- 
ity in  favor  of  the  club-moss ;  for  the  examination  of  these,  which  are 
found  in  great  abundance,  and  known  under  the  name  olLepidostrobus 
(scale  -  cone),  has  shown  that  they  bear  in  the  axils  of  their  scales 
spores  like  club-mosses,  and  not  seeds  like  conifers.  Also,  like  club- 
mosses,  there  are  in  these  plants  two  kinds  of  spores1 — microspores  and 
1  Williamson,  Nature,  vol.  viii.,  p.  498. 


FIG.  480  —Restoration  of  a  Lepi- 
dodendron, by  Dawson. 


356 


PALEOZOIC   SYSTEM   OF   ROCKS. 


macrospores — corresponding  to  stamens  and  pistils    (Fig.  489).     This 
would  again  all}-  them  very  closely  with  conifers.     The  external  ap- 


ill 


FIG.  486.  FIQ.4S5.  FIG.  438.  FIG.  457. 

FIGS.  481-488.— LEPIDODEJTDRIDS  :    4S1.  Lepidodendron  modulatum  (after  Lesqnereux).     482. 

dendron  diplotigioidcs  (after  Lesquereux).  483.  Lepidodendron  politum  (after  Lesquereux).  4S4. 
Lepidodendron  corrugatum,  branch  and  loaves  (after  Dawson).  4S5.  Lepidodendron  corrugatum, 
branch  and  fruit  (after  Dawson).  486.  Lepidodendron  rigens  (after  Lesquereux).  4M.  Lepido- 
phloios  Acadianus,  fruit  (after  Dawson).  483.  Lepidostrobus  (after  Lesquereux). 


PLANTS  OF  THE   COAL. 


357 


pearance  and  inflorescence,  therefore,  indicate  that  they  are  Lycopods, 
with  very  strong  coniferous  affinities. 

This  conclusion  is  entirely  borne  out  by  the  internal  structure.   Fig. 


FIG.  489.— Lepidodendron  compared  with  Club-Moss:  a,  club-moss;  ft,  a  scale  enlarged;  c.  microspores; 
d,  macrospores ;  x,  lepidostrobus ;  y  and  2,  the  scales  containing  spores ;  m,  microspores ;  n,  ma- 
crospores  (after  Balfour). 

490  represents  an  ideal  cross  and  longitudinal  section  of  the  stem  of  a 
Lepidodendron.  It  is  seen  that  the  stem  consists  of  a  dense  outer  bark 
or  rind,  inclosing  a  great  mass  of  loose 
cellular  tissue  or  inner  bark,  through 
the  centre  of  which  runs  a  compara- 
tively small  fibro-vascular  cylinder, 
with  very  distinct  pith.  Bundles  go 
from  the  cylinder  outward  to  form 
the  venation  of  the  leaves.  Now,  the 
structure  of  a  club-moss  is  almost  the 
same,  except  that  the  fibro-vascular 
cylinder  is  solid,  and  there  is,  there- 


FIG.  490.—  Ide 
pith;  b,  v 


l  Section  of  a  Lepidodendron:  a, 
scular  cylinder:  c,  inner  bark; 

'  *  ^^ 


358 


PALAEOZOIC   SYSTEM   OF   ROCKS. 


fore,  no  pith.  The  presence  in  Lepidodendron  of  a  distinct  pith  is  an 
important  character,  placing  it  far  above  modern  Lycopods,  and  allying 
it  most  decidedly  with  Exogens. 

4.    Sigillarids. — The   typical   genus   of   this   family   is  Sigillaria. 
These  plants  are  found,  like  Lepidodendrids,  mostly  as  flattened  masses, 


wm^rnijm 

^''•••'^^-•^'^^r-.^fc1:-^-1 


fjm^Km'W 

~  nli  ^•^-s--vi:.'?^-----  v 

•^ 


FIGS.  401-495.— SIOILLARIDS:  491.  Sigillaria  roticn.ata  (after  Lesquereux).  492.  Siarillaria  Grspseri. 
493.  Sigillaria  lievigata  (European).  494.  Sigillaria  obovata  (after  Lesquereux).  495.  Leaf  of  Sigil- 
laria elegans  (after  Dawson). 

which  are  portions  of  trunks,  but  also  as  roots  and  leaves.     The  trunk- 
impressions  are  distinguished  from  those  of  Lepidodendrids  by  longi- 


PLANTS   OF  THE   COAL.  359 

tudinal  ribbings  or  flutings,  ornamented  with  seal-like  impressions 
(sigilla,  a  seal),  in  vertical  rows  (Figs.  491-494).  Little  is  known  of 
their  leaves,  though  they  seem  to  have  been  similar  to  those  of  Lepido- 
dendron  (Fig.  495). 

The  best  general  conception  which  we  can  form  of  the  Sigillaria 
would  represent  it  as  a  tall,  gently-tapering  trunk,  longitudinally  fluted 
like  a  Corinthian  column,  and  ornamented  with  seal-like  impressions  in 
vertical  ranks,  representing  the  phyllotaxis ;  unbranched  or  else  dividing 
only  into  a  few  large  branches,  clothed  thickly  with  long,  stiffish,  taper- 
ing leaves.  From  the  base  of  the  trunk  extended  large,  radiating  roots, 
branching  dichotomously  and  sparsely,  with 
many  long,  thread-like  rootlets  penetrating 
the  soil  below.  The  stumps  of  Sigillaria  and 
Lepidodendrons,  with  these  large,  horizontally- 
spreading  roots  and  thread-like  appendages, 
are  very  common  in  the  underclay,  and  were 
long  supposed  to  be  a  peculiar  plant,  and 
called  Stigmaria,  on  account  of  the  round 
spots  (stigma)  on  their  surface.  They  are 
now  known  to  belong  to  Sigillarids  and  Le- 
pidodendrids,  and  are  either  roots  or  spread- 
ing rhizomes  (underground  branches). 

T      j.1.      e  11        •         c.  /At\n\     j.  f  FIG.  496. — Stigmaria  flcoides  (after 

In  the  following  ngure  (497),  taken  irom  Lesquereux). 

Dawson,  we   have   attempted  to  realize  the 

general  appearance  of  a  Sigillaria.  Their  trunks  were  sometimes  of 
prodigious  length  and  diameter.  They  were  probably  the  largest 
trees  of  the  time.  In  a  coal-seam  in  Dauphin  County,  Pennsylvania, 
flattened  stems  were  found  four  feet  and  even  five  feet  in  width.  Some 
of  these  were  exposed  for  fifty  feet,  with  but  little  apparent  diminution. 
One  was  exposed  sixty-five  feet,  and  was  estimated  to  have  extended  at 
least  thirty  feet  more.  Another  was  exposed  seventy  feet,  and  was  es- 
timated to  have  been  eighty  to  one  hundred  feet  when  growing.1 

The  Sigillarids  are  regarded  as  closely  allied  to  the  Lepidodendrids. 
Indeed,  the  two  families  shade  into  each  other  in  such  wise  that  there 
are  many  genera  the  position  of  which,  whether  in  the  one  family  or  in 
the  other,  is  doubtful.  The  typical  Sigillaria,  however,  differs  in  gen- 
eral port  from  the  typical  Lepidodendron,  chiefly  in  possessing  a  more 
Palm-like,  or  Cycas-like,  or  Dracena-like  stem.  They  are  evidently,  like 
the  Lepidodendrids,  closely  allied  to  Lycopods,  but  their  alliance  with 
higher  classes  is  even  stronger  than  that  of  Lepidodendrids. 

The  internal  structure  of  the  stem  entirely  confirms  this  conclusion. 
A  cross-section  (Fig.  498)  of  a  Sigillaria-stem  shows  a  hard  external 
rind,  d,  inclosing  a  great  mass  of  loose,  cellular  tissue  (inner  bark), 

1  Taylor,  "Statistics  of  Coal,"  pp.  149,  150;   Williamson,  Nature,  vol.  viii.,  p.  447. 


360 


PALAEOZOIC  SYSTEM   OF  ROCKS. 


c  c,  through  the  centre  of  which  runs  a  comparatively  small  woody  cylin- 
der, b  b,  and  in  the  centre  of  this  again  a  large  pith,  a  a.  From  the 
woody  cylinder  go  bundles  of  fibro-vascular  tissue,  ff,  through  the  cel- 
lular tissue  of  the  inner  bark,  to  the  leaves,  e  e.  Thus  far  the  descrip- 


FIQ.  49T.— Restoration  of  Sigillaria,  by  Dawsoc. 

tion  is  like  the  Lepidodendron,  except  that  the  woody  cylinder  is  larger 
and  thicker ;  but  closer  examination  shows,  in  addition,  the  woody  cyl- 
inder divided  into  woody  wedges  by  medullary  rays,  ff  ff,  in  true  ex- 


FIG.  498.— Ideal  Section  of  a  Sipillaria-Stem  :  a, 
woody  cylinder;  c,  inner  bark;  </,  rind;  e, 
leaves ;  /,  vascular  thread  running  to  the  le 
medullary  rays. 


pith;  ft, 
bases  of 


FIG.  499. -Cross-Section  of 
Stem  of  Cycas. 


ogenous  style,  though  the  concentric  rings  characteristic  of  Exogens 
are  wanting.  Still  closer  examination  with  the  microscope  shows  a  true 
gymnospermous  tissue  (p.  316,  Figs.  373-375);  both  on  cross  and  longi- 
tudinal section.  Now,  there  is  no  plant  living  which  combines  gym- 


PLANTS   OF  THE   COAL. 


3G1 


nospermous  tissue  with  a  general  stem-structure  at  all  similar  to  this, 
except  Cycads  (Cycas,  Zamia,  etc.).  For  sake  of  comparison,  we  have 
given  (Fig.  499)  a  cross-section  of  a  Cycas ;  the  letters  represent  the 
same  as  in  the  previous  figure.  There  can  be  no  reasonable  doubt,  there- 
fore, of  the  close  alliance  of  the  Sigillarids  with  the  Cycads*  But  their 
close  connection  with  Lepidodendrids  shows  an  equally  close,  or  closer, 
alliance  with  Lycopods.  So  thoroughly  are  they  a  connecting  type  that 
some  paleontological  botanists  (Dawson)  regard  them  as  Cycads  with 
strong  Lycopod  affinities,  while  most  regard  them  as  Lycopods  with 
strong  Cycad  affinities. 

5.  Calamites. — These  are  plants  having  long,  slender,  tapering,  reed- 
like  stems,  jointed  and  hollow,  or  else  with  large  pith.     The  exterior 


FIG.  503.  FIG.  504. 

FIGS.  500-504. -CALAMITES  AND  THEIR  ALLIES:  500.  Lower  End  of  Stem  of  Calamites  from  Nova  Scotia. 
501.  Lo-rer  End  of  Stem  of  Calamites  cannaeformis.  50'2.  Sphenophyllum  erosum  (after  Dawson). 
503.  Asterophyllites  foliosus,  England  (after  Nicholson).  504.  Annularia  inflata  (after  Lesquereux). 


362  PALEOZOIC   SYSTEM  OF  ROCKS. 

surface  of  the  stem  is  finely  striated  or  fluted,  but  the  striaa  are  not  con- 
tinuous nor  marked  with  leaf-scars  like  the  flutings  of  the  Sigillaria,  but 
are  interrupted  at  the  joints  in  the  manner  shown  in  Figs.  5UO  and  501. 
At  the  joints  are  attached  in  whorls  the  leaves,  which  are  either  scale- 
like,  or  strap-like,  or  thread-like.  Sometimes  at  the  joints  of  the  main 
stem  come  out  in  whorls  thread-like,  jointed  branches,  bearing  scale-like 
or  thread-like  leaves.  At  the  lower  end  of  the  stem,  the  joints  grow 
rapidly  smaller  and  shorter,  so  that  this  end  was  conical.  From  these 
short,  rapidly-tapering  joints  came  out  the  thread-like  roots. 

What  I  have  said  thus  far  applies  word  for  word  to  Equisetas ;  but 
the  Equisetae  of  the  present  day  are  small,  rush-like  plants,  never  much 
thicker  than  the  finger,  and  seldom  more  than  three  or  four  feet  high, 
although  in  South  America  (Caracas)  they  grow  thirty  feet  high,  but 
are  very  slender  ;  while  Calamites  were  certainly 
two  feet  or  more  in  diameter,  and  thirty  feet  high. 
Fig.  505  is  an  attempt  to  reconstruct  the  general  ap- 
pearance of  a  Calamite  by  Dawson. 

The  internal  structure  of  Calamites  still  further 
removes  them  from  Equisetas ;  for  they  seem  to  have 
had  (some  of  them,  at  least)  a  thick,  woody  cylinder 
of  exogenous  structure  and  gymnospermous  tissue. 
And  if,  as  Williamson  supposes,1  many  of  the  striated 
jointed  stems  called  Calamites  are  only  casts  of  the 
pith,  the  stems  must  have  been  even  much  larger  than 
stated  above. 

Thus,  as  Lepidodendrids  connected  Lycopods  with 
Conifers,  and  Sigillarids  connected  Lycopods  with 
Cycads,  so  these  connected  Equisetas  with  Conifers. 
General  Conclusion. — The   conclusion   which  we 
draw  from  this  examination  of  Coal  plants  is:  1.  That 
they  belong  to  the  highest  Cryptogams,  viz.,  Vascu- 
FIG.  505.— Restoration  of  lar  Cryptogams,  and  the   lowest  Phaanogams,  viz., 
acaiamite (after Daw-  Gymnosperms  ;  2.  That  they  were  intermediate  be- 
tween these  now  widely-separated  classes,  and  con- 
nected them  closely  together.     These  facts  are  strictly  in  accordance 
with  the  law  already  announced  (p.  332),  viz.,  that  the  earliest  repre- 
sentatives of  any  class  or  order  are  not  typical  representatives  of  that 
class  or  order,  but  connecting  or  comprehensive  types — that  is,  types 
which,  along  with  their  distinctive  classic  or  ordinal  character,  united 
others  which  connected  them  with  other  classes  or  orders.     Thus  the 
now  widely-separated  classes  and  orders  of  organisms,  when  traced 
backward,  in  time  approach  each  other  more  and  more,  and  probably 
unite  in  one  common  stem,  although  we  are  seldom  able  to  find  the 
1  Nature,  vol.  viii.,  p.  447. 


THEORY  OF  THE  ACCUMULATION  OF  COAL.  353 

point  of  actual  union.  Thus,  in  this  case,  the  now  widely-separated 
Cryptogams  and  Phasnogams,  when  traced  backward,  approach  until  in 
the  Coal  they  are  nearly,  if  not  completely,  united.  The  organic  king- 
dom may  be  compared  to  a  tree  whose  trunk  is  probably  to  be  found, 
if  found  at  all,  in  the  lowest  strata  ;  its  main  branches  begin  to  separate 
in  the  Palaeozoic,  the  secondary  branches  in  the  Mesozoic,  and  so  the 
branching  continues  until  the  extreme  ramification,  but  also  the  flower 
and  fruit,  are  found  in  the  fauna  and  flora  of  the  present  day.  The 
duty  of  the  evolutionist  is  to  trace  each  bough  to  its  fellow-bough,  and 
each  branch  to  its  fellow-branch,  and  thus  gradually  to  reconstruct  this 
tree  of  life,  and  determine  the  law  and  the  cause  of  its  growth. 

Theory  of  the  Accumulation  of  Coal. 

There  is  no  question  connected  with  the  Carboniferous  period  con- 
cerning which  there  has  been  more  discussion  than  the  mode  in  which 
coal  has  been  accumulated.  There  are  some  things,  however,  about 
which  there  is  little  difference  of  opinion.  These  we  will  state  first, 
and  thus  narrow  the  field  of  discussion. 

Presence  of  Water. — That  coal  has  been  accumulated  in  the  pres- 
ence of  water,  or  at  least  of  abundant  moisture,  is  evident :  a.  From  the 
preservation  of  the  organic  matter.  By  aerial  decay  vegetable  matter 
is  either  entirely  consumed,  or  else  crumbles  into  dust.  Only  in  the 
presence  of  water  is  it  preserved  and  accumulated  in  larger  quantities. 
b.  The  interstratified  sand  and  clays  and  limestones  have,  of  course, 
been  deposited  like  all  strata  in  water,  c.  The  coal  itself  is  not  un- 
frequently  distinctly  and  finely  stratified,  d.  The  plants  found  in  con- 
nection with  the  coal-seams  are  mostly  such  as  grow  in  moist  ground. 

Thus  far,  then,  theorists  agree,  but  from  this  point  opinions  diverge, 
and  until  recently  have  very  widely  diverged.  Some  have  thought 
that  coal  has  accumulated  by  the  growth  of  plants  " in  situ"  as  in 
peat-bogs  and  peat-swamps  of  the  present  day.  Others  have  sup- 
posed that  it  has  accumulated  by  driftage  of  vegetable  matter  by 
rivers,  like  the  rafts  now  found  at  the  mouths  of  great  rivers  of  the 
present  day.  According  to  the  one  view,  a  coal-seam  is  an  ancient 
peat-swamp;  according  to  the  other,  it  is  an  immense  buried  raft. 
The  one  is  called  the  "Peat-bog  theory?  the  other,  the  '•'•Estuary  or 
raft  theory." 

Recently,  however,  scientific  opinions  have  converged  toward  a 
common  belief.  We  will  not,  therefore,  discuss  these  two  rival  theo- 
ries, but  simply  bring  out  what  is  most  certain  in  the  present  views  on 
this  subject. 

1.  Coal  has  been  accumulated  by  growth  of  vegetation  in  situ,  as 
in  peat-swamps  of  the  present  day.  This  fact  is  now  demonstrable. 
The  reasons  for  believing  it  are  the  following  :  a.  The  purity  of  coal. 


364 


PALEOZOIC  SYSTEM   OF   ROCKS. 


The  coal  of  the  American  coal-fields  are,  with  few  exceptions,  abso- 
lutely pure,  i.  e.,  the  amount  of  ash  is  not  greater  than  would  result 
from  the  ash  of  the  plants  of  which  it  is  composed.  The  same  is  true  of 
coals  of  most  extensive  coal-fields  everywhere.  Now,  it  has  already  been 
shown  (p.  135)  that  in  extensive  peat-swamps,  like  the  Great  Dismal 
Swamp,  absolutely  pure  vegetable  accumulations  unmixed  with  sedi- 
ment occur ;  but  in  buried  rafts  or  drifted  vegetable  matter  of  any  kind 
there  must  be  a  large  admixture  of  mud.  b.  The  preservation  of  the 
most  complex  and  delicate  parts  of  the  plant  in  their  natural  relations 
to  each  other.  Large  fronds  are  spread  out  and  pressed  as  in  a  bota- 
nist's herbarium.  Delicate  leaves  are  preserved  with  all  their  finest 
venation  perfectly  visible.  This  is  exactly  what  we  would  expect  if 
they  lay  where  they  fell,  but  is  incompatible  with  driftage  by  rapid 
currents  to  long  distances,  c.  The  position  of  these  perfect  specimens 
only  on  the  upper  part  of  the  seam,  as  would  be  the  case  with  the  last 
fallen  leaves,  instead  of  mixed  throughout  the  seam,  as  would  be  the 
case  with  drifted  matter,  d.  The  presence  of  stumps  with  their  spread- 
ing roots  penetrating  the  underclay  exactly  as  they  grew.  This  is  not 
an  occasional  phenomenon,  but  is  found  in  the  underclay  of  nearly  every 
coal-seam.  In  South  Wales  there  are  100  seams  of  coal,  every  one  of 
which  is  underlaid  by  clay  crowded  with  roots  and  sometimes  with 
stumps.  In  Nova  Scotia  there  are  seventy-six  seams,  twenty  of  which 
have  erect  stumps  standing  in  their  original  position  with  spreading 
roots  still  penetrating  the  underclay.  The  other  seams  have  each  its 
underclay  filled  with  stigm aria-roots.  Besides  these  seams  there  are 
many  dark  bands  (dirt-beds)  indicating  old  forest -grounds. 

The  following  section  (Fig.  506)  shows  some  of  these  seams  and 
dirt-beds  or  forest-grounds,  with  penetrating  roots  and  erect  trunks. 

Fig.  507  shows  an  area  of 
about  one-quarter  acre  of 
surface  of  the  underclay  of 
an  English  coal-seam  in 
which  there  are  seventy- 
three  stumps  in  situ.  This 
last  evidence  (d)  is  demon- 
strative. Beneath  every 
coal-seam  there  is  a  fossil 
soil  —  an  ancient  forest- 
ground. 

^Recapitulation.  —  We 
may  sum  up  the  evidence, 
and  at  the  same  time  make  it  clearer,  by  describing  a  section  of  a 
peat -bog,  and  comparing  with  a  coal-seam.  In  such  a  section  we  have 
always  an  underclay,  on  which  accumulated  the  moisture,  and  on 


FIG.  506.— Erect  Fossil  Trees,  Coal-Measures,  Nova  Scotia. 


THEORY  OF  THE  ACCUMULATION  OF  COAL. 


365 


which  grew  the  original  trees  of  the  locality.  This  underclay  is  often 
full  of  roots  and  stumps  of 
the  original  growth.  Above 
this  is  a  fine,  structureless, 
carbonaceous  mass,  corre- 
sponding to  the  coal-seam. 
On  this  are  the  last-fallen 
leaves,  not  yet  disorganized, 
and  the  still-growing  vege- 
tation. Now,  imagine  this 
overwhelmed  and  buried  by 
mud  or  sand,  the  whole  sub- 
jected to  powerful  pressure, 
and  a  slow  subsequent  pro- 
cess Of  bituminization  ;  and  FIG.  507.-Ground-Plan  of  a  Fossil  Forest,  Parkfield  Colliery. 

we  have  a  complete  repro- 
duction of  the  phenomena  of  a  coal-seam  with  its  accompanying  under- 
clay filled  with  roots,  and  its  black  shale  filled  with  leaf  and  branch 
impressions. 

2.  Coal  has  been  accumulated  at  the  mouths  of  rivers,  and  therefore 
in  localities  subject  to  floods  by  the  river  and  incursions  by  the  sea.  It 
is  otherwise  impossible  to  account  for  the  clays  and  sands  (often  inclos- 
ing drift-timber),  and  limestones,  interstratified  with  the  coal.  The  phe- 
nomena of  an  individual  seam  prove  the  accumulation  by  growth  in 
situ;  the  general  phenomena  of  a  coal-basin,  with  its  succession  of 
strata,  prove  that  this  took  place  at  the  mouths  of  rivers.  Thus,  the 
field  of  discussion  is  narrowed  to  very  small  limits. 

We  conclude,  therefore,  that  coal  has  been  accumulated  in  extensive 
peat-swamps  at  the  mouths  of  great  rivers,  and  therefore  subject  to  oc- 
casional floodings  by  the  river,  and  inundations  by  the  sea.  That  pure 
peat  may  accumulate  under  these  circumstances,  is  sufficiently  proved  by 
the  fact  mentioned  by  Lyell,  that  over  large  tracts  of  ground  in  the  river- 
swamp  and  delta  of  the  Mississippi  pure  peat  is  now  forming,  in  spite 
of  the  annual  floods;  the  sediments  being  all  stopped  by  the  thick  jungle- 
growth  surrounding  these  spots,  and  deposited  on  the  margins,  while 
only  pure  water  reaches  the  interior  portions.1 

But  if  coal  has  indeed  been  formed  at  the  mouths  of  great  rivers,  we 
ought  to  find  at  least  something  analogous  to  a  coal-field  in  sections  of 
great  river-deltas.  And  so,  indeed,  we  do.  We  have  seen  (p.  130)  that 
a  great  river-delta,  like  that  of  the  Mississippi  or  the  Ganges,  consists 
of  alternate  layers  of  river-sediments  (sands  and  clays)  and  marine  sedi- 
ments (limestones),  with  thin  layers  of  peaty  matter,  and  old  forest* 

1  Lyell,  "Elements  of  Geology,"  p.  488. 


366  PALAEOZOIC  SYSTEM  OF  ROCKS. 

grounds  with  stumps  and  roots.  It  is,  in  other  -words,  a  coal-field, 
though  an  imperfect  one,  in  the  process  of  formation.  It  will  be  remem- 
bered, also,  that  we  accounted  for  this  alternation,  not  by  oscillations, 
but  by  the  operation  of  two  opposing  forces,  one  depressing  (subsi- 
dence), the  other  up-building  (river-deposit),  with  varying  success. 
When  the  up-building  by  river-deposit  prevailed,  the  area  was  reclaimed, 
and  became  covered  with  thick  jungle  vegetation ;  when  the  subsidence 
prevailed,  it  was  again  covered  with  water,  and  buried  in  river-sediments, 
etc.  Now  and  then,  when  the  subsidence  was  unusually  great,  the  sea 
invaded  the  same  area,  and  limestone  was  formed.  It  is  substantially 
in'  this  way  that  coal-fields  were  probably  formed. 

Application  of  the  Theory  to  the  American  Coal-Fields :  a.  Appalachian 
Coal-Field. — A  glance  at  the  map  (p.  278)  will  show  that,  during  Carbo- 
niferous times,  there  was  high  land  to  the  north,  east,  and  west  of  this 
field,  and  the  black  area,  representing  the  Coal-measures,  was  then  a 
trough,  into  which,  therefore,  drained  rivers  from  every  side  except  the 
south.  This  trough  was  sometimes  a  coal-swamp,  sometimes  a  lake 
emptying  southward,  sometimes  an  arm  of  the  sea  connecting  with  the 
ocean  southward.  When  it  was  a  coal-marsh,  a  coal-seam  was  formed  ; 
when  a  lake,  sands  and  clays  were  deposited  by  the  rivers ;  when  an 
arm  of  the  sea,  marine  deposits — limestones — were  formed. 

This  alternation  of  conditions  we  explain  as  follows  :  There  were 
three  forces  at  work  on  this  area :  1.  A  general  continental  upheaval, 
affecting  this  along  with  all  other  parts  of  the  continent ;  2.  An  up- 
building by  sedimentary  deposit ;  3.  A  local  subsidence.  The  evidence 
of  all  these  is  complete.  The  continental  upheaval,  as  we  have  already 
seen,  was  unceasing  throughout  the  previous  periods,  and,  as  we  shall 
see,  continued  throughout  the  subsequent  periods.  The  up-building  by 
sediments  and  the  pari  passu  subsidence  are  as  clearly  marked  as  in 
deltas  of  the  present  day,  by  shore-marks ,  by  shalloic-icater  fossils,  and 
especially  \>y  forest-grounds  repeated  through  several  thousand  feet  of 
vertical  thickness.  The  existence  of  these  three  forces,  therefore,  is 
not  a  doubtful  hypothesis.  Now,  the  first  two  would  tend  to  reclaim, 
the  third  to  submerge,  the  area.  When  the  reclaiming  forces  pre- 
dominated, the  area  became  swamp-land,  and  covered  with  coal  vegeta- 
tion, and  the  river-water,  strained  through  the  thick  growth,  slowly  went 
southward  by  a  kind  of  seepage.  When  the  submerging  forces  pre- 
dominated, the  area  became  a  lake,  and  sediments  in  great  quantities 
were  brought  down  by  the  rivers.  It  is  possible,  perhaps  probable,  that 
correlative  with  the  more  rapid  local  subsidence  which  formed  the  lake 
there  was  also  a  more  rapid  elevation  of  the  high  lands  on  all  sides,  pro- 
ducing more  torrential  river-currents  and  greater  sedimentary  deposits. 
Now  and  then,  at  long  intervals,  the  subsidence  would  bring  the  area 
below  sea-level,  and  would  thus  form  an  interior  sea,  or  mediterranean. 


ESTIMATE   OF  TIME.  367 

During  such  times,  limestones  would  be  formed,  and  marine  animals 
would  be  imbedded  as  fossils. 

b.  Western  Coal-Fields. — The  Central  and  Western  coal-fields  may  be 
regarded  as  owe,  having  been  subsequently  separated  by  denudation. 
This  immensely  extensive  field  may  have  been,  like  the  Appalachian,  a 
hollow  surrounded  on  all  sides  by  higher  land.  If  so,  the  western  land 
has  since  been  submerged,  and  covered  by  more  recent  deposits.  Or  it 
may  have  been  an  extensive  jungly  flat,  bordering  a  western  sea,  with 
many  small  rivers  with  inosculating  deltas,  flowing  westward  and  seep- 
ing through  the  thick,  marshy  vegetation.  There  were  here  far  less 
mechanical  sediments,  because  less  high  land,  and  far  more  marine  de- 
posits, because  there  was  a  larger  and  opener  sea ;  but,  in  other  re- 
spects, the  process  may  be  regarded  as  similar. 

Appalachian  Revolution. — This  state  of  oscillation  and  incertitude 
was  cut  short  bv  the  Appalachian  revolution.  At  the  end  of  the  Coal 
period,  the  sediments  which  had  been  so  long  accumulating  in  the  Appa- 
lachian region,  until  their  aggregate  thickness  had  now  reached  40,000 
feet,  at  last  yielded  to  the  horizontal  pressure  produced  by  interior  con- 
traction of  the  earth  (p.  252),  and  were  crumpled,  and  mashed,  and 
thickened  up  into  the  Appalachian  chain.  At  the  same  time  the  Western 
coal-swamps  were  upheaved  sufficiently  to  become  permanent  dry  land. 
This  revolution  closed  the  Carboniferous  age  and  the  Palseozoic  era. 

Estimate  of  Time. 

We  have  already  said  (p.  264)  that  it  is  important  that  the  mind 
should  become  familiarized  with  the  idea  of  the  immense  time  necessary 
to  explain  geological  phenomena.  We  therefore  embrace  this  oppor- 
tunity of  making  a  rough  approximative  estimate  of  the  Coal  period. 
The  estimate  may  be  made  either  by  taking  the  whole  amount  of  coal 
in  a  coal-field  as  the  thing  to  be  measured,  and  the  rate  at  which  vigor- 
ous vegetation  now  makes  organic  matter  as  the  measuring-rod  ;  or  else 
by  taking  the  whole  amount  of  sediments  in  a  coal-basin  as  the  thing 
to  be  measured,  and  the  rate  of  accumulation  of  sediments  by  large 
rivers  as  the  measuring-rod.  We  will  give  both,  though  the  latter  is 
probably  the  more  reliable. 

1.  From  Aggregate  Amount  of  Coal. — A  vigorous  vegetation— as, 
for  example,  an  average  field-crop  or  a  thick  forest — makes  about  2,000 
pounds  of  dried  organic  matter  per  annum  per  acre,  or  200,000  pounds 
or  100  tons  per  century.  But  100  tons  of  vegetable  matter  pressed  to 
the  specific  gravity  of  coal  (1.4),  and  spread  over  an  acre,  would  make  a 
layer  less  than  two-thirds  of  an  inch  in  thickness.  But,  according  to 
Bischof,  vegetable  matter  in  changing  to  coal  loses,  on  an  average,  four- 
fifths  of  its  weight  by  the  escape  of  C02,  CH4,  and  H2O  (p.  345),  only 


368  PALAEOZOIC   SYSTEM   OF   ROCKS. 

one-fifth  remaining.  Therefore,  vigorous  vegetation  at  present  could 
make  only  about  one-eighth  of  an  inch  of  coal,  specific  gravity  1.4,  per 
century.  To  make  a  layer  one  foot  thick  would  require  nearly  10,000 
years.  But  the  aggregate  thickness  in  some  coal-basins  is  100  feet  and 
even  150  feet  (p.  338).  This  would  require — the  former  near  1,000,000, 
the  latter  1,400,000  years.  It  is  probable,  however,  that  coal  vegeta- 
tion was  more  vigorous  than  the  present  vegetation.  Our  measuring- 
rod  may  be  too  short ;  we  will  try  the  other  method : 

2.  From  Amount  of  Sediment. — We  are  indebted  to  Sir  Charles 
Lyell  for  the  following  estimate  of  the  time  necessary  to  accumulate 
the  Nova  Scotia  Coal-measures.  This  coal-field  is  selected  because  the 
evidences  of  river-sediments  are  very  clear  throughout.  The  area  of 
this  coal-basin  is  given  on  page  339  as  18,000  square  miles ;  but  the 
identity  in  character  of  portions  now  widely  separated  by  seas — e.  g., 
on  Prince  Edward's  Island,  Cape  Breton,  Magdalen  Island,  etc. — plainly 
shows  that  all  these  are  parts  of  one  original  field,  which  could  not 
have  been  less  than  36,000  square  miles.  The  thickness  at  South  Jog- 
gins  is  13,000  feet.  At  Pictou,  100  miles  distant,  it  is  nearly  as  great. 
We  will  certainly  not  err  on  the  side  of  excess,  therefore,  if  we  take  the 
average  thickness  over  the  whole  area  as  7,500  feet.  This  would  give 
the  cubic  contents  of  the  original  delta  deposit  as  about  51,000  cubic 
miles.  Now,  the  Mississippi  River,  according  to  Humphrey  and  Abbot, 
carries  to  its  delta  annually  sediment  enough  to  cover  a  square  mile 
268  feet  deep,  or  nearly  exactly  one-twentieth  of  a  cubic  mile.  There- 
fore, to  accumulate  the  mass  of  sediment  mentioned  above  would  take 
the  Mississippi  about  1,000,000  years. 

It  may  be  objected  to  this  estimate  that  it  is  founded  on  a  particu- 
lar theory  of  the  accumulation  of  the  Coal-measures.  The  answer  to 
this  is  plain.  Any  other  mode  would  only  extend  the  time,  for  this 
mode  is  more  rapid  than  any  other.  Again,  it  may  be  objected  that  we 
have  evidence  of  a  very  rapid  accumulation  in  stumps  and  logs  and 
erect  trunks,  either  bituminized  or  petrified,  and  which,  therefore,  must 
have  been  completely  buried  before  they  could  decay.  The  answer  is, 
that  these  are  only  examples  of  local  rapid  deposit,  and  do  not  at  all 
affect  the  general  result.  Precisely  the  same  happens  now  in  river- 
deltas.  Again,  it  maybe  objected  that  the  agencies  of  Nature  were  far 
more  energetic  then  than  now.  This  objection  has  already  been  an- 
swered on  page  264. 

We,  therefore,  return  to  our  estimate  with  increased  confidence 
that  it  is  far  within  limits.  But  the  Coal  period,  as  already  said  (p. 
334),  is  not  more  than  one-thirtieth  of  the  recorded  history  of  the 
earth ;  beyond  which,  again,  lies  the  infinite  abyss  of  the  unrecorded. 


PHYSICAL   GEOGRAPHY  AND   CLIMATE   OF  THE   COAL   PERIOD.       369 

Physical  Geography  and  Climate  of  the  Coal  Period. 

Physical  Geography. — In  the  eastern  part  of  the  American  Conti- 
nent the  area  of  land  during  this  period  is  approximately  shown  in  the 
map  (p.  278).  It  included  the  Laurentian,  the  Silurian,  and  Devonian 
areas,  during  the  whole  age.  In  the  sub-Carboniferous  period  the  sub- 
Carboniferous  and  Carboniferous  areas  were  covered  by  the  sea,  but  in 
fhe  Carboniferous  period  proper  the  sub-Carboniferous  area  was  land, 
and  the  Carboniferous  area,  as  already  seen,  was  in  an  uncertain  state, 
sometimes  above  and  sometimes  below  the  sea-level.  It  is  probable, 
also,  that  the  Eastern  border-land  extended  then  much  beyond  the  line 
of  the  Tertiary  deposits  (see  map,  p.  278),  perhaps  beyond  the  present 
coast-line,  and  was  partly  submerged  in  the  elevation  of  the  Appa- 
lachian chain,  at  the  end  of  the  Coal  period. 

In  the  Rocky  Mountain  region  there  were  considerable  bodies  of 
land,  mainly  in  the  Basin  region,  but  their  limits  are  not  accurately 
known. 

Again,  it  is  almost  certain  that  all  the  lands  were  comparatively 
low.  None  of  the  great  mountain-chains  of  the  continent  were  yet 
formed.  It  is  also  probable  that  the  same  was  true  of  the  other  con- 
tinents. Nearly  all  the  high  mountain-chains  are  either  more  recent 
in  their  origin,  or  else  in  their  principal  growth.  In  general  terms, 
then,  the  lands  were  smaller  and  lower,  and  the  conditions  more 
oceanic,  than  at  present. 

Climate. — The  climate  of  the  Coal  period  was  undoubtedly  charac- 
terized by  greater  warmth,  humidity,  uniformity,  and  a  more  highly 
carbonated  condition  of  the  atmosphere,  than  now  obtain.  Most  of 
these  characteristics,  if  not  all,  are  indicated  by  the  nature  of  the  vege- 
tation: 

1.  The  icarmth  is  shown  by  the  existence  of  a  tropical  or  ultra- 
tropical  vegetation.  Of  the  present  flora  of  Great  Britain  about  one- 
thirty-fifth  are  Ferns,  and  none  of  these  Tree-ferns.  Of  the  Coal  flora  of 
Great  Britain  about  one-half  were  Ferns,  and  many  of  these  Tree-ferns. 
At  present  in  all  Europe  there  are  not  more  than  sixty  known  species 
of  Ferns :  in  European  Coal-measures  there  are  nearly  350  *  species,  and 
these  are  certainly  but  a  fraction  of  the  actual  number  then  existing. 
That  this  indicates  a  tropical  climate  is  shown  by  the  fact  that  out  of 
1,500  species  of  living  Ferns  known  twenty  years  ago,  1,200,  or  four- 
fifths,  were  tropical  species.  The  number  of  known  living  Ferns  is  now 
about  3,000,"  but  the  proportion  of  tropical  species  is  still  probably  the 
same.  Even  in  the  tropics,  however,  the  proportion  of  Ferns  is  far  less 
than  in  Great  Britain  during  the  Coal  period.  Again,  Tree-ferns,  arbo- 
rescent Lycopods,  Cycads,  and  Araucarian  Conifers,  are  now  wholly  con- 
1  Lesquereux.  *  Nature,  August,  1876. 

24 


370  PALAEOZOIC  SYSTEM   OF  ROCKS. 

fined  to  tropical  or  sub-tropical  regions.  The  prevalence  of  these  tropi- 
cal families  and  their  immense  size,  compared  with  their  congeners  of  the 
present  day,  would  seem  to  indicate  not  only  tropical  but  wftra-tropical 
conditions.  And  these  conditions  prevailed  not  only  in  the  United 
States  and  Europe,  but  northward  to  75°  north  latitude,  for  in  Mellville 
Island  have  been  found  coal-strata  containing  Tree-ferns,  gigantic 
Lycopods,  Calamites,  etc. 

2.  The  humidity  is  indicated  by  the  fact  that  Tree-ferns  and  arbo- 
rescent Lycopods  are  most  abundant  now  on  islands  in  the  midst  of  the 
ocean ;  and  further  by  the  great  extent  of  the  Coal  swamps,  and  per- 
haps also  by  the  general  succulence  of,  or  the  predominance  of  cellular 
tissue  in,  the  plants  of  that  period. 

3.  The  uniformity  is  proved  by  the  great  resemblance  and  often 
identity  of  the  species  in  the  most  widely-separated  regions.     Accord- 
ing to  Lesquereux,  out  of  434  American  and  440  European  species,  176 
are  common,  and  the  remainder  far  less  diverse  in  character  than  the 
species  of  the  two  florae  at  present.     Again,  in  all  latitudes,  from  the 
tropics  to  75°  north  latitude,  Coal  species  are  extremely  similar.     Such 
uniformity  of  vegetation  shows  a  remarkable  uniformity  of  climate.  From 
the  earliest  times  until  the  present  there  has  been  probably  a  gradual 
evolution  of  continents — a  gradual  differentiation  of  land  and  water, 
a  consequent  differentiation  of  climates,  and  a  corresponding  differen- 
tiation of  faunae  and  florae. 

4.  The  carbonated  condition  of  the  atmosphere  is  proved  by  the 
large  quantity  of  carbon  laid  up  in  the  form  of  coal,  the  whole  of  which 
was  withdrawn  from  the  atmosphere  in  the  form  of  carbonic  acid.    It  is 
also  indicated  by  the  nature  and  the  luxuriance  of  the  vegetation.     The 
proportion  of  carbonic  acid  in  the  atmosphere  is  now  about  -fa  per  cent. 
(20*o7).     Now,  since  carbonic  acid  is  the  necessary  food  of  plants,  it  is 
natural  to  expect  that  up  to  a  certain  limit  the  increase  of  atmospheric 
carbonic  acid  would  increase  the  luxuriance  of  vegetation.     Experi- 
ments prove  that  this  is  true  for  vascular  Cryptogams,  but  not  for 
Phaenogams. 

We  may  therefore  picture  to  ourselves  the  climate  of  this  period  as 
warm,  moist,  uniform,  stagnant  (for  currents  of  air  are  determined  by 
difference  of  temperature),  and  stifling,  from  the  abundance  of  carbonic 
acid.  Such  physical  conditions  are  extremely  favorable  to  vegetation, 
but  unfavorable  to  the  higher  forms  of  animal  life. 

Cause  of  this  Climate. — The  moisture  and  uniformity  were  the 
necessary  result  of  the  physical  geography  already  given.  They  were 
due  to  the  wide  extent  of  ocean  and  the  absence  of  large  continents  and 
high  mountains.  High  mountains  are  the  precipitating  points  for  the 
atmosphere — points  through  which  it  discharges  its  superabundant 
.moisture.  As  these  did  not  exist,  the  atmosphere  was  always  highly 


PHYSICAL   GEOGRAPHY  AND   CLIMATE   OF  THE   COAL  PERIOD.       371 

charged.  The  prevalence  of  the  ocean  also,  as  is  well  known,  produces 
uniformity. 

The  greater  warmth  of  high  latitudes  is  partly  explained  by  the 
uniformity.  But  there  is  good  reason  to  believe  that  there  was  then 
a  higher  mean  temperature  than  now  exists.  This  was  probably  due 
to  the  constitution  of  the  atmosphere.  This  may  be  shown  as  follows  : 

The  surface-temperature  of  the  earth  is  now  almost  wholly  due  to 
external,  not  to  internal,  causes.  It  has  been  calculated  that  only  one- 
twentieth  of  a  degree  Fahr.  is  now  due  to  the  latter  cause.  In  going 
downward,  the  heat  increases  about  1°  Fahr.  for  every  50  to  60  feet, 
i.  e.,  the  internal  heat  for  every  50  feet  of  depth  increases  twenty  times 
the  surface  temperature.  Now,  it  has  been  shown  by  Fourier  and 
Hopkins  that  the  same  would  be  true  whatever  be  the  surface-tempera- 
ture from  internal  causes.  For  example,  if  the  surface-temperature 
from  internal  causes  be  1°,  then  for  every  fifty  feet  of  depth  the  interior 
heat  would  increase  20°.  If  the  surface-temperature  from  internal 
causes  be  10°,  then  for  everv  50  feet  of  depth  the  interior  heat  would 
increase  200° — a  condition  of  things  entirely  inconsistent  with  the 
growth  of  plants,  since  all  the  springs  would  be  boiling.  We  cannot, 
therefore,  attribute,  as  many  have  done,  even  a  few  degrees'  increase 
of  mean  temperature  to  causes  interior  to  the  earth.  In  fact,  it  seems 
almost  certain  that  during  the  whole  recorded  history  of  the  earth,  i.  e., 
during  the  time  it  has  been  inhabited  by  organisms,  the  surface-tem- 
perature of  the  earth  has  been  almost  wholly  due  to  external  causes. 
Now,  the  composition  of  the  atmosphere  is  an  external  cause,  which 
greatly  affects  the  surface-temperature,  but  which  has  hitherto  been 
almost  wholly  neglected.  The  thorough  explanation  of  this  point  will 
require  some  discussion  of  the  properties  of  transparent  media  in  rela- 
tion to  light  and  heat. 

Many  bodies  which  are  transparent  to  light  are  opaque  to  heat. 
Such  bodies,  however,  will  freely  transmit  heat,  if  the  heat  be  accom- 
panied with  intense  light.  It  is  as  if  the  light  carried  the  heat  through 
with  it.  Heat  thus  associated  with  light  is  sometimes  called  light-heat, 
while  that  which  is  not  thus  associated  is  called  dark  heat.  Now,  the 
bodies  spoken  of  are  transparent  to  light-heat,  but  opaque  to  dark  heat. 
Glass  is  such  a  body.  If  a  pane  of  glass  be  held  between  the  face  and 
the  sun,  the  heat  passes  freely  and  burns  the  face,  but  the  same  pane 
would  act  as  a  partial  screen  before  a  fire,  and  as  a  perfect  screen  be- 
fore a  hot,  but  not  incandescent,  cannon-ball. 

It  is  in  this  way  we  explain  the  fact  that  a  glass  greenhouse,  even 
in  the  coldest  sunshiny  winter's  day,  becomes  insupportably  warm  if 
shut  up.  The  sun-light  and  heat  pass  freely  through  the  glass,  and 
heat  the  ground,  the  benches,  the  flower-pots ;  but  the  light-heat  thereby 
becomes  converted  into  dark  heat,  and  thus  is  imprisoned  within. 


372  PALAEOZOIC  SYSTEM   OF   ROCKS. 

Now,  the  earth  and  its  atmosphere  are  such  a  greeenhouse.  The  light- 
heat  passes  readily  through,  warms  the  ground,  changes  into  dark  heat, 
and  is  in  a  measure  imprisoned  by  the  partial  opacity  of  the  atmosphere 
to  this  kind  of  heat.  The  atmosphere  is  a  kind  of  blanket  put  about 
the  earth  to  keep  it  warm.  So  much  has  long  been  recognized.  But 
Tyndall  has  shown '  that  the  property  of  opacity  to  dark  heat  in  the 
case  of  the  atmosphere  is  due  wholly  to  the  small  quantity  of  carbonic 
acid  and  aqueous  vapor  present ;  that  oxygen  and  nitrogen  are  trans- 
parent to  dark  heat,  and,  therefore,  if  the  atmosphere  consisted  only  of 
these  two  gases,  it  would  not  be  heated  by  radiation  from  the  earth, 
and  the  ground  would  lose  all  its  heat  by  radiation  during  the  night, 
and  become  intensely  cold  like  space.  In  other  words,  the  blanket  put 
about  the  earth  to  keep  it  warm  is  woven  of  carbonic  acid  and  aqueous 
vapor. 

Now,  we  have  seen  that  during  the  Coal  period  the  quantity  of  car- 
bonic acid  and  aqueous  vapor  in  the  air  was  far  greater  than  now. 
The  atmosphere  was  then  a  double  blanket,  and  therefore  kept  the 
young  earth  much  warmer.  We  believe  that  Prof.  T.  S.  Hunt*  was 
the  first  to  apply  this  discovery  of  Tyndall  to  the  explanation  of  the 
climate  of  the  Coal  period.  E.  B.  Hunt  had  previously  attributed  it  to 
greater  density  of  the  air  (Dana,  "  Manual,"  p.  353) ;  but  this  is  a  wholly 
different  principle.3 

Thus  the  physical  geography  explains  the  humidity  and  uniformity, 
and  the  greater  humidity  and  the  carbonic  acid  explain  the  greater 
mean  temperature.  But  there  is  still  the  carbonic  acid  to  be  accounted 
for. 

The  more  highly-carbonated  condition  of  the  atmosphere  must  be 
attributed  to  the  original  constitution  of  the  air.  All  carbonic-acid- 
producing  causes,  such  as  animal  respiration,  combustion,  general  decay 
of  organic  matter,  volcanoes,  carbonated  springs,  etc.,  only  return  to  the 
air  what  has  been  previously  taken  from  it.  There  can  be  no  doubt  that 
all  the  carbon  in  the  world,  whether  in  the  form  of  organic  matter,  or  of 
coal,  or  of  bitumen,  or  of  carbonates,  existed  once  as  carbonic  acid  in 
the  air,  and  has  been  progressively  withdrawn.  First  immense  quanti- 
ties were  withdrawn  and  fixed  as  carbonates,  especially  as  carbonate  of 
lime  (limestone),  and  the  air  correspondingly  purified.  Again,  immense 
quantities  were  withdrawn  by  the  luxuriant  vegetation  of  the  Coal  pe- 
riod, and  fixed  as  coal.  In  this  latter  method  of  withdrawal  the  oxygen 
of  the  carbonic  acid  is  returned,  and  the  oxygenation  of  the  air  is  in- 

1  "  Proceedings  of  the  Royal  Society,"  vol.  xi.,  p.  100 ;  American  Journal,  second 
series,  vol.  xxxvi.,  p.  99. 

*  "  Chemical  and  Geological  Essays,"  p.  42. 

8  According  to  Buff,  "Archives  des  Sciences,"  vol.  Ivii.,  p.  293,  the  opacity  to  dark  heat 
of  carbonic  acid  and  aqueous  vapor  has  been  greatly  exaggerated  by  Tyndall. 


IRON-ORE   OF  THE   COAL-MEASURES.  373 

creased.  We  shall  see  hereafter  that  the  process  of  purification  did  not 
cease  with  the  Coal  period  ;  for  large  quantities  were  again  withdrawn 
and  laid  down  as  coal  and  lignite  in  the  Jurassic,  the  Cretaceous,  and 
Tertiary  periods.  There  can  be  no  doubt  that  this  progressive  purifica- 
tion of  the  air,  by  the  withdrawal  of  superabundant  carbonic  acid  and 
returning  the  pure  oxygen,  fitted  it  for  the  purposes  of  higher  and  higher 
animals. 

Iron-  Ore  of  the  Coal- Measures. 

We  have  already  stated  that  the  Coal-measures  consist  of  alternat- 
ing layers  of  sandstones,  shales,  and  limestones,  containing  seams  of 
coal  and  bands  of  iron-ore.  We  have  already  discussed  the  mode  of 
occurrence,  the  varieties,  and  the  theory  of  accumulation  of  the  coal. 
We  come  now  to  discuss  the  same  points  in  regard  to  the  iron-ore. 

Mode  of  Occurrence. — The  mode  of  occurrence  of  iron-ore  is,  in 
many  respects,  like  that  of  coal.  Like  coal,  it  is  found  in  seams,  which 
vary  in  thickness  from  a  fraction  of  an  inch  to  forty  or  fifty  feet.  Like 
coal,  these  very  thick  seams  are  apt  to  be  impure,  being  largely  mixed 
with  clay.  Seams  pure  enough  to  work  profitably  are  seldom  more 
than  three  or  four  feet  thick.  Like  coal,  the  seams  are  repeated  many 
times  in  the  same  section  (Fig.  433,  p.  335),  but  without  any  discoverable 
order  of  succession.  Like  coal,  the  seam  is  usually  underlaid  by  clay. 

Kinds  of  Ore. — The  form  of  iron-ore  found  in  all  strata,  except  those 
containing  coal,  is  usually  ferric  oxide,  either  hydrated  (brown  hema- 
tite— limonite),  or  anhydrous  (red  hematite),  or  else  magnetic  oxide ; 
but  in  the  Coal-measures  of  this  period,  and  in  the  Coal-measures  of 
every  other  period — i.  e.,  in  all  .strata  containing  coal,  the  iron  is  in 
the  form  of  ferrous  carbonate.  This  is  usually  mixed  with  clay,  and 
therefore  called  clay  iron-stone.  It  is  often  nodular  and  mammillated, 
and  called  kidney  iron-ore.  Sometimes  it  is  mixed  intimately  with  car- 
bonaceous matter,  and  is  called  black-band  ore.  This  last  very  valuable 
ore  is  found  in  Pennsylvania  and  in  Scotland. 

The  importance  of  the  association  of  coal  and  iron  in  the  same  strata 
cannot  be  over-estimated.  For  this  reason,  the  raising  of  coal  and  the 
manufacture  of  iron  are  conducted  in  connection  with  each  other,  and 
the  smelting-furnaces  are  often  situated  at  the  mouths  of  the  coal-mines. 
It  is  easy  to  understand,  therefore,  why  Great  Britain,  the  greatest 
coal-producing  country  in  the  world,  should  be  also  the  greatest  iron- 
producing  country.  Nearly  all  the  iron-ore  worked  in  Great  Britain  is 
taken  from  her  coal-mines.  In  this  country,  much  iron  is  made  from 
the  iron  carbonates  of  the  coal-mines,  but  much  also  from  the  peroxide 
ores  found  elsewhere,  especially  in  Laurentian  strata  (p.  273). 

The  following  table  gives  a  comparative  view  of  the  annual  iron- 
production,  in  tons,  of  the  principal  iron-producing  countries  of  the 
world.  It  will  be  seen  that  Great  Britain  makes  about  half  the  iron  of 


374 


PALEOZOIC  SYSTEM  OF  ROCKS. 


the  world.     The  rapid  increase  in  the  production  of  this  great  agent  of 
civilization  is  also  seen. 


IRON. 

1845. 

1856. 

1864. 

18T1. 

1S73. 

Great  Britain  

2  200,000 

3  500  000 

5  000  000 

6  566  000 

United  States  

502,000 

1,000,000 

1,200,000 

2  560  000 

France  

450,000 

1,217,000 

1  381  000 

1  664  000 

World.."  

7,000,000 

14,485,000 

Theory  of  the  Accumulation  of  the  Iron-Ore  of  the  Coal-Measures. 
— We  have  already  explained  (p.  136)  how  iron-ore  is  now  accumu- 
lated by  the  agency  of  decaying  organic  matter.  We  have  also  shown 
that  if  the  organic  matter  is  consumed  in  doing  the  work  of  accumula- 
tion, the  iron-ore  is  left  in  the  form  of  iron  peroxide  ;  but,  if  it  is  accu- 
mulated in  the  presence  of  excess  of  organic  matter,  it  retains  the  form 
of  ferrous  carbonate.  We  will  now  give  additional  evidence,  taken 
from  the  occurrence  of  iron-ore  in  the  strata  of  the  earth,  that  the  same 
agency  has  accomplished  the  same  results  in  all  geological  times. 

1.  Immense  beds  of  iron-ore  are  found  in  the  strata  of  all  geological 
ages ;   but,  wherever  we  find  them,  we  find  also  associated  a  corre- 
sponding amount  of  strata,  decolorized  or  leached  of  their  iron  coloring- 
matter.    Contrarily,  wherever  we  find  the  rocks  extensively  red,  we  find 
also  an  absence  of  valuable  beds  of  iron-ore.     We  are  thus  led  to  con- 
clude that  the  iron-ore  of  iron-beds  have  been  washed  out  of  the  strata, 
which  are  thereby  left  in  a  decolorized  condition. 

2.  That  this  has  been  done  by  the  agency  of  organic  matter  is  shown 
by  the  fact  that,  wherever  we  find  evidences  of  organic  matter,  whether 
in  the  form  of  fossils  or  of  coal,  we  find  the  sandstones  and  shales  are 
white  or  gray — i.  e.,  leached  of  coloring-matter.     Conversely,  red  rocks 
are  always  barren  of  fossils  or  of  coal.    For  example,  all  the  sandstones 
of  the  coal-measures,  or  of  all  other  strata  containing  coal,  are  gray, 
while  the  Old  Red  sandstone  below  the  coal,  and  the  New  Red  sandstone 
above  the  coal,  and,  in  fact,  all  red  sandstones,  are  very  poor  in  fossils 
or  evidences  of  organic  matter  of  any  kind.    Thus,  evidences  of  organic 
matter,  and  the  decoloring  of  the  strata,  and  the  accumulation  of  iron- 
ore,  are  closely  associated  as  cause  and  effect. 

3.  In  all  the  strata,  whether  older  or  newer,  in  which  there  is  no 
coal,  i.  e.,  in  which  there  is  no  excess  of  organic  matter  in  a  state  of 
change,  the  iron-ore  is  peroxide  (ferric  oxide)  ;  while  in  coal-measures 
of  all  periods,  whether  Carboniferous,  or  Jurassic,  or  Cretaceous,  or  Ter- 
tiary, or  in  all  cases  where  there  is  organic  matter  in  excess  in  a  state 
of  change  (not  graphite),  the  iron-ore  is  in  the  form  of  carbonate  pro- 
toxide, or  ferrous  carbonate  (FeCO3). 


IRON-ORE   OF  THE   COAL-MEASURES.  375 

Therefore,  we  conclude  that  both  now  and  always  iron-ore  is,  and 
has  been,  accumulated  by  organic  agency;  again,  that  both  now  and 
always  there  are,  and  have  been,  three  conditions  of  iron-ore,  each  as- 
sociated with  the  absence  or  presence  in  smaller  or  larger  quantities  of 
changing  organic  matter  :  1.  It  may  be  universally  diffused  as  a  color- 
ing matter  of  rocks  and  soils,  and  unavailable  for  industries ;  in  this 
case  there  has  been  no  organic  matter  to  leach  it  out  and  accumulate 
it.  2.  It  may  be  accumulated  as  ferric  oxide ;  in  this  case  there  has  been 
organic  matter  only  sufficient  to  do  the  work  of  accumulation,  and  was 
all  consumed  in  doing  that  work.  3.  It  may  be  accumulated  as  ferrous 
carbonate  ;  in  this  case  there  is  excess  of  organic  matter,  in'  the  form 
of  coal. 

This  much  is  certain ;  but,  as  to  the  exact  mode  and  time  of  the 
leaching  and  accumulation,  there  is  some  difference  of  opinion.  There 
are  two  ways  in  which  the  accumulation  may  have  occurred  :  It  may 
have  accumulated  in  the  coal-marshes  during  the  Coal  period,  being 
then  leached  out  of  the  surrounding  soils,  which  were  therefore  left  in 
a  decolorized  condition,  and  in  this  condition  subsequently  washed  down 
as  sediments  into  the  coal-marshes.  Or,  it  may  have  been  brought 
down  as  the  coloring-matter  of  red  sands  and  clays  ;  and  afterward, 
perhaps  after  the  Coal  period,  leached  out  by  percolating  waters  con- 
taining organic  matter  from  the  coal-beds,  carried  downward  until 
stopped  by  an  impervious  clav-stratum,  and  accumulated  there.  The 
former  mode  is  the  most  probable. 

But,  in  any  case,  organic  matter  has  been  the  agent ;  and,  there- 
fore, in  this  case,  as  in  all  other  cases,  iron-ore  is  the  sign  of  organic 
matter,  and  the  measure  of  the  amount  of  organic  matter  consumed  in 
its  accumulation.  There  are,  therefore,  three  signs  of  the  previous 
existence  of  organisms  used  by  geologists :  they  are  coal,  iron-ore,  and 
fossils. 

We  cannot  dismiss  this  subject  without  making  one  passing  reflec- 
tion suggested  by  the  mention  of  these  three  signs  of  life  : 

The  organic  kingdom  is  so  much  matter  taken  from  the  atmosphere, 
embodied  for  a  brief  space  in  individual  living  forms,  to  be  again 
dissolved  by  death,  and  returned  to  the  atmosphere  whence  it  came. 
The  same  material  is  again  taken  by  the  next  generation,  embodied, 
and  again  returned  at  its  death.  The  same  small  quantity  of  matter  in 
the  atmosphere  is  embodied  and  disembodied,  again  embodied  and 
disembodied,  and  thus  worked  over  and  over  again  by  constant  circula- 
tion thousands,  yea,  millions  of  times,  in  the  history  of  the  earth. 
Now,  in  this  constant  circulation  of  the  elements  of  organic  matter, 
besides  the  work  done  in  the  fact  of  circulation  itself,  viz.,  the  wonderful 
but  fleeting  phenomena  of  vegetable,  animal,  vea,  of  human  life,  there 
was  another  work,  the  results  of  which  accumulated  from  age  to  age — 


376  PALEOZOIC   SYSTEM   OF  ROCKS. 

a  work,  too,  of  the  greatest  importance  to  the  well-being  of  the  human 
race.  A  portion  of  this  circulating  matter,  in  its  course  downward 
from  the  organic  to  the  mineral  kingdom,  stopped  half-way,  and  was 
accumulated  as  great  beds  of  coal — reservoirs  of  stored  force.  As  cir- 
culating water  descending  seaward  is  stopped  and  stored  in  reservoirs 
to  complete  its  descent  under  the  control  of  man,  and  do  his  work,  so 
circulating  organic  matter  descending  is  stopped  and  stored,  and  is 
now  completing  its  descent  under  the  control  of  man,  and  doing  his 
work,  and  thus  becomes  the  great  agent  of  modern  civilization. 

A  second  portion  of  circulating  organic  elements  completes  its  de- 
scent, but  in  doing  so  accumulates  iron-ore,  the  second  great  civilizer  of 
the  human  race. 

A  third  portion  also  completes  its  descent,  but  accumulates  neither 
coal  nor  iron-ore ;  but  it  accomplishes  a  work  far  more  subtile  and  beau- 
tiful than  either  of  the  others.  As  each  particle  of  organic  matter  re- 
turns to  the  atmosphere,  it  compels  a  particle  of  mineral  matter  to  take 
its  place,  thus  completely  reproducing  its  form  and  structure.  Thus 
fossils  are  formed,  and  thus  is  the  history  of  the  organic  kingdom  self- 
recorded.  Thus,  while  the  other  two  portions  have  subserved  the  mate- 
rial wants  of  man,  this  portion  has  subserved  his  higher  intellectual 
wants. 

Bitumen  and  Petroleum. 

The  origin  of  bitumen  and  petroleum  is  so  closely  connected  with 
that  of  coal,  that  although  not  confined  to,  nor  even  found  principally 
in,  the  Coal-measures,  the  subject  is  best  taken  up  in  this  connection. 

It  is  well  known  that  coal  or  any  organic  matter,  by  suitable  distilla- 
tion, may  be  broken  up  into  a  great  variety  of  products:  some  solid,  as 
coal-pitch;  some  tarry,  as  coal-tar;  some  liquid,  as  coal-oil;  some  vola- 
tile, as  coal-naphtha  ;  and  some  gaseous,  as  coal-gas.  Now,  we  find  col- 
lected, in  fissures  beneath  the  earth,  or  issuing  from  its  surface,  a  very 
similar  series  of  products:  some  solid,  as  asphalt;  some  tarry,  as  bitu- 
men; some  liquid,  as  petroleum;  some  volatile,  as  rock-naphtha ;  and 
some  gaseous,  as  marsh-gas  and  carbonic  acid  of  burning  springs. 
There  can  be  no  doubt  that  these  also  are  of  organic  origin. 

Geological  Relations. — Bitumen  and  petroleum  are  found  in  all  fos- 
siliferous  rocks,  from  the  lowest  Silurian  to  the  uppermost  Tertiary,  under 
certain  conditions,  among  which  are  the  local  abundance  of  organisms 
from  which  these  substances  are  formed,  and  the  absence  of  great  meta- 
morphism.  The  signs  of  their  presence  in  any  locality  are  iridescent 
scums  on  the  water  of  springs  (oil-show),  and  the  issuing  of  combustible 
gases  (burning  springs).  In  regard  to  the  first  sign,  it  must  be  remem- 
bered that  iridescent  scums  are  produced  by  many  other  substances  be- 
sides petroleum.  The  second  sign  is  considered  the  best,  although  com- 
bustible gases  may  issue  from  decomposing  organic  matter  of  any  kind, 


BITUMEN   AND  PETROLEUM. 


3T7 


or  from  coal.  Some  of  the  burning  springs  in  the  oil-region  of  Ken- 
tucky are  said  to  produce  a  flame  twenty  to  thirty  feet  long.  It  is  a 
curious  fact  that  petroleum  is  often  associated  with  salt.  It  is  so  in 
Pennsylvania,  in  Virginia,  and  in  many  other  localities. 

Oil-Formations. — I  have  said  that  petroleum  and  bitumen  are  found 
in  all  fossiliferous  formations,  but  in  each  country  there  are  certain  for- 
mations where  it  especially  abounds :  in  Europe  it  is  found  principally 
in  the  Tertiary ;  in  Eastern  United  States  it  is  found  almost  wholly  in 
the  Palaeozoic,  below  the  Coal-measures ;  in  California  it  is  found  in 
the  Tertiary. 

Principal  Oil-Horizons  of  the  United  States.— In  Pennsylvania  and 
Kentucky  oil  is  found  in  the  Upper  Devonian ;  in  Canada  and  Michigan, 
in  the  Lower  Devonian ;  in  Western  Virginia  it  is  found  in  the  sub- 
Carboniferous  ;  in  Ohio,  in  Lower  Coal-measures,  though  it  probably 
originates  below  ;  in  California  it  is  found  in  Miocene  Tertiary  of  the 
Coast  Range,  all  the  way  from  Los  Angeles  to  Cape  Mendocino.  These 
have  been  called  oil-horizons. 

Laws  of  Interior  Distribution. — The  mode  of  interior  distribution 
of  petroleum  and  bitumen  is  similar  to,  yet  different  from,  that  of 
water.  Like  water,  it  occurs  in.  porous  strata  and  collected  in  fissures 
and  cavities  ;  'like  water  and  with  water,  it  issues  in  hill-side  springs  ; 
like  water  and  with  water,  it  collects  in  ordinary  wells,  or  sometimes 
spouts  in  immense  quantities  from  artesian  wells.  Some  of  the  great 
spouting-wells  of  Pennsylvania,  when  first  opened,  yielded  3,000  bar- 


rels per  day.  But,  unlike  water,  there  is  no  perennial  large  supply ; 
the  accumulations  of  ages  being  exhausted  in  a  few  months  or  a  few 
years.  Unlike  water,  the  force  of  ejection  in  great  spouting-wells  is 
not  hydrostatic  pressure,  but  the  pressure  of  elastic  gases  generated 
from  the  petroleum.  The  great  spouting-wells  are,  therefore,  the  for- 
tunate tappings  of  reservoirs  in  large  fissures  or  cavities,  which,  having 
been  accumulating  for  millenniums,  are  enormously  productive,  but 


378 


PALAEOZOIC  SYSTEM  OF  ROCKS. 


also  rapidly  exhausted.  In  the  case  of  less  productive  but  more  per- 
manent wells,  the  oil  is  contained  in  more  numerous  but  smaller  fissures 
and  pores.  In  all  cases  of  collection  in  large  fissures  and  cavities,  these 


reservoirs  are  occupied  also  by  water  and  gas  ;  and  the  three  materials 
arrange  themselves  in  the  order  of  their  relative  specific  gravities,  as  in 
Figs.  508  and  509. 

These  facts  easily  account  for  the  many  curious  phenomena  con- 
nected with  oil-wells.  Thus,  if  the  well  a  (Fig.  508)  taps  the  reser- 
voir, only  gas  will  escape,  and  oil  and  water  can  be  gotten  only  by 
pump.  But  if  the  well  be  at  b,  oil  will  spout ;  and  afterward,  when  the 
gas  has  escaped,  oil  and  water  may  be  pumped.  If  the  well  be  at  c, 
then  water  will  spout  first  and  afterward  oil.  If  the  cavity  be  irregu- 
lar, with  more  than  one  chamber  containing  compressed  gas  (Fig.  509), 
and  the  well  be  at  a,  then  gas  will  escape  first,  and  afterward  oil  and 
water  will  spout. 

Kinds  Of  Rocks  which  bear  Petroleum. — As  already  stated,  petro- 
leum, like  water,  is  found  principally  in  pores  and  fissures  and  cavities. 
The  same  kinds  of  rocks,  therefore,  which  are  water-bearing  are  also 
oil-bearing,  viz.,  limestones  and  sandstones.  In  Canada  it  is  found  in 
limestone,  in  Pennsylvania  in  sandstone.  The  intervening  shales  are 
usually  barren.  In  Pennsylvania  there  are  three  oil-bearing  sandstones, 
separated  by  about  200  feet  of  intervening  shales.  If  a  well  reaches 
the  first  sandstone  without  obtaining  oil,  the  boring  is  continued  to 


BKAOYS    BEND 


the  second,  or  even  to  the  third.  Fig.  510  (taken  from  Lesley)  rep- 
resents a  section  through  the  Pennsylvania  oil-regions,  showing  the 
three  principal  oil-horizons  of  the  United  States,  viz.,  the  Venango 


ORIGIN  OF  PETROLEUM  AND  BITUMEN,  379 

County  (Pennsylvania)  horizon  with  its  three  sandstones ;  the  Virginia 
sub-Carboniferous  horizon  above  ;  and  the  Canada  horizon  below. 

Petroleum  (especially  the  lighter  oils)  is  found  only  in  horizontal 
or  gently-folded  strata,  because  strongly-folded  and  crumpled  strata  are 
always  metamorphic,  and  the  heat  which  produced  metamorphism  has 
also  concreted  the  oil  into  bitumen  or  asphalt.  Also  the  outcropping 
of  the  edges  of  highly-inclined  strata  favors  the  escape  of  gas  and 
the  concretion  of  the  oil.  It  is  hardly  probable,  therefore,  that  a  light 
oil  will  ever  be  found  in  the  California  oil-region. 

In  gently-folded  strata  the  most  productive  portions  seem  to  be 
along  a  line  of  anticline  ;  because  there  we  may  expect  large  fissures, 
and  also,  perhaps,  because  the  oil  working  up  on  the  surface  of  water 
is  apt  to  accumulate  under  the  saddles  of  the  strata. 

Origin  of  Petroleum  and  Bitumen. 

We  have  seen  that  the  whole  petroleum  and  bitumen  series  may  be 
made  artificially  by  destructive  distillation  of  coal.  There  seems  also 
to  be  little  doubt  that  certain  organic  matters  at  ordinary  temperature, 
in  presence  of  abundant  moisture,  and  out  of  contact  of  air,  will  undergo 
a  species  of  decomposition  or  fermentation  by  which  an  oily  or  tarry 
substance,  similar  to  bitumen,  is  formed.  In  the  interior  of  heaps  of 
vegetable  substance  such  bituminous  matter  is  often  found.  Taking 
the  composition  of  petroleum  as  CnH2n,  the  reaction  by  which  it  is 
formed  from  vegetable  matter  is  expressed  in  the  following : 

Cellulose C36H60030 

Subtract  |  !g^  i C24H3603o 

CiaHS4  =  petroleum. 
Or, 

Cellulose C36H60030 

Subtract  P2CO,  ?  ...0,^,0,. 


C24H48  =  petroleum. 

There  are  therefore  two  general  theories  of  the  origin  of  petroleum : 
one,  that  it  is  produced  by  the  distillation  at  high  temperature  of  bitu- 
minous coal  by  volcanic  heat,  the  coal  being  left  as  anthracite  ;  the 
other,  that  it  is  formed  at  ordinary  temperature  by  a  peculiar  decompo- 
sition of  certain  organic  matters.  The  evidence  in  favor  of  the  first 
view  is  the  similarity  between  the  artificial  and  the  natural  series  ;  the 
objection  to  it  is  that  the  occurrence  of  petroleum  seems  to  have  no 
necessary  connection  with  the  occurrence  below  of  coal-seams,  and  also 
that  petroleum  is  found  mostly  in  strata  which  have  not  been  subjected 
to  any  considerable  heat. 


380  PALAEOZOIC  SYSTEM  OF  ROCKS. 

The  argument  for  the  other  view  is  the  fact  that  we  actually  find 
fossil  cavities  in  solid  limestone  containing  bitumen,  evidently  formed 
by  decomposition  of  the  animal  matter.  So,  also,  shales  have  been 
found  in  Scotland  filled  with  fishes,  which  have  changed  into  bitumen. 

The  most  probable  view  seems  to  be  that  both  coal  and  petroleum 
are  formed  from  organic  matter,  but  of  different  kinds  and  under  slight- 
ly different  conditions  —  that  coal  is  formed  from  terrestrial  woody 
plants,  in  the  presence  of  fresh  water,  while  bitumen  and  petroleum  are 
formed  from  more  perishable  cellular  plants  and  animals,  in  the  presence 
of  salt-water.  We  have  already  noticed  the  frequent  association  of 
petroleum  and  salt. 

Origin  Of  Varieties. — However  formed,  there  can  be  no  doubt  that 
the  different  varieties  of  this  series  are  formed  from  one  another  by  a 
subsequent  process.  It  is  certain  that  from  all  varieties  CH4  is  con- 
stantly passing  off,  and  that  the  result  of  this  is  a  slow  consolidation. 
By  this  process  light  oil  is  changed  into  heavy  oil,  heavy  oil  into  bitu- 
men, and  bitumen  into  asphalt.  Some  of  the  grandest  fissure-reservoirs 
of  oil  have  thus  been  changed  into  solid  asphalt.  In  the  upper  barren 
Coal-measures  of  West  Virginia  there  is  a  vein  of  asphalt  four  feet 
thick,  over  3,000  feet  long,  and  of  unknown  depth.  It  fills  a  great 
fissure  which  breaks  through  the  rocks  nearly  perpendicularly,  and  out- 
crops on  the  surface. 

There  are,  therefore,  two  series  of  substances  formed  from  organic 
matter,  viz.,  the  coal  series  and  the  oil  series.  In  each  series  the  pro- 
portion of  carbon  increases  by  subsequent  change  until,  perhaps,  pure 
carbon  may  be  reached.  In  the  coal  series  we  have  fat  coal,  bituminous 
coal,  semi-anthracite,  anthracite,  and,  finally,  graphite.  In  the  oil  series 
we  have  light  oil,  heavy  oil,  bitumen,  asphalt,  probably  jet,  and  possi- 
bly, finally,  diamond:  for  Liebig  has  suggested  that  diamond  is  most 
probably  formed  by  crystallization  of  carbon  from  a  liquid  hydro-carbon, 
in  which  the  proportion  of  carbon  is  constantly  increasing  by  loss  of 
CH4. 

Area  of  Oil-bearing  Strata  in  the  Eastern  United  States.— The 
amount  of  oil  in  the  United  States  is  practically  inexhaustible.  The 
finding  of  great  reservoirs,  producing  spouting-wells,  has  always  been, 
and  always  will  be,  very  uncertain  ;  but  a  moderate  return  for  industry 
and  capital  is  certain  for  an  unlimited  time.  A  large  portion  of  the 
Palaeozoic  basin,  including  an  area  of  about  200,000  square  miles,  is  un- 
derlaid by  rocks  which  are  more  or  less  oil-bearing.  The  eastern  por- 
tion of  the  United  States  is  the  great  oil-bearing,  as  it  is  the  great 
coal-bearing,  country  of  the  world. 


FAUNA  OF  THE   CARBONIFEROUS  AGE. 


381 


Fauna  of  the  Carboniferous  Age. 

As  heretofore,  we  will  disregard  the  subdivisions,  and  treat  of  the 
fauna  of  the  whole  age,  at  least  of  sub-Carboniferous  and  Carboniferous, 
together.  It  must  be  borne  in  mind,  however,  that  most  of  the  lower 
marine  animals  mentioned  are  from  the  sub-Carboniferous,  while  most  of 


FIG.  512.  FIG.  513. 

FIGS  511-513. — CARBONIFEROUS  CORALS:  511.  Lithostrotion  Californiense  (after  Meek).  512.  Clisto- 
phyllum  Gabbi  (after  Meek).  513.  a,  Archimedes  Wortheni  (after  Hall) ;  6,  Portion  of  same,  enlarged 
to  show  structure. 


382 


PALAEOZOIC  SYSTEM  OF  ROCKS. 


the  fresh-water  and  land  animals  are  from  the  Coal-measures.  We  can 
notice  only  what  important  families  are  going  out,  what  important  fami- 
lies are  coming  in,  and  a  few  which  are  very  characteristic. 


FIG.  516. 


FIG.  518 


FIG.  519. 


FIGS.  514-520.—  ECHTNODERMS  OF  THE  CABBONTFEKOTT8  AGE  —  BlosUcls:  514.  Pentremitcs  Burlingto- 
niensis  (after  Meek).  515.  Pentremites  gracilis  (after  Meek).  51  H.  Pentremites  cervinus  (after  Hall). 
517.  Pentremltes  pyriformis  (after  Hall).  Crinids:  518.  Batoerinus  Chrystii  (alter  Meek).  519. 
Scaphiocrinus  scalaris  (after  Meek.)  520.  Forbesiocrinus  Wortheni  (after  Meek). 


Among  corals  the  same  general  characteristic  Palaeozoic  t3?pe  (Qua- 
dripartita)  continues  to  prevail,  though  in  greatly-diminished  variety 
of  families  ;  for  the  Favositidas  and  Halysitidas  have  passed  away,  and 


FAUNA  OF  THE   CARBONIFEROUS  AGE. 


383 


only  the.  Cyathophy Holds,  or  cup-corals,  remain.  The  most  beautiful 
and  characteristic  are  the  Columnar  Lithostrotion  (Fig.  511),  a  polyp- 
coral,  and  the  curious  corkscrew-like  Archimedes  (Fig.  513),  a  Bryozoan. 
Among  Crinoids,  the  Cystids  no  longer  exist,  for  they  passed  out 
with  the  Silurian,  but  the  Blastids  and  Crinids  increase  in  number  and 


FIGS.  521-524  —ECHINODERMS  or  THE  CARBONIFEROUS  Aa-R  —  Crinid:  521.  Zeaorinus  elepans  Cafter 
Hall).  EOwnouu  and  Asteroids:  522.  OHgOporns  nobilis,  x  V  (after  Meek).  523.  a,  Archicocidaris 
Wortheni  (after  Hall) ;  &,  Spine  of  same,  natural  size.  524.  Onyctiaster  flexilis  (after  Meek) 

beauty.     Also,  the  free  Echinoderms  (Echinoids,  and  Asteroids)  begin 
to  be  more  abundant. 

Among  Brachiopods,  the  straight-hinged  or  square-shouldered  kinds 
continue,  but  pass  out  almost  wholly  with  this  age. 


384: 


PALAEOZOIC   SYSTEM   OF  ROCKS. 


Land  and  fresh-water  shells,  as  might  have  been  expected,  are  be- 
ginning to  be  found  in  great  abundance  in  the  Coal-measures.  The  genus 
Pupa,  a  land  air-breathing  gasteropod,  and  the  genus  Cyclas,  a  fresh- 


FIG.  526. 


FIG.  528. 

PIGS.  525-528.  —  CAKBCWTFETiors  BRAOHIOPODS  AND  HETEROPODS  :  525.  Spirifer  plenus  (after  Hall)  ; 
a,  dorsal  view ;  &,  side-view.  f>2(>.  Chonetes  Balmaniana.  f>27.  Productus  punctatus  (after  Meek). 
528.  Productus  mesialis  (after  Hall);  a,  ventral  view;  6,  side-view. 

water  bivalve,  and  the  genus  Cypris,  a  little  crustacean  bivalve,  all  of 
which  are  still  represented  by  living  species,  are  found. 


FAUXA  OF   THE   CARBONIFEROUS  AGE. 


385 


Of  course,  marine  species,  both  Lamellibranchs  and  Gasteropods,  are 
abundant.     Some  figures  of  these  are  given  below. 


FIG.  530. 


FIG.  53L 


FIG.  529.  FIG.  532. 

FIGS.  529-532.— CARBONIFEROUS  FKESH-WATEK  SHELLS  :  529.  Pupa  vetusta  (after  Dawson)— a  Land- 
Shell;  a,  natural  size;  b,  enlarged.  5iO.  Cypris  (after  Dawson);  a,  natural  size.  531.  Spirobis 
(alter  Dawson) ;  a,  natural  size.  532.  Naiadites  (after  Dawson). 

Among  Cephalopods,  Orthoceratites  still  continue,  but  in  diminished 
number,  variety,  and  size.     Goniatites,  introduced  in  the  Devonian,  also 


FIG.  534. 


FIG.  535. 

FIGS.  533-536.— CARBOXIFEROVS  LAMELLIBRANCHS  (after  Meek)  : 
risma  ventricosa.    535.  Aloriszna  pleuropistha.    £ 

35 


FIG.  536. 

!.  Solenomya  anodontoides.   534.  Alo- 
Astartella"  Newberryi. 


386 


PALAEOZOIC  SYSTEM   OF   ROCKS. 


continue,  but  both  may  be  said  to  pass  out  with  this  age,  although  a 
few  seem  to  pass  into  the  Lower  Triassic. 

Trilobites  and  Eurypterids  also  continue  ready  to  disappear  at  the 


FIGS.  53T-640.— CARBONIFEROUS  GASTEROPODS  (after  Meek) :  537.  Macrocheilus  Newberryi.    588.  Pleu- 
rotomaria  scitula.    539.  Euomphaius  subquadratus.    540.  Bellerophon  sublcevis  (after  Hall). 


FIGS.  541,  542.— CARBONIFEROUS  GONIATITES:  541.  Goniatites  Lyonl  (after  Meek)  ;  a,  side-view;  6,  end- 
view.    542.  Goniatites  crenistria  (European) ;  a,  side-view ;  6,  end-view. 


FIG.  543.— CARBONIFEROUS  CRUSTACEAN  :  Euproops  Dan®  (after  Meek  and  Worthen). 


FAUNA   OF   THE   CARBONIFEROUS   AGE. 


387 


end,  but  an  advance  in  the  Crustacean  class  is  observed  in  the  introduc- 
tion here  of  Limuloids  (king-crabs),  Fig.  543,  and  of  Macrourans — 
long-tailed  Crustaceans  (lobsters,  crawfish,  shrimps,  etc.),  Figs.  545-547. 
Insects  now  for  the  first  time  seem  to  be  in  considerable  abundance 
and  variety.  Their  appearance  in  connection  with  abundant  land-vege- 
tation seems  natural.  Nearly  all  the  principal  orders  of  insects  are  rep- 


FIG.  544. 


FIG.  546. 


FIG.  54T. 


FIGS.  544-547.— CARBONIFEBOFS  CRUSTACEANS  :  544.  Phillipsia  Lodiensis  (after  Meek  and  Worthen). 
545.  Acanthotelson  Stiinpsoni  (after  Meek  and  Worthen).  546.  Palseocarus  typus  (after  Meek 
and  Worthen).  54T.  Anthrapatemon  graciiis  (after  Meek  and  Worthen). 


resented,  viz.,  dragon-flies  (Neuropters),  Fig.  551 ;  grasshoppers,  cock- 
roaches, etc.  (Orthopters),  Figs.  549  and  550;  spiders  and  scorpions 
(Arachnids),  Fig.  548;  beetles  (Coleopters)  and  centipedes  (Myriapods), 
Figs.  552  and  553.  About  thirty  species  have  been  described  from  the 
American  Coal-measures,  of  which  eight  are  Orthopters ;  eleven,  Neu- 
ropters; four,  Arachnids;  and  seven,  Myriapods  (Scudder). 


388 


PALAEOZOIC  SYSTEM  OF  ROCKS. 


Vertebrates  (Fishes). — The  great  Ganoids  and  Placoids  continue  in 
undiminished  or  even  increased  numbers,  size,  and  variety.  They  are 
still  the  rulers  of  the  seas.  Of  Placoids,  one  has  been  found  with  dorsal 


FIGS.  54S-658.  CARBONIFEROUS  INSECTS  :  548.  Eoscorpius  earbonarius  (after  Meek  and  Worthen).  549. 
Blatta  Maderw,  Wing-case*  (after  Lesquerenx).  650.  Blattina  venusta.  Wing-cases  (after  Leg- 
qnereux).  551.  Miamia  Danse  (after  Scndder).  552.  Enphoberia  annigera  (after  Meek  and 
Worthen).  558.  Zylobius  sigillarte  (after  Dawson). 

spine  eighteen  inches  long,  another  with  spine  three  inches  broad  and 
nine  and  a  half  inches  long,  although  much  of  the  point  is  broken  off. 
Their  teeth,  too,  are  beginning  to  assume  more  of  the  character  of  true 
shark's-teeth.  They  are  no  longer  wholly  Cestracionts  (Fig.  558),  but 
also  now  Hybodonts,  having  teeth  somewhat  like  modern  sharks,  but 
rounded  on  the  edges  (Figs.  560  and  561).  Among  Ganoids,  the 
well-protected  but  sluggishly-moving  JPlacoderms  have  passed  away, 


FAUNA   OF   THE   CARBONIFEROUS   AGE.  389 

but  the  Sauroids  continue  in  increased  numbers  and  size.  Bony,  enam- 
eled scales  of  the  Megalichthys  and  Holoptychius  are  found,  two  to 
three  inches  across;  and  jaws  of  the  Holoptychius,  a  foot  or  more 


FIG.  560.  FIG.  561.  FIG.  559. 

FIGS.  554-561.— CARBOXTFEBOFS  FISHES—  Placoids :  554.  Edestus  vorax  (after  Newberry).  555.  Plenrt- 
canthus— a  Ray  (after  Nicholson).  556.  Gyvacanthus  (after  Nicholson).  557.  Ctenacanthus  (after 
Nicholson).  558.  Coehliodus  contortua.  559.  Petalodus  destructor  (after  Newberry).  560.  Clado- 
dus  spinosus  (after  Newberry).  561.  Orodus  mammllare  (after  Newberry). 


390 


PALAEOZOIC  SYSTEM  OF  ROCKS. 


long,  armed  with  Saurian  teeth,  two  inches  in  length  (Fig.  563).     Also, 
as  we  approach  the  time  for  the  appearance  of  Reptiles,  some  of  these 


FIG.  562. 


FIGS.  662,  563.— CARBONIFEROUS  FISHES— Ganoids :  562.  Amblypterus  macropterus      563  Tooth  of 
Holoptychius  Hibberti,  natural  size. 

Sauroid  fishes  seem  to  become  still  more  reptilian  in  character,  while 
others  become  more  fish-like. 

Reptiles — Amphibians. — The  first  known  appearance  of  the  class  of 
Reptiles  on  the  earth  was  in  this  age :  not  yet,  however,  in  as  great 
numbers  or  size,  or  as  high  in  the  scale  of  organization,  as  in  the  next 
age.  The  reign  of  Reptiles  had  not  yet  commenced. 

The  class  of  Reptiles  may  be  divided  into  two  sub-classes,  viz.,  True 
Reptiles  and  Amphibians.  The  Amphibians  differ  so  greatly  from 
other  Reptiles,  that  they  are  now  usually  made  a  distinct  class,  inter- 
mediate between  Fishes  and  True  Reptiles.  Of  these  two  sub-classes 
only  the  Amphibians  are  certainly  known  to  have  been  represented  in 
the  Carboniferous,  although  probably  True  Reptiles  also  existed  in  the 
last  portion  of  this  period.  Again,  Amphibians  are  subdivided  into 
four  orders,  viz.:  1.  Tailless  Batrachians  (Anoura),  such  as  frogs,  toads, 
etc. ;  2.  Tailed  Batrachians  ( Urodela),  such  as  tritons,  salamanders, 
sirens,  etc.;  3.  The  rare  snake-like  forms  (Ophiomorpha  or  Gymno- 
phiona);  and  4.  Labyrinthodonts.  Of  these,  only  the  Labyrinthodonts 
were  represented  in  the  Carboniferous.  The  other  three  orders  still 
exist,  but  the  last  has  been  long  extinct.  The  Labyrinthodonts  were 
very  large,  often  gigantic,  reptiles.  They  were  most  of  them  salaman- 
driform,  with  long  tail,  weak  limbs,  and  sluggish  movement.  Some 
were  pisciform,  and  had  paddles  instead  of  feet. 

We  can  only  briefly  describe  a  few  representatives  of  the  class,  and 
draw  some  conclusions. 

1.  Reptilian  Footprints. — In  the  sub-Carboniferous  of  Pennsylvania, 


FAUNA  OF  THE   CARBONIFEROUS  AGE. 


391 


near  Pottsville,  have  been  found  tracks  of  a  four-footed,  crawling  ani- 
mal, having  thick,  fleshy  feet  about  four  inches  long,  and  making  a 
stride  of  about  thirteen  inches.  The  impression  of  a  dragging  tail  is 
also  visible.  The  surface  of  the  slab  on  which  the  tracks  are  found  is 
marked  with  distinct  ripple-bars  and  rain-prints.  "We  thus  learn," 
says  Dana,  "  that  there  existed  in  the  region  about  Pottsville,  at  that 
time,  a  mud-flat  on  the  border  of  a  body  of  water ;  that  the  flat  was 
swept  by  wavelets,  leaving  ripple-marks ;  that  the  ripples  were  still 
fresh  when  a  large  amphibian  walked  across  the  place;  that  a  brief 


FIG.  564.— Fossil  Bain-prints  of  the  Coal  Period. 

shower  of  rain  followed,  dotting  with  its  drops  the  half-dried  mud ; 
that  the  waters  again  flowed  over  the  flat,  making  new  deposits  of  de- 
tritus, and  so  buried  the  records." 

Similar  tracks  have  also  been  found  in  the  Coal-measures  of  Penn- 
sylvania, on  a  slab  affected  with  sun-cracks  (Fig.  565).  The  reptile 
had  evidently  walked  on  the  cracked  and  half-dried  mud  at  low  tide. 
Tracks  have  also  been  found  in  the  Coal-measures  of  Illinois,  Indiana, 
Kansas,  and  Nova  Scotia,  and  in  the  latter  place  beautiful  specimens  of 
rain-prints  (Fig.  564). 

There  can  be  little  doubt  that  the  reptiles  making  the  tracks  men- 
tioned above  were  Labyrinthodonts. 

2.  Dendrerpeton. — In  the  Coal-measures  of  Nova  Scotia  have  been 
found  quite  a  number  of  small  reptiles,  belonging  to  several  genera. 
Among  these  one  is  especially  interesting,  on  account  of  the  conditions 
under  which  it  seems  to  have  been  preserved.  It  is  called  the  Den- 
drerpeton— tree-reptile — (Fig.  566),  because  it  was  found  by  Dawson 
and  Lyell  in  sandstone,  filling  the  hollow  stump  of  a  Sigillaria  (Fig.  567), 
along  with  another  small  species  of  reptile,  a  number  of  land-shells 
— pupa,  etc. — (Fig.  529,  p.  385),  and  a  myriapod  (Fig.  553,  p.  388). 


392  PALJSOZOIC  SYSTEM   OF  ROCKS. 

The  Sigillaria  possessed  a  thick,  strong  bark,  which  was  more  resistant 
of  decomposition  than  the  cellular  interior.     Stumps  of  these  trees  are 


FIG.  565.— Slab  of  Sandstone  with  Reptilian  Footprints,  from  Coal-measures  of  Pennsylvania;   x  J. 

often  found,  consisting  only  of  coaly  bark  filled  with  sandstone,  evi- 
dently deposited  within  the  hollow.     These  sands  are  rich  repositories 


PIG.  566.— Jaw  of  Dendrerpeton  Acadeanum,  and  Section  of 
Tooth,  enlarged  (after  Dawson). 


FIG.  5«7.— Section  of  Hollow  Sigil- 
laria Stump,  filled  with  Sand- 
stone (after  Dawson). 


of  organic  remains.     We  can  easily  imagine  the  circumstances  under 
which  the  Dendrerpeton  was  preserved.     A  dead  Sigillaria  tree,  rotted 


FAUNA  OF  THE   CARBONIFEROUS  AGE.  393 

to  the  base  and  only  its  hollow  stump  remaining,  stood  on  the  margin 
of  a  coal-swamp;  river-floods  filled  the  stump  with  sand;  in  the  stump 
lived  and  perished  a  Dendrerpeton ;  or  else  the  dead  body  of  the  reptile, 
together  with  shells  and  other  organic  remains,  was  floated  into  the 
hollow  stump  and  buried  there.  This  reptile  was  probably  a  Labyrintho- 
dout,  but  with  strong  alliances  with  true  reptiles,  especially  Lacertians. 
3.  ArchegOSailTUS  (Primordial  Saurian). — In  the  Bavarian  Coal- 
measures  has  been  found  the  almost  perfect  skeleton  of  a  reptile,  about 
three  and  a  half  feet  long,  which  combines  in  a  remarkable  degree  the 
characters  of  Amphibians  with  those  of  Ganoid  Fishes.  It  seems  to  have 
been  a  Labyrinthodont  Amphibian,  with  general  form  and  structure 
adapted  for  a  purely  aquatic  life.  It  had,  certainly  in  the  early  stages 
of  its  life,  probably  throughout  life,  both  gills  and  lungs,  and  therefore, 
like  all  the  Amphibians  of  the  present  day  at  this  stage,  or  like  Perenni- 
branchiate  Amphibians  throughout  life,  breathed  both  air  and  water. 
The  locomotive  organs  were  paddles,  adapted  for  swimming,  not  for 
walking.  The  body  was  covered  with  imbricated  ganoid  scales  (Fig. 


FIG.  568. — Archegosaurus. 

568,  A),  and  the  head  with  ganoid  plates.  The  structure  of  the  teeth  (B) 
was  also  ganoid.  The  bodies  of  the  vertebras  were  not  ossified  nor  even 
cartilaginous,  but  retained  the  early,  embryonic,  fibrous  condition  of  a 
notochord.  It  was  apparently  a  connecting  link  between  the  lowest  Pe- 
rennibranchiate  Amphibians  and  the  Sauroid  Fishes  (Owen),  with,  per- 
haps, some  alliances  with  the  marine  Saurians  which  afterward  appeared. 
It  was  so  distinct  from  other  Labyrinthodonts  that  Prof.  Owen  puts  it  in 
a  distinct  order,  which  he  calls  Ganocephala.  The  skeleton  of  this  ani- 
mal is  given  above  (Fig.  568),  with  the  limbs  (C  and  D)  and  jaw  (E) 
of  a  Proteus — a  perennibranchiate  amphibian — for  comparison. 

4.  Eosaurus. — In  the  Coal-measures  of  Nova  Scotia,  in  1861,  Prof. 
Marsh  found  the  vertebrae  of  what  he  thinks,  with  much  reason,  was  a 
marine  Saurian;  an  order  which  is  largely  developed  in  the  Mesozoic. 
But  as  only  the  bodies  of  a  few  vertebrae  have  been  found,  and  as  the 
bi-concavity  of  these  is  the  chief  evidence  of  marine  Saurian  affinity, 
and  as  bi-concavity  also  exists  among  Labyrinthodonts,  Huxley  believes 
this  was  also  a  Labyrinthodont.  There  is,  therefore,  still  some  doubt  as 


394 


PALAEOZOIC   SYSTEM   OF  ROCKS. 


to  the  true  affinity  of  this  animal,  but  the  weight  of  evidence  seems  in 
favor  of  a  marine  Saurian.  The  size  of  some  of  the  vertebrae  was  two 
and  a  half  inches,  indicating  a  reptile  of  gigantic  dimensions. 

Many  other  genera  have  been  described  by  authors  both  in  Europe 


FIG.  569.— Two  Vertebrae  of  Eosaurns  Acadiensis  (after  Dawson). 

and  America.  Among  these,  Baphetes,  Raniceps,  Hylerpeton,  Hylono- 
inus,  and  Amphibamus  from  America,  and  Anthracosaurus,  Ophiderpe- 
ton,  and  Apateon  from  Europe,  are  best  known.  The  Baphetes  and  the 
Anthracosaurus  attained  gigantic  size. 


FIG.  570. — Ptyonlus  (after  Coj 


Very  recently  a  large  number  (thirty-four  species  referable  to  seven- 
teen genera)  of  small  Amphibians  have  been  brought  to  light  by  the 
Ohio  Survey,  and  described  by  Cope.  These  are  all,  or  nearly  all,  Laby- 
rinthodonts  (Stegocephali,  Cope).  Some  of  them  have  the  usual  broad 


FAUNA   OF  THE   CARBONIFEROUS  AGE. 


395 


heads  of  Amphibians,  but  a  large  number  are  remarkable  for  their  long, 
limbless,  snake-like  forms  and  pointed  heads.  These  are  evidently  among 
the  lowest  form  of  Amphibians,  and  have  strong  affinities  also  with 
Ganoid  fishes.  Figs.  570  and  571  represent  two  of  the  Ohio  Amphibians. 

Some  General  Observations  on  the  Earliest  Reptiles. — With  the  pos- 
sible exception  of  the  Eosaurus,  all  the  reptiles  of  the  Carboniferous 
were  Labyrinthodonts.  They  are  so  called  on  account  of  the  extraordi- 
nary labyrinthine  structure  of  their  teeth,  produced  by  the  intricate 
infolding  of  the  surface  and  of  the  cavity.  The  same  structure  is  ob- 
served in  ganoid  teeth,  but  in  a  far  less  degree.  The  simple  infoldings 
of  Ganoids  (Fig.  432,  p.  331)  become  intricate  in  Labyrinthodonts  (Fig. 
572). 

The  Labyrinthodonts  were  probably  the  most  complete  example  of  a 
connecting  type  which  has  yet  been  discovered.  First,  they  were  true 


FIG.  571. — Tuditanus  radiatus,  x  j-  (after  Cope). 


-Section  of  Tooth  of  a  Labvrinthodont. 


Amphibians  in  the  strictest  sense,  having  all  of  them  in  the  early 
stages  of  their  life— some  throughout  life — both  lungs  and  gills,  and 
thus  connecting  water-breathers  with  air-breathers.  Again,  they  were 
very  different  from  the  slimy-skinned  Amphibians  of  the  present  day,  in 
being  covered,  at  least  partly,  with  bony  scales  over  the  body,  and  with 
closely-fitting  bony  plates  over  the  head.  Again,  they  differed  wholly 
from  the  present  Amphibians  in  having  jaws  thoroughly  armed  with  very 
large  and  powerful  teeth,  the  structure  of  which  is  labyrinthine.  All 
of  these  characters  connected  them  with  Sauroid  fishes  which  preceded 
them,  and  the  great  Saurian  reptiles  which  succeeded  them.  Finally, 
they  seemed  to  possess  also  characters  connecting  them  with  several 
orders  of  subsequently-existing  reptiles.  In  the  Labyrinthodonts  and 
Sauroid  fishes  we  can  almost  find  the  point  of  separation  of  the  two 
great  branches,  Reptile  and  Fish,  of  the  vertebrate  stem ;  and  in  the 


396  PALAEOZOIC  SYSTEM   OF   ROCKS. 

former  the  commencing  differentiation  of  the  several  orders  of  Rep- 
tiles. 

Some  General  Observations  on  the  Whole  Palaeozoic. 

We  have  defined  geology  as  the  history  of  the  evolution  of  the  earth. 
Evolution,  therefore,  is  the  central  idea  of  geology.  It  is  this  idea 
alone  which  makes  geology  a  distinct  science.  This  is  the  cohesive 
principle  which  unites  and  gives  significance  to  all  the  scattered  facts 
of  geology ;  which  cements  what  would  otherwise  be  a  mere  inco- 
herent pile  of  rubbish  into  a  solid  and  symmetrical  edifice.  It  seems 
appropriate,  therefore,  that  at  the  end  of  the  long  and  eventful  Paleo- 
zoic era  we  should  glance  backward  and  briefly  recapitulate  the  evi- 
dences of  progressive  change  (evolution),  physical,  chemical,  and  vital. 

Physical  Changes. — The  Palaeozoic  era  opened  on  this  continent 
with  a  V-shaped  mass  of  land — the  Laurentian  area — to  the  north; 
also,  a  land-mass  of  Laurentian  rocks,  of  unknown  shape  and  extent,  on 
the  eastern  border,  and  probably  some  islands  and  masses  of  larger  ex- 
tent in  the  Rocky  Mountain  region.  This  condition  of  things  is  repre- 
sented on  the  map  on  page  279.  Throughout  the  Palaeozoic  era  there 
was  a  steady  accretion  of  land  to  this  nucleus  by  upheaval  of  contiguous 
sea-bottoms ;  a  steady  development  of  the  continent  southward  (and 
perhaps  northward)  from  the  northern  area,  and  both  eastward  and 
westward  from  the  eastern  border  area,  until  at  the  end  of  the  Palaeo- 
zoic the  eastern  half  of  the  continent  included  certainly  all  the  Lau- 
rentian, Silurian,  Devonian,  and  Carboniferous  areas  shown  on  the  map 
on  page  278,  and  probably  also  some  on  the  eastern  and  western  border 
of  this  area,  which  was  subsequently  covered  by  the  sea,  and  is  there- 
fore now  concealed  by  more  recent  deposits.  The  loss  of  Palaeozoic 
land  on  the  eastern  border  probably  took  place  during  the  Appalachian 
revolution.  In  the  Rocky  Mountain  region  the  development  was  prob- 
ably less  steady.  Unconformity  of  Carboniferous  on  Silurian  strata 
shows  extensive  land-areas  there  during  Devonian  times.  Thus  it  is 
seen  that  the  continent  was  already  sketched  in  the  beginning  of  the 
Palaeozoic,  and  the  process  of  development  went  on  during  that  era,  so 
that  at  the  end  the  outlines  of  the  continent  were  already  unmistakable. 
We  shall  trace  the  further  development  hereafter. 

Chemical  Changes. — Progressive  changes  in  chemical  conditions  are 
no  less  evident.  At  first,  i.  e.,  before  the  Archaean  era — before  the  ex- 
istence of  life  on  the  earth — the  atmosphere,  as  shown  by  Hunt  ("  Essays," 
p.  40,  et  seq.),  was  loaded  with  carbonic  acid,  representing  all  the  car- 
bon and  carbonates  in  the  world;  with  sulphuric  acid  representing  all 
the  sulphur  and  sulphates  ;  with  hydrochloric  acid  representing  all  the 
chlorides ;  and  with  aqueous  vapor  representing  all  the  water  in  the 
world.  Of  course,  such  a  condition  rendered  life  impossible.  From 
this  primeval  atmosphere,  by  cooling,  the  strong  acids  were  first  pro- 


GENERAL   OBSERVATIONS   ON   THE   WHOLE   PALAEOZOIC.          397 

cipitated  with  the  water;  and  afterward  more  slowly  the  carbonic 
acid,  by  the  action  of  this  acid  upon  the  primeval  silicates,  with  the 
formation  of  carbonates,  especially  limestone.  All  limestones,  there- 
fore, represent  so  much  carbonic  acid  withdrawn  from  the  air.  This 
withdrawal  proceeded  through  the  whole  Archsean,  Silurian,  and 
Devonian.  During  the  Carboniferous,  the  purification  of  the  air 
was  accelerated  by  the  growth  of  vegetable  matter  and  its  preserva- 
tion as  coal,  as  already  explained,  page  344.  In  this  method  of  with- 
drawal the  oxygen  of  the  carbonic  acid  is  returned,  and  the  air  becomes 
more  oxygenated. 

Progressive  Change  in  Organisms.  —  Corresponding  with  these 
changes,  physical  and  chemical,  it  is  natural  to  expect  changes  in  spe- 
cies, genera,  families,  etc.,  of  organisms  :  and  such  we  find.  The  law  of 
continuance  or  geological  range  of  species,  genera,  families,  orders,  is 
very  similar  to  that  of  extent  or  geographical  range  of  the  same  groups 
(p.  157) ;  i.  e.,  the  laws  of  distribution  in  time  are  similar  to  those  of 
distribution  in  space.  The  period  of  continuance  (range  in  time)  of 
species  is,  of  course,  less  than  that  of  genera,  and  that  of  genera  less 
than  that  of  families,  etc.  According  to  Prof.  Hall,  there  have  been  in 
the  Silurian  and  Devonian  ages  alone  at  least  thirty  complete  changes 
of  species.  The  changes  of  genera  are,  of  course,  much  less  numerous, 
and  those  of  families  still  less  than  those  of  genera.  These  general 
laws  may  be  illustrated  by  any  Palaeozoic  order ;  but  I  select  the  order 
of  Trilobites,  because  they  are  very  numerous,  very  diversified,  and 
well  studied,  and  because  they  came  in  with  the  Palaeozoic,  continued 
throughout  the  whole  era,  and  then  passed  away  forever. 

The  following  diagram  illustrates  these  laws  in  the  order  of  Trilo- 
bites. It  is  seen  that  this  order  continues  through  the  whole  era,  com- 
mencing in  small  numbers,  reaching  its  highest  development  in  the 
middle  Silurian,  and  declining  to  the  end.  But  the  families  are  changed 
several  times.  Six  groups  are  given,  to  show  how  they  come  and  go 
successively.  If  we  should  attempt  the  distribution  of  genera,  the 
changes  would  be  much  more  numerous,  and  of  species  still  more  so. 
In  the  lower  portion  of  the  diagram  we  have  attempted  to  show  in  a 
very  general  way  how  the  distribution  of  species  of  Calymene  and 
Acidaspis  might  be  represented. 

General  Comparison  of  the  Fauna  of  Palaeozoic  with  that  of  Neozoic 

Times. — The  greatest  change  which  has  ever  occurred  in  the  history  of 
the  organic  kingdom  took  place  at  the  end  of  the  Palaeozoic  era.  As 
human  history  is  primarily  divided  into  Ancient  and  Modern,  so  the 
whole  history  of  the  earth  may  be  properly  divided  into  Palaeozoic  and 
Neozoic  times.  We  wish  to  contrast  broadly  the  faunas  of  these  two 
great  divisions  of  time.  In  the  diagram  (p.  399),  the  vertical  line 
represents  the  dividing  line  between  the  old  and  the  new  time-world. 


PAIuEOZOIC   SYSTEM   OF  ROCKS. 


In  this  country  it  is  appropriately  called  the  Appalachian  revolution. 
On  the  left  is  the  Palaeozoic,  on  the  right  the  Neozoic.  When  families 
or  orders  of  animals  are  placed  on  one  or  the  other  side  without  mark, 
it  means  that  they  are  the  only  kind  of  the  contrasted  families  found 
on  that  side,  or  nearly  so.  If  the  orders  or  families  so  placed  are 


1.  Paradoxides 


2.  Bathyurus,  Agnos- 
tus,  etc 


8.  Asaphus,      Eemo- ) 
pleurides,      Tri-  y  -- 
nucleus,  etc ) 

4.  Calymene,  Acidas-  ) 

pis,  etc j    — 


5.  Homalonotus,     Li- 
chas,  etc 


6.  Phillipsia,  Griffithides 


7.  Distribution  of  ape-) 
cies  of  Calymene,  y 
etc \ 


FIG.  578.— Diagram  illustrating  Distribution  of  Families,  etc.,  in  Time. 


marked  with  the  sign  +,  it  means  that  they  are  the  predominant  kinds. 
For  example,  among  Cephalopods,  the  Tetrabranchs,  or  shelled  fam- 
ily, are  the  only  kinds  found  in  the  Palaeozoic  ;  in  the  Neozoic,  both 
families  exist,  but  the  Dibranchs  or  naked  ones  vastly  predominate. 

General  Picture  of  Palaeozoic  Times. 

Perhaps  it  is  not  inappropriate  to  group  some  of  the  more  impor- 
tant facts  in  a  very  brief  outline-picture  of  Palaeozoic  times.  We  must 
imagine,  fhen^wide  seas  and  low  continents  of  small  extent;  a  hot,  moist, 
still  air,  loaded  with  carbonic  acid,  stifling  and  unsuited  for  the  life  of 
warm-blooded  animals.  If  an  observer  had  walked  along  those  early 
beaches  he  would  have  found  cast  up,  in  great  numbers,  the  shells  of 
Brachiopods ;  clinging  to  the  rocks  and  hiding  away  among  their  hollows, 
instead  of  sea-urchins  arid  star-fishes  and  crabs,  he  would  have  found 


GENERAL   PICTURE   OF  PALAEOZOIC  TIMES. 


399 


Palaeozoic  times '. Neozoic  times. 

RADIATA. 

| 
Corals. 

Quadripartita |  .  .Sexpartita. 

Echinoderms. 

+  +  Stemmed,  or  Crinoids |  .  .Free,  or  Echinoids  and  Asteriods  +  +. 

Crinoids. 
+  Armless,  or  simple  arms I  .  .Plumose  arms. 

I 

MOLLUSKS. 

Bivalves. 

+  Brachiopods |  . .  Lamellibranchs  +  + . 

Brachiopods. 

+  Square-shouldered |  .  .Sloping-shouldered. 

Lamellihranchs. 

+  Unsiphonated |  . .  Siphonated  + . 

Gasteropoda. 

Marine |  . .  Land,  fresh-water,  and  marine. 

Marine. 

Unbeaked— Herbivorous |  . .  Beaked — Carnivorous  + . 

Cephalopods. 

Shelled,  or  Tetrabranchs I  . .  Naked,  or  Dibranchs  +  4- . 

Shelled. 

+  Straight I  . .  Coiled. 

Orthoceratites. 

Goniatites. 

I         Ceratites. 

Ammonites. 

N  a  u  t  i  1  u  s. 

| 

I 
ARTICULATA. 

Crustacea. 

Entomostraca Malacostraca  + . 

Trilobites. 

Limu  oids. 

Macrourans. 

Brachyourans. 

VERTEBRATA. 

Fishes. 

Heterocercals I  . .  Homocercals  + . 

Ganoids  and  Placoids. ...  | Teleosts  +. 

Placoids. 
Cestracionts. 

Hybodonts. 

Squalodonts. 
Reptiles. 
Amphibians |  . .  True  Reptiles. 


crinoids  and  trilobites.     In  the  open  sea  he  would  have  found  as  rulers, 
instead  of  whales  and  sharks  and  teleosts  and  cuttlefish,  huge  cuirassed 


400      '  PALAEOZOIC  SYSTEM  OF  ROCKS. 

Sauroids  and  the  straight-chambered  Orthoceras.  Turning  to  the  land, 
he  would  have  seen  at  first  only  desolation ;  for  there  were  no  land- 
plants  until  the  Devonian,  and  almost  no  land-animals  until  the  Coal. 
During  the  Coal  there  were  extensive  marshes,  overgrown  with  great 
trees  of  Sigillaria,  Lepidodendron,  and  Calamites,  with  dense  under- 
brush of  Ferns,  inhabited  by  insects  and  amphibians ;  no  umbrageous 
trees,  no  fragrant  flowers  or  luscious  fruits,  no  birds,  no  mammals. 
These  "  dim,  watery  woodlands "  are  flowerless,  fruitless,  songless, 
voiceless,  except  the  occasional  chirp  of  the  grasshopper.  If  the  ob- 
server were  a  naturalist,  he  would  notice,  also,  the  complete  absence  of 
modern  types  of  plants  and  animals — it  would  be  like  another  world. 

This  long  dynasty  was  overthrown,  this  reign  of  Fishes  ended,  the 
physical  conditions  described  above  changed,  and  the  whole  fauna  and 
flora  destroyed  or  transmuted,  by  the  Appalachian  revolution.  At  the 
end  of  the  Palceozoic,  the  sediments  which  had  been  so  long  accumulating 
in  the  Appalachian  region  at  last  yielded  to  the  slowly-increasing  horizon- 
tal pressure,  and  were  mashed  and  folded  and  thickened  up  into  the  Ap- 
palachian chain,  and  the  rocks  metamorphosed.  In  America,  this  chain 
is  the  monument  of  the  greatest  revolution  which  has  taken  place  in 
the  earth's  history.  Similar  and  very  extensive  changes  in  physical 
geography  must  have  taken  place  in  other  portions  of  the  globe,  other- 
wise we  cannot  account  for  the  enormous  changes  in  physical  conditions 
and  fauna  and  flora.  Many  of  these  have  been  traced,  but  we  cannot 
yet  trace  them  as  clearly  as  in  America. 

Transition  from  the  Palaeozoic  to  the  Mesozoic — Permian  Period. 

The  Permian  a  Transition  Period. — The  Palaeozoic  era  was  closed  and 
the  Mesozoic  inaugurated  by  the  Appalachian  revolution.  All  the  great 
revolutions  in  the  earth's  history  are  periods  of  oscillations.  Such  oscil- 
lations produce  unconformity.  They  also  produce  changes  of  climate, 
and  therefore  of  fauna  and  flora.  We  find,  therefore,  that  the  Mesozoic 
rocks  are  universally,  so  far  as  known,  unconformable  on  the  Carbonifer- 
ous ;  and,  corresponding  with  this  unconformity,  there  is  a  wonderful 
change  in  fauna  and  flora — a  change  the  greatness  of  which  we  have  at- 
tempted to  show  in  the  contrast  on  the  preceding  page.  Now,  the  older 
geologists  regarded  this  change  as  one  of  instantaneous  destruction  and 
recreation,  because  they  took  no  account  of  a  lost  interval.  But  we 
have  already  shown  (pp.  179,  280)  that  in  all  cases  of  unconformity 
there  is  such  a  lost  interval,  which  in  some  cases  is  very  large.  In 
order  to  account  for  the  very  great  change  in  the  organic  world,  it  is 
only  necessary  to  suppose  that  periods  represented  by  unconformity  are 
critical  periods  in  the  earth's  history  —  periods  of  rapid  change  in 
physical  geography,  climate,  and  therefore  of  rapid  change  in  fauna 
and  flora,  by  the  passing  out  of  old  types  and  the  differentiation  of  new 


PERMIAN  PERIOD.  401 

types.  Unfortunately,  in  the  earth's  history,  as  in  human  history,  it  is 
exactly  these  critical  periods — these  periods  of  change  and  revolution — 
the  record  of  which  is  apt  to  be  lost.  In  both  histories,  too,  this  is 
truer  the  farther  back  we  go.  Of  the  long  interval  between  the  Archae- 
an and  Palaeozoic,  not  a  leaf  of  record  has  been  yet  recovered  ;  but  of 
the  interval  now  under  discussion  many  leaves  of  record  have  been 
recovered.  These  have  been  bound  together  in  a  separate  volume  or 
chapter  and  called  the  Permian.  I  shall  regard  the  Permian,  therefore, 
as  essentially  a  transition  period ;  its  rocks  were  deposited  during  the 
period  of  commotion ;  its  fossil  types  are  in  a  state  of  change,  though 
more  nearly  allied  to  the  Palaeozoic. 

From  what  has  just  been  said,  it  will  be  anticipated  that  the  uncon- 
formity of  the  Mesozoic  on  the  Palaeozoic  sometimes  takes  place  be- 
tween the  lowest  Mesozoic  and  the  Permian,  and  sometimes  between 
the  Permian  and  the  Coal.  The  Permian,  therefore,  is  sometimes  con- 
formable with  the  Coal,  as,  e.  g.,  in  this  country  ;  sometimes  conform- 
able with  the  Triassic,  as  in  England.  It  thus  allies  itself  stratigraphi- 
cally  sometimes  with  the  Palaeozoic,  sometimes  with  the  Mesozoic. 
Paleontologically  it  is  always  more  allied  to  the  Palaeozoic.  The 
English  section,  and  the  history  of  opinion  concerning  it,  admirably  il- 
lustrate this  point.  Fig.  574  is  an  ideal  section  through  the  Devonian, 


the  Coal  and  Triassic  (Lower  Mesozoic)  of  England.  Lying  uncon- 
formably  on  the  eroded  surface  of  the  Coal,  J,  there  is  seen  a  continu- 
ous and  perfectly  conformable  series  of  strata,  a.  This  series,  more- 
over, is  lithologically  characterized  throughout,  especially  the  lower 
part,  by  frequent  alternations  of  Red  sandstones,  and  therefore  has  been 
called  New  Red  sandstone,  to  distinguish  it  from  the  Devonian,  which 
is  often  called  Old  Red  sandstone.  It  is  further  distinguished  through- 
out, especially  the  upper  part,  by  variegated  shales,  and  therefore 
called  altogether  Poikilitic  group.  It  is  also  distinguished  through- 
out by  the  presence  of  salt,  and  therefore  called  the  Saliferous  group. 
Here,  then,  there  were  the  strongest  reasons  for  regarding  the  whole 
as  one  group,  distinctly  separated  by  unconformity  from  the  underlying 
Coal.  The  line  of  unconformity  was,  therefore,  naturally  believed  to 
be  the  line  between  Palaeozoic  and  Mesozoic.  Unfortunately,  the  lower 
portion  is  very  barren  of  fossils,  and  this  means  of  correcting  the 
stratigraphic  conclusion  was  at  first  nearly  wanting.  When  fossils 
26 


402  PALEOZOIC   SYSTEM   OF  ROCKS. 

were  discovered  in  sufficient  numbers,  however,  they  showed  a  greater 
alliance  with  the  unconformable  Coal  below  than  with  the  conformable 
strata  above.  Thus,  if  we  make  the  division  between  Palaeozoic  and 
Mesozoic  on  stratigraphical  grounds,  we  would  find  it  between  the 
Coal  and  the  overlying  strata;  while,  if  we  make  it  on  paleontological 
grounds,  we  would  have  to  draw  the  line  through  the  midst  of  the 
conformable  strata,  a,  giving  one  half  to  the  Palaeozoic  and  the  other 
half  to  the  Mesozoic.  It  is  the  lower  Palaeozoic  half  which  is  called  the 
Permian. 

As  a  broad  general  fact,  therefore,  the  great  commotion  which  is 
called  the  Appalachian  revolution  took  place,  or  commenced  to  take 
place,  at  the  end  of  the  Coal  period.  But  the  fauna  and  flora  were  not 
immediately  exterminated,  but  struggled  on,  maintaining,  as  it  were,  a 
painful  existence  under  changed  conditions,  themselves  meanwhile 
changing,  until  complete  and  permanent  harmony  was  reestablished 
with  the  opening  of  the  Mesozoic.  If  we  may  use  an  illustration,  the 
Appalachian  revolution  was  the  death-sentence  of  Palaeozoic  types,  but 
the  sentence  was  not  instantly  executed.  This  transition  period,  this 
period  between  the  sentence  and  the  execution  of  Palaeozoic  types,  is 
the  Permian. 

It  is  well  here  to  draw  attention  to  the  fact  of  this  great  change  of 
organisms,  the  greatest  in  the  whole  history  of  the  earth,  taking  place 


FIGS.  575-579.-PERMIAN  SHELLS  (after  Meek) :  575.  Eumlcrotis  Hawnii.    576.  Myalina  Permiana. 
577.  Bakewellia  parva.    578.  Pleurophorus  subcuneatus.    679.  A  Gasteropod. 

in  the  midst  of  conformable  strata  (Fig.  574,  a).  Evidently  the  change 
must  have  been  comparatively  rapid. 

We  have  given  the  history  of  change  of  opinion  in  regard  to  the 
English  section  (Fig.  574),  because  it  is  a  tpye  of  many  discussions  and 
changes  which  have  occurred  and  will  still  occur  in  geological  opinion. 

The  Permian  has  been  found  in  the  United  States,  in  Kansas,  bor- 
dering on,  and  conformable  with,  the  coal  of  that  region  (map,  p.  278), 
and  perhaps  in  other  parts  of  the  Plains ;  but  nothing  of  importance 
or  interest  has  been  found  in  it  except  a  few  shells  (Figs.  575-579). 


PERMIAN  PERIOD. 


403 


Fio.  580.— Walchia  piniformis  (Permian  of  Europe). 


Fra.  682. 


Fio.  581.  Fio.  583.  Fio.  584. 

FIGS.  581-584.— PERMIAN  BRACHIOPODS:  581.  Product*  horrida.    582.  Lingula  Credneri.    583.  Terebra- 
tula  elongata.    594.  a,  6,  Camarophoria  globulina  (after  Nicholson). 


FIG.  585.— Restoration  of  Palseoniscus. 


Fio.  536.— Platysomus  gibbosus  (Permian  of  Europe). 


404  .MESOZOIC  ERA— AGE   OF  REPTILES. 

In  Europe  the  flora  consists  principally  of  Ferns,  Calamites,  and 
Lepidodendrids,  closely  allied  to  those  of  the  Coal,  and  several  species 
of  Walchia  (Fig.  580),  Voltzia,  Ulmannia,  genera  of  Conifers.  In  fact, 
Conifers  are  more  abundant  and  varied  than  in  the  Coal. 

In  the  fauna,  Trilobites  and  Goniatites  are  gone,  but  a  few  Ortho- 
ceratites  and  a  few  square-shouldered  Brachiopods,  such  as  Productus 
(Fig.  581)  and  Spirifer,  are  still  found,  as  also  are  several  genera  of 
Ganoids  observed  in  the  Coal  (Fig.  585),  and  some  characteristic  of  this 
period  (Fig.  586). 

Along  with  Labyrinthodonts,  already  found  in  the  Coal,  are  also 
found  now  some  Thecodont  (socket-toothed)  reptiles,  allied  to  Crocodil- 
ians,  which  show  a  decided  advance  on  the  Coal  reptiles.  Unless  we 
except  the  Eosaurus,  these  are  the  first  true  reptiles  found.  They  are 
probably  the  progenitors  of  the  crocodiles,  though  they  have  also  affini- 
ties with  the  Dinosaurs  (Huxley). 


CHAPTER  IV. 

MESOZOIC  ERA— AGE   OF  REPTILES. 

THE  Palaeozoic  era,  we  have  seen,  was  very  long,  and  very  diversi- 
fied in  dominant  types,  of  both  animals  and  plants.  It  was  during  this 
long  era  that  originated  nearly  all  the  great  branches,  and  even  sub- 
branches,  of  the  organic  kingdom.  "We  have  during  this  era,  therefore, 
three  very  distinct  ages:  an  age  of  Invertebrates,  an  age  of  Fishes,  and 
an  age  of  Acrogens  and  Amphibians.  The  Mesozoic  was  far  less  long 
and  far  less  diversified  in  dominant  types.  It  consists  of  only  one  age, 
viz.,  the  age  of  Reptiles.  Never  in  the  history  of  the  earth,  before  or 
since,  did  this  class  reach  so  high  a  point  in  numbers,  variety  of  form, 
size,  or  elevation  in  the  scale  of  organization. 

General  Characteristics. — The  general  characteristics  of  the  Meso- 
zoic era  are  the  culmination  of  the  class  of  Reptiles  among  animals,  and 
of  Cycads  among  plants,  and  the  first  appearance  of  Teleosts  (common 
osseous  fishes),  JHrcls,  Mammals  among  animals,  and  of  Palms  and 
Dicotyls  among  trees. 

Subdivisions. — The  Mesozoic  era  is  divided  into  three  periods,  viz. : 
1.  Triassic,  because  of  its  threefold  development  where  first  studied  in 
Germany;  2.  Jurassic,  because  of  the  splendid  development  of  its 
strata  in  the  Jura  Mountains ;  3.  Cretaceous,  because  the  chalk  of  Eng- 
land and  France  belongs  to  this  period. 


TRIASSIC  PERIOD.  405 

f  3.  Cretaceous  period. 
Mesozoic  Era.  •<  2.  Jurassic  period. 

[_  1.  Triassic  period. 

In  this  country  the  Triassic  and  'Jurassic  are  not  so  distinctly  sepa- 
rable as  they  are  in  Europe,  nor  as  they  are  from  the  Cretaceous.  They 
form,  in  fact,  one  series,  and  if  the  Mesozoic  had  been  studied  first  in 
this  country,  the  whole  would  probably  have  been  divided  into  only  two 
periods.  We  shall  therefore  speak  of  the  Mesozoic  of  this  country  as 
consisting  of  two  periods,  viz.,  the  Jura- Trias  and  the  Cretaceous.  On 
account  of  their  fuller  development  in  Europe,  it  will  be  best  to  speak, 
first,  of  the  Triassic  generally ',  then  of  the  Jurassic  generally,  taking  our 
illustrations  mainly  from  European  sources,  and  then  of  the  Jura-Trias 
in  America.  Also,  on  account  of  the  comparative  poverty  of  the  Trias 
in  remains,  we  will  dwell  much  less  on  this  period  than  on  the  subse- 
quent Jurassic ;  for  in  this  latter  period  culminated  all  the  distinctive 
characters  of  the  Reptilian  age. 

SECTION  1. — TRIASSIC  PERIOD. 

As  already  stated,  the  Triassic  strata  are  always  unconformable  with 
the  Coal,  and  the  period  opens  with  a  fauna  and  flora  wholly  and  strik- 
ingly different  from  the  preceding.  In  some  places,  however,  there  is 
found  an  intermediate  series,  the  Permian,  sometimes  conformable  with 
the  Coal  and  unconformable  with  the  Trias,  sometimes  conformable 
with  the  Trias  and  unconformable  with  the  Coal.  Its  fauna  and  flora 
are  also  to  some  extent  intermediate,  though  more  nearly  allied  to 
those  of  the  Coal.  The  explanation  of  this  has  already  been  given. 

Subdivisions. — The  subdivisions  of  the  Triassic  rocks  and  period  in 
several  countries  are  given  below: 


GERMAN. 

FRENCH. 

ENGLISH. 

3.  Keuper. 

Marne  irise'e. 

Variegated  marl. 

2.  Muschelkalk. 

Muschelkalk. 

Wanting. 

1.  Bunter  Sandstein. 

Gres  bigan-6. 

Upper  New  Red  sandstone. 

The  flora,  of  the  Trias  is  very  imperfectly  known.  We  find,  how- 
ever, no  longer  the  great  coal-making  trees  of  the  Carboniferous — Sigil- 
larids,  Lepidodendrids,  and  Calamites — though  Tree-ferns  still  continue 
in  abundance.  The  forest-trees  seem  to  have  been  principally  Tree- 
ferns,  Cycads,  and  Conifers,  although  the  last  two  did  not  reach  their 
highest  development  until  the  next  period.  For  this  reason  we  will  put 
off  the  fuller  discussion  of  them  until  we  come  to  that  period. 


406 


MESOZOIC  ERA— AGE   OF  REPTILES. 


FiGfl.  587-689.— TBIASSIC  CONIFERS  AND  CYCADS  :  587.  Voltzia  heterophylla.    588.  PterophyUum  Ja>geri. 
539.  Podozamites  lanceolatns  (after  Nicholson). 


Animals. — Among  Echinoderms  we  find  no  longer 
any  Cystids  and  Blastids ;  but  Crinids^  beautiful  lily 
EncriniteS)  with  long  plumose  arms,  are  very  abundant 
(Fig.  590).  Among  Brachiopods  the  familiar  square- 
shouldered  forms,  including  the  Spirifer  family,  the 
Strophomena,  family,  and  the  Productus  family,  are 
almost  if  not  wholly  gone;  only  a  few  Spirifers  re- 
main. Among  Cephalopods,  we  find  no  longer  Ortho- 
ceratites  or  Goniatites,  but  Ceratites  (Fig.  598)  take 
their  place,  and  Ammonites  begin.  In  Ceratites,  the 
suture  is  more  complex  than  in  Goniatites,  but  not 
so  complex  as  the  subsequent  Ammonite.  Among 


FIG.  590.-Encrinus  liliformis. 


FIG.  591.— Aspidura  loricate. 


TRIASSIC  PERIOD. 


407 


Crustaceans,  we  6nd  no  longer  TriloMtes  nor  huge  Eurypterids,  but 
Macrourans,  which  began  in  the  Carboniferous,  are  now  more  abundant, 
and  of  more  modern  forms  (Fig.  599). 

Fishes. — Among  fishes,  still  we  find  no  Teleosts,  only  Ganoids  and 


FIGS.  592-597.— LAMELLIBRANCHS  :  592.  Daonella  Lommellii.    593.  Pecten  Valoniensis.    594.  Myophpria 
lineata.    595.  Cardium  Rhseticum.    596.  Avicula  contorta.    597.  Avicula  socialis  (.after  Nicholson). 

Placoids ;  but  while  the  Ganoids  are  some  of  them  heteroceral  or  ver- 
tebrated-tailed  like  the  Palaeozoic  Ganoids,  some  are  only  slightly  ver- 


FIG.  598.— Ceratites  nodosns. 


FIG.  599.— Pemphyx  Sueurii. 


tebrated,  and  some  wholly  non-vertebrated-tailed,  or  homoceral.  The 
Ceratodus,  a  remarkable  genus  of  fishes,  one  species  of  which  still  lives 
in  Australian  rivers  (Pig.  424,  p.  329),  is  traced  back  to  this  period.  Being 


408 


MESOZOIC  ERA— AGE   OF  REPTILES. 


FIG.  601. 


FIG.  602. 


FIGS.  600-602.—  TRIASSTC  FISHES  :  600.  a,  Dental  Plate  of  Ceratodus  serratus  ;  6,  Dental  Plate  of  Cer&- 
todus  altus,  Keuper  (after  Agassiz).    601.  Acrodus  minimus.    602.  Hybodus  apicalis  (after  Agassiz). 


FIG.  604. 

FIGS.  603.  604.— TRIASSIC  REPTILES—  Labyrinthodontx :  603.  Mastodonsaurus  Jcegeri.     604.  Trema- 
tosaurus  (after  Huxley). 


TRIASSIC   PERIOD. 


409 


known  in  a  fossil  state  only  by  the  curious  palatal  teeth  (Fig.  600),  it 
has  heretofore  been  classed  with  Placoids.  The  Placoids  are  partly 
Cestracionts  (Fig.  601),  and  partly  Hybodonts  (Fig.  602). 

Reptiles. — This  class  was  represented  by  Labyrinthodonts,  Enalio- 
saurs  (marine  Saurians),Rhynchosaurs  (beaked  Saurians),  and  Lacertians 
(lizards). 

Marine  Saurians  readied  their  culmination  in  the  next  period,  and 
we  will  therefore  put  off  discussion  of  them  until  then.  Labyrintho- 
donts have  already  been  described  in  connection  with  the  Carbonifer- 
ous, where  they  first  occur.  They  cul- 
minated^ however,  in  the  Triassic,  and 
then  became  extinct.  They  reached  in 
the  Triassic  gigantic  proportions.  The 
head  of  the  Labyrinthodon  (Mastodon- 
saurus)  Jaegeri  (Fig.  603)  was  more 
than  three  feet  long  and  two  feet  wide, 
and  one  of  the  teeth  was  three  and  one- 
half  inches  beyond  the  jaw,  and  one 
and  a  half  inch  in  diameter  at  base 
(Owen,  Figs.  605,  606).  Tracks  made 
by  Labyrinthodonts  have  been  found 
in  England  and  in  Germany,  in  rocks 


FIGS.  605,  606.— TBIASSIO  REPTILES— Labyrinthodonts  :  605.  Tooth  of  Labyrinthodon,  natural  size. 
606.  Section  of  same  enlarged,  showing  structure. 

of  this  period.  The  unknown  animal  was  at  first  called  Cheirotherium 
(hand-beast),  because  of  the  resemblance  of  the  track  to  the  impression. 
of  a  very  fat  human  hand  (Fig.  607).  Both  the  tracks  and  the  skeleton, 
show  that  the  hind  limbs  were  much  longer  than  the  fore.  In  the 


410 


MESOZOIC  ERA— AGE   OF   REPTILES. 


tracks  figured  below,  the  hind-tracks  are  eight  inches  and  the  fore- 
tracks  about  four  inches  long.  Others  have  been  found  of  much  greater 
size. 

The  beaked  Saurians,  also  called  Anomodonts  (lawless-toothed), 


FIG.  607— Tracks  of  a  Cheirotheriuin— a  Labyrinthodont. 


FIG.  608. 


FIGS.  608-611. — TRTASSIC  REPTILES  (after  Owen) — Anomodonts  and  Theriodonte  :  60S.  Dieynodon 
lacerticeps.  609.  Oudenodon  Bainii.  610.  a  b,  Lycosaurus.  611.  Canine  Tooth  of  Cynodracon, 
natural  size. 


TRIASSIC  PERIOD. 


411 


are  peculiar  to  this  period.  The  most  extraordinary  of  this  remarkable 
group  is  the  Dicynodon  (two-canine-toothed).  This  was  a  saurian  with 
the  head  and  nipping,  horny  beak  of  a  tortoise,  and  with  two  long  curved 
overhanging  canine  teeth  from  the  upper  jaw  (Fig.  608).  Several 
species  have  been  found,  in  one  of  which  (the  tigriceps)  the  head  was 
twenty  inches  long  and  eighteen  inches  wide.  They  have  been  found 
only  in  the  fresh-water  Triassic  of  South  Africa  (Karoo  beds).  Several 
other  genera  of  the  same  order  (Anomodonts)  have  been  found  in  the 
same  locality.  The  Oudenodon  had  a  nipping,  horny  beak  (Fig.  609), 
without  teeth  of  any  kind. 

According  to  Prof.  Owen,  this  remarkable  order  combined  the  char- 
acters of  crocodiles,  tortoises,  and  lizards. 

Verv  recently  from  the  same  South  African  strata  (Karoo  beds) 
Prof.  Owen  has  described  a  great  number  of  remarkable  reptiles,  in- 
cluding Lycosaurus,  Cynodracon,  Tigrisuchus,  Cynosuchus,  and  many 
others,  which,  from  some  mammalian  characters,  especially  in  the  teeth, 
he  calls  Theriodonts  (beast-tooth).  The  strata  in  which  they  have  been 
found  are  usually  assigned  to  the  Triassic,  but.  they  may  be  Permian,  as 
similar  reptiles  have  been  found  in  the  Permian  of  the  Ural.  Figs. 
510  and  511,  taken  from  Owen,  show  the  characters  of  these  reptiles. 

Birds. — No  Birds  have  yet  been  found  in  the  strata  of  the  Triassio 
age,  unless  we  except  the  so-called  bird-tracks  of  the  Connecticut  Val- 
ley sandstone,  which  we  will  discuss  further  on. 


FIR.  612,-Tooth  of  the  Microlestes  antiquus, 


Fio.  613.—  Myrmecobius  fasciatus,  Banded  Ant-eater  of  Australia. 


Mammals. — Remains  of  two  or  three  small  insectivorous  Marsupials 
have  been  found  in  the  uppermost  Triassic,  both  of  Europe  and  of  the 


412  MESOZOIC   ERA— AGE   OF   REPTILES. 

United  States.  Figures  of  a  tooth  of  one  of  these,  Mierolestes  antiquus, 
are  given  (Fig.  612),  and  also  a  figure  of  what  is  regarded  as  its  nearest 
living  congener  (Fig.  613).  But  as  these  are  found  in  very  small  num- 
bers in  the  uppermost  Triassic  beds,  and  as  similar  animals  are  found 
in  much  greater  numbers  in  the  Jurassic,  it  seems  best  to  regard  these 
as  anticipations,  and  to  put  off  the  discussion  of  the  affinities  of  the 
earliest  mammals  until  we  take  up  that  period. 

Mammals  probably  preceded  Birds.  This  is  not  a  little  remarkable. 
But  it  must  be  remembered  that  Birds  are  very  closely  allied  to  Reptiles, 
and  may  be  regarded  as  a  secondary  offshoot  of  the  reptilian  branch. 

Origin  of  Eock-Salt. 

Neither  rock-salt  nor  coal  is  confined  to  the  rocks  of  any  particular 
age.  Both  have  been  formed  in  every  age;  both  are  forming  now. 
But  as  the  subject  of  the  origin  of  coal-deposits  was  discussed  in  con- 
nection with  that  age  during  which  it  was  accumulated  in  the  greatest 
abundance — the  Carboniferous — so  the  origin  of  rock-salt  is  best  dis- 
cussed in  connection  with  the  so-called  Saliferous  or  Triassic. 

Age  of  Rock-Salt. — As  already  stated,  rock-salt  is  found  in  strata 
of  all  ages,  and  is  forming  now.  Moreover,  there  is  none  which  deserves 
the  name  Saliferous  to  the  same  extent  that  the  Carboniferous  deserves 
its  name.  The  salt  of  Syracuse,  New  York,  is  found  in  the  Upper  Si- 
lurian ;  that  of  Canada,  which  exists  in  immense  beds  100  feet  thick,  is 
found  in  the  Upper  Silurian  or  Lower  Devonian ;  that  of  Pennsylvania 
is  Upper  Devonian ;  of  Southwest  Virginia  is  sub-Carboniferous ;  of 
Petite  Anse,  Louisiana,  is  uppermost  Cretaceous  or  lowest  Tertiary 
(Hilgard).  In  Europe,  the  English  salt-beds  are  Triassic,  the  German 
beds  Triassic  and  Jurassic ;  the  celebrated  Polish  beds  at  Cracow  are 
Tertiary. 

Mode  of  Occurrence. — Salt  occurs  in  immense  beds  of  pure  rock-salt, 
or  else  impregnating  strata.  It  is  obtained  by  direct  mining,  or  else  by 
boiling  down  the  saline  waters  either  of  natural  springs  or  of  artesian 
wells  sunk  into  the  salt-bearing  strata.  The  further  explanation  of 
its  mode  of  occurrence  is  best  and  most  concisely  given  by  comparing 
it  with  coal. 

1.  Like  coal,  it  occurs  in  isolated  basins,  but  these  are  far  more 
limited  than  the  great  coal-fields.  2.  Like  coal,  it  is  interstratified  with 
sands  and  clays,  the  whole  series  repeated  often  many  times.  In  Ga- 
licia,  for  example,  there  are  found  seven  salt-beds  in  the  same  section. 
3.  But  it  differs  from  coal,  in  the  great  thickness  of  the  beds.  In  Can- 
ada the  salt-bed  is  100  feet  thick  (Gibson).1  In  Cheshire,  England, 
there  are  two  beds,  one  100  feet,  the  other  90  feet  thick,  separated  by 
thirty  feet  of  shale.  At  Stassfurt  a  salt-bed  has  been  penetrated 
1  American  Journal  of  Science,  vol.  v.,  p.  362,  1873. 


ORIGIN  OF  ROCK-SALT.  413 

1,000  feet,  and  the  bottom  not  yet  reached.1  4.  Recollecting  the  some- 
what limited  extent  of  basins,  it  is  evident  that  salt-beds  thin  out  far 
more  rapidly  than  coal.  The  English  salt-beds  thin  out  fifteen  feet  per 
mile.  Coal,  therefore,  lies  in  extensive  sheets,  salt  in  lenticular  masses. 
5.  Coal  has  its  characteristic  valuable  accompaniment  in  iron-beds, 
salt  in  beds  of  gypsum.  Thus,  as  coal-measures  consist  of  repetitions 
of  sands,  clays,  occasional  limestones,  with  valuable  beds  of  coal  and 
iron-ore  many  times  repeated,  so  salt-measures  consist  of  sands,  clays, 
and  occasional  limestones,  with  valuable  beds  of  salt  and  gypsum  many 
times  repeated.  Gypsum-beds  are  often  entirely  separate  from  salt- 
beds,  but  each  salt-bed  is  apt  to  be  underlaid  by  gypsum.  6.  While 
coal-measures  are  remarkable  for  the  abundance  of  organic  remains, 
both  vegetable  and  animal,  salt-measures  are  equally  remarkable  for 
extreme  poverty  in  this  respect.  The  presence  of  these  remains  in 
the  one  case,  and  their  absence  in  the  other,  are  the  cause  of  the  dif- 
ference in  the  color  of  the  sandstones.  Coal-measure  sandstones  are 
white  or  gray,  being  leached  of  their  oxide  of  iron  by  organic  matter. 
Salt-measure  sandstones  are  usually  red,  the  iron  being  diffused  as 
coloring-matter. 

Theory  of  Accumulation. — We  have  already  seen  (p.  73)  that  salt- 
lakes  are  evaporated  residues  of  river-water  or  sea-water  in  dry  cli- 
mates, and  are  now,  most  of  them,  depositing  salt:  also,  that  sea- 
water  evaporated  deposits  first  gypsum,  then  salt :  also,  that  these 
deposits  of  salts  and  gypsum  alternate  annually  with  sediments  of  sand 
and  clay — the  salt  or  gypsum  deposit  representing  the  dry  season,  and 
the  mechanical  deposits  representing  the  season  of  floods.  It  is,  there- 
fore, natural  to  look  in  this  direction  for  an  explanation  of  salt  and 
gypsum  deposits  —  to  think  that  salt-basins  are  dried-up  salt-lakes. 
But  the  immense  thickness  of  the  beds  plainly  shows  that  there  must 
have  been  important  modifications  of  this  process.  It  is  plain  that 
the  alternations  of  salt  and  sedimentary  deposit  were  not  annual  but 
secular. 

The  conditions  under  which  salt-measures  were  formed  were  prob- 
ably as  follows  :  Imagine  a  low,  flat  coast,  with  salt  lagoons  or  lakes, 
connected  periodically  with  the  sea,  by  changing  direction  of  winds,  or 
at  longer  intervals  by  oscillations  of  the  earth-crust ;  and  subjected  to 
hot  sun  and  dry  climate,  and  without  contiguous  mountains  furnishing 
abundant  sediment.  Under  these  conditions  either  gypsum  alone,  or 
gypsum  first  and  then  salt,  might  accumulate  by  deposit  indefinitely. 
If  the  water  of  the  lagoon  was  kept,  by  periodic  fresh  supply  of  sea- 
water,  just  below  the  saturating  point  for  salt,  gypsum  only  would  con- 
tinue to  deposit;  but  if  the  concentration  should  reach  the  point  of 

1  Bischof,  "  Chemical  Geology,"  vol.  i.,  p.  383. 


414  MESOZOIC  ERA— AGE   OF   REPTILES. 

saturation  for  salt,  then  salt  would  deposit  indefinitely,  since  fresh  sup- 
plies would  come  in  from  the  sea. 

Something  like  this  is  said  to  take  place  now,  in  portions  of  the 
delta  of  the  Indus  (Lyell's  "  Principles  ").  A  low,  flat  country  of  7,000 
square  miles  (Runn  of  Cutch)  is  covered  by  sea-water  a  portion  of 
every  year  by  the  action  of  the  monsoons,  and  dry  another  portion  of 
every  year  by  the  change  of  wind.  Salt  seems  to  be  depositing  there 
without  limit.  The  region  is,  of  course,  utterly  desolate,  and  the 
lagoon-water  almost  wholly  destitute  of  life  of  any  kind,  vegetable  or 
animal. 

In  the  deposits  of  salt-lakes  or  saturated  lagoons  we  would  not 
expect  to  find  many  animal  remains,  but  the  tracks  of  animals  walking 
along  their  muddy  shores,  as  also  sun-cracks  and  rain-prints,  would  be 
found  as  on  other  shores.  Now,  in  the  strata  associated  with  salt, 
although  organic  remains,  as  already  said,  are  rare,  shore-marks  of 
all  kinds  are  common. 

SECTION  2.— JTTEASSIC  PERIOD. 

This  is  the  culminating  period  of  the  Mesozoic  era  and  Reptilian 
age.  In  it  all  the  characteristics  of  this  age  reach  their  highest  de- 
velopment. We  must  discuss  this  somewhat  more  fully  than  the  last. 

The  strata  belonging  to  this  period  are  magnificently  developed  in 
the  Jura  Mountains,  and  hence  the  name  Jurassic.  These  mountains 
are  an  admirable  illustration  of  the  manner  in  which  ridges  and  valleys 
are  formed  by  the  folding  of  strata  (Fig.  614) ;  they  also  abound  in 


PIG.  614.— Section  of  the  Jura  Mountains. 

fossils  of  this  period.     They  have  always  been  regarded  by  geologists, 
therefore,  as  classic  ground. 

English  geologists  call  the  period  Oolite  (egg-stone),  on  account 
of  the  abundant  occurrence  in  that  country  of  a  peculiar  limestone 
composed  often  wholly  of  small  rounded  grains  like  the  roe  of  a  fish. 
They  divide  the  whole  period  into  three  epochs,  viz. :  1.  Lias;  2. 
Oolite  proper ;  3.  Wealden.  They  also  subdivide  the  Oolite  proper 
into  Lower,  Middle,  and  Upper  Oolite,  separated  by  intervening  Oxford 


JT7RASSIC  PERIOD.  415 

and  Kimmeridge  clays.  All  these  divisions  and  subdivisions  are  well 
shown  in  the  following  section  passing  from  London  westward.  This 
section  is  interesting  not  only  as  exhibiting  all  the  divisions  and  sub- 
divisions of  the  Oolitic  period,  but  also  as  showing  their  conformity 

Lower  Middle  Upper  London 

Oolite.  Oolite.  Oolite.  Chalk,     clay. 


Lias.  Oxford  Clay.  Kimmeridge      Gault. 

Clay. 
FIG.  615. 

among  themselves  and  with  the  overlying  chalk,  and  the  unconformity 
of  these  with  the  overlying  Tertiary.  It  also  shows  how  parallel  ridges 
and  intervening  hollows  are  formed  by  the  outcropping  of  a  series  of 
strata  alternately  hard  and  soft. 

Origin  of  Oolitic  Limestones. — Oolitic  limestones  are  now  forming 
on  coral  shores  by  cementation  of  rolled  and  rounded  coral-sand  grains 
(p.  148.)  But  oolitic  grains  often  contain  small  foreign  particles  around 
which  the  limestone  is  arranged  concentrically.  In  such  cases  the 
rounded  grains  "  seem  to  have  been  gathered  by  attraction,  out  of  the 
calcareous  mud,  round  nuclei  of  previously-solidified  matter  "  (Phillips). 

Jurassic  Coal-Measures. — In  the  Jurassic  times  we  have  reproduced 
on  a  large  scale  the  conditions  favorable  for  luxuriant  growth  of  plants, 
and  for  their  accumulation  and  preservation  in  the  form  of  coal.  Hence 
in  many  countries  we  have  Jurassic  coal-fields.  To  this  period  belong 
the  Yorkshire  coal  of  England  and  the  Brora  coal  of  Scotland.  To  this 
or  the  previous  period  belong  the  coal-fields  of  North  Carolina  and 
Eastern  Virginia,  and  some  of  the  coal-fields  of  India  and  China.  The 
fine  coal-measures  of  New  South  Wales,  Australia,  covering  an  area  of 
20,000  square  miles,  have  been  usually  referred  to  this  period,  but  they 
are  probably  Permian  or  Carboniferous.  Jurassic  coal-measures  have  a 
general  structure  similar  to  those  of  the  Carboniferous.  Like  the  lat- 
ter, they  consist  of  alternations  of  sands  and  clays,  and  occasional  lime- 
stones, containing  seams  of  coal  and  beds  of  iron-ore.  The  iron-ore  too 
is  of  the  same  kind,  viz.,  day  iron-stone.  We  find  here  also  underclays, 
with  stumps  and  roots,  and  roof-shales  filled  with  leaf-impressions.  It 
is  fair  to  conclude,  therefore,  that  the  mode  of  accumulation  was  similar 
to  that  already  described,  viz.,  in  marshes  subject  to  occasional  floods. 
Jurassic  coal,  though  perhaps  inferior  as  a  general  rule  to  Carbonifer- 
ous, is  often  of  good  quality,  occurring  in  thick  and  profitable  seams. 

Dirt-Beds—Fossil  Forest-Grounds. — Coal-seams  with  their  under- 
lying clays  are  fossil  swamp-grounds  ;  dirt-beds  are  fossil  soils  or  forest- 
grounds.  The  one  graduates  insensibly  into  the  other,  and  both  are 
occasionally  found  in  all  strata,  from  the  Devonian  upward.  In  the 


416 


MESOZOIC  ERA— AGE   OF  REPTILES. 


Upper  Oolite  of  England,  at  the  Isle  of  Portland  and  elsewhere,  there 
occurs  an  interesting  example  of  such  a  fossil  forest-ground  with  the 
erect  stumps  and  ramifying  roots  still  in  situ,  though  silicified,  and  the 
logs,  also  silicified,  still  lying  on  the  fossil  soil  (Figs.  616,  617).  It  is  evi- 
dent that  the  sequence  of  events  at  this  place  in  Jurassic  times  was  as 
follows:  1.  The  place  was  sea-bottom,  and  received  sediment  which 
consolidated  into  Portland-stone.  2.  After  being  flooded  and  covered 
with  river-deposit,  it  was  raised  to  land  and  became  forest-ground,  cov- 
ered with  trees  and  other  vegetation  peculiar  to  that  time,  the  decaying 


FIG.  61 6. -Section  In  Cliff  east  of  Lul- 
worth  Cove:  a,  Dirt-bed. 


T 1 

FIG.  617.— Section  in  the  Isle  of  Portland : 
aLDirt-bed. 


leaves  of  which  accumulated  as  a  rich  and  thick  vegetable  mould.  3.  It 
became  flooded  with  fresh  water,  and  the  trees  therefore  died  and  rotted 
to  stumps.  4.  The  whole  ground,  with  its  stumps  and  logs,  became 
covered  with  mud,  which  hardened  into  slates.  5.  Finally,  the  whole 
was  raised  into  high  land,  and  in  the  first  figure  (Fig.  616)  tilted  at 
considerable  angle. 

Thus,  we  have  here  not  only  an  old  forest-ground  with  its  vegetable 
mould,  but  also  the  stumps  and  logs  of  the  trees  which  grew  there, 
still  in  place ;  and  closer  examination  easily  detects  the  kinds  of  trees 
which  grew  in  the  forest.  They  are  Cycads  and  Conifers  (Figs.  618-625). 


FIG.  618.— Zamia  spiralis,  a  living  Cycad  of  Australia. 

Still  further,  there  is  good  reason  to  believe  that  the  remains  of  some 
of  the  animals  which  roamed  these  forests  have  been  found.  Of  these 
we  will  speak  in  their  proper  place. 


PLANTS. 
Plants. 


417 


Although  the  conditions  under  which  coal  was  accumulated  were 
probably  similar  in  all  geological  periods,  yet  the  kinds  of  plants  out 


FIG.  619.— Cycas  circinalis,  x  Ti5,  a  living  Cycad  of  the  Moluccas  (after  Decaisne). 

of  which  the  coal  was  made  varied.  As  already  seen,  the  principal 
coal-plants  of  the  Carboniferous  period  were  vascular  Cryptogams.  On 
the  contrary,  the  principal  coal-plants  of  the  Jurassic  period  were  ferns, 


FIG.  G20.-Stem  of  Cycadeoidea  megalophylla. 

Cycads,  and  Conifers.     The  Jurassic  may  be  called  the  age,  of  Gym- 
nosperms,  as  the  Carboniferous  was  the  age  of  Acrogens.     The  Gym- 

27 


418 


MESOZOIC  ERA— AGE   OF  REPTILES. 


nosperms,  especially  the  family  of  Cycads,  reached  here  their  highest 
development.  This  is  shown  in  Fig.  245  on  page  270.  The  leaves  (Fig. 
621)  and  short  stems  of  Cycas  and  Zamia  (Fig.  620)  are  found  very 


FIG.  621. 


FIGS.  621-624. — JITRASSIC  PLANTS — Cycads  and  Ferns:  621.  Pterophyllum  comptum  (a  Cycad).    622. 
Hemitelites  Brownii  (a  Fern).    628.  Coniopteris  Murrayana.    624.  Pachypteris  lanceolate. 


abundantly  in  connection  with  the  coal-bearing  strata.     It  is  probable, 
therefore,  that  the  coal  is  composed  largely  of  these  plants.      Some 


ANIMALS.  419 

remains  of  Jurassic  plants  are  given  (Figs.  620-626),  and  also  of  living 
Cycads  (Figs.  618,  619),  for  comparison. 


FIG.  625.  FIG 

FIGS.  625,  626.— JURASSIC  PLANTS— Conifers :   625.  Cone  of  a  Pine.    626.  Cone  of  an  Araucaria. 

-Animals. 

The  animals  of  the  Jurassic,  both  marine,  fresh-water,  and  land, 
were  very  abundant,  and  have  been  well  preserved.  It  is  impossible, 
therefore,  in  the  lower  departments,  to  do  more  than  touch  lightly  the 
most  salient  points.  In  the  higher  departments  we  will  dwell  a  little 
longer. 

Corals  have  assumed  now  the  modern  type  and  style  of  partitions 
(Fig.  627).  Among  Echinoderms,  the  Crinids,  or  plumose-armed  Cri- 
noids,  are  very  abundant  and  very  beautiful ;  in  fact,  they  seem  to  have 
reached  their  highest  point  in  abundance,  diversity,  and  gracefulness 
of  form  (Figs.  628,  629).  But  the  free  forms,  Echinoids  and  Asteroids, 
are  now  equally  abundant  (Figs.  630-632). 

BracMopods  are  still  abundant,  though  far  less  so  than  formerly ; 
but  they  now  belong  almost  wholly  to  the  modern  or  sloping-shoul- 
dered types,  such  as  Terebratula  and  Rhynchonella.  Only  a  very  few 
small  specimens  of  the  Palaeozoic  type  linger  until  the  Lias. 

Lamellibranchs,  or  common  bivalves,  are  extremely  abundant. 
Among  the  common  and  characteristic  forms  are  Trigonia,  Gryphsea, 
and  Exogyra,  belonging  to  the  oyster  family ;  and  the  strangely-shaped 
Diceras.  It  is  interesting,  also,  to  observe  here  the  first  appearance  of 
the  genus  Ostrea  (oyster). 

Cephalopods. — One  of  the  most  striking  characteristics  of  the  Juras- 
sic period  is  the  culmination  of  the  class  of  Cephalopods  in  number, 
diversity  of  forms,  and,  if  we  except  some  of  the  Silurian  Orthocera- 
tites,  in  size.  They  were  represented  by  the  Ammonites  and  the  Be- 
lemnites,  the  one  belonging  to  the  order  of  Tetrabranchs,  or  shelled, 


420 


MESOZOIC  ERA— AGE   OF  REPTILES. 


the  other  to  the  Dibranchs,  or  naked  Cephalopods.     It  is  important  to 
observe  that  the  highest  order  of  Cephalopods,  the  Dibranchs,  by  far 


FIG.  631. 


FIG.  623. 

FIGS    627-632.-JntA8STC  CORALS  AND  ECHINODERMS  :   fi-27.  Prionastrea  ohlonpata.    623.  Apiocrinus 
Roissianus.      (529.  Saoeoeoma  pectinata  (- *~-  *"--"•'       -»-     •   • 
Plotii.    632.  a  6,  Hemicidaris  crenukris. 


ANIMALS. 


421 


the  most  abundantly  represented  at  the  present  time,  were  introduced 
here  for  the  first  time. 


FIG.  68T.  FIG.  638.  FIG.  639.  FIG.  640. 

FIGS.  633-640.  —  JURASSIC  LAMELLIBRANCHS  AND  BRACHIOPODS  OF  ENGLAOT)  :  638.  Astarte  excavate. 
634.  Trigonia  clavellata.  635.  Ostrea  Sowerbyi.  636.  Pecten  flbrosus.  637.  Ostrea  Marshii.  638. 
Khynchonella  varians.  639.  Terebratula  sphieroldalis.  640.  Terebratula  digona  (after  Nicholson). 

Ammonites. — The  Ammonite  family,  which  is  distinguished,  as  already 
explained  (pp.  306, 320),  by  the  dorsal  position  of  the  siphon  and  the  com- 
plexity of  the  suture,  is  represented  in  extreme  abundance  by  the  type- 


FIG.  641.— Ammonites  Humphreysianus. 

genus  Ammonites.  About  500  species  of  this  genus  are  known,  ranging 
in  time  from  the  Triassic  through  the  Cretaceous.  They  are  therefore 
characteristic  of  the  Mesozoic.  They  varied  extremely  in  shape,  and  in 


422 


MESOZOIC  ERA— AGE   OF  REPTILES. 


size  from  half  an  inch  to  a  yard  or  more  in  diameter.     Below,  and  on 
page  421,  we  give  figures  of  some  of  the  most  common  species. 

In  the  genus  Ammonites  the  distinguishing  character  of  the  familv, 
viz.,  the  complexity  of  the  suture,  reached  its  highest  point.     In  this 


FIGS.  642-645.  —  JUEASSIC  CEPHALOPODS—  Ammonites:  642.  Ammonites  bifrons.  643.  Ammonites  mar- 
garitanus.  644.  Ammonites  Jason:  a,  side-view;  b,  showing  suture.  645.  Ammonites  cordatns: 
a,  side-view  ;  6,  showing  suture. 


Nautilus. 


FIG.  646.— Diagram  showing  the  Form  of  the  Suture  and  the  Position  of  the  Siphon  in  Cephalopods. 


ANIMALS. 


423 


genus,  the  edge  of  the  septa,  which  was  only  plaited  in  Goniatite,  and 
lobed  in  the  Ceratite,  becomes  most  elaborately  frilled.  We  give  above 
(Fig.  646)  the  form  of  suture  in  the  type-genera  of  the  different  orders 
of  shelled  Cephalopods,  the  four  lower  in  the  order  of  their  first  appear- 
ance. In  each  case  the  suture  is  supposed  to  be  divided  on  the  ventral 
surface  and  spread  out,  so  that  the  central  part  in  the  figure  represents 
the  dorsal  portion,  and  the  two  extremities  the  ventral.  In  the  Ammo- 
nite family,  which  includes  the  second,  third,  and  fourth,  the  gradual 
evolution  of  this  structure  is  well  shown.  The  corresponding  figures 
on  the  left  are  sections  showing  the  position  of  the  siphon. 

The  order  in  which  these  several  genera  appeared,  and  their  contin- 
uance, are  shown  in  the  diagram  (Fig.  656)  on  page  425. 

Belemnites. — The  Belemnite  (^e/Ujitvov,  a  dart)  was  nearly  allied  to 
the  squid  and  cuttle-fish  of  the  present  day.     Like  the  squid,  it  had  an 

internal  bone  (the  pen  of  the  squid), 
except  that  the  bone  is  much  larger 
and  heavier  in  the  Belemnite.  It  is 
this  bone,  or  the  lower  portion  of 
it,  which  is  usually  fossilized  (Figs. 
651-654).  When  perfect  it  is  ex- 
panded and  hollow  at  the  upper  end, 
and  in  the  hollow  is  a  small,  coni- 


FIG.  647.  FIG.  648.  FIG.  649. 

FIGS.  647-649.— 64T.  Internal  Shell  of  Belemnite  (restored  by  d'Orbigny).    648.  The  Animal  (restored 
by  Owen).    649.  A  living  Sepia  for  comparison. 


424 


MESOZOIC  ERA— AGE   OF   REPTILES. 


cal,  chambered,  siphuncled  shell,  the  Phragmocone.  Fig.  647,  a  and  £>, 
shows  the  perfect  bone,  and  Fig.  651  the  upper  part  broken  and  the 
phragmocone  in  place.  Like  the  squid,  too,  it  had 
an  ink-bag,  from  which  it  doubtless  squirted  the 
inky  fluid  to  darken  the  water  and  escape  its  enemy. 
These  ink-bags  are  often  well  preserved  (Fig.  650), 
and  the  fossil  ink  has  been  found  to  make  good  pig- 
ment (sepia),  and  drawings  of  these  extinct  animals 
have  actually  been  made  with  the  fossil  ink  of  their 
own  ink-bags  (Buckland).  Belemnites  were  some  of 
them  of  great  size,  and  evidently  formidable  animals. 
The  bone  of  the  Belemnites  giganteus  has  been 
found  two  feet  long  and  three  to  four  inches  in 


FIG.  651.— Belemnites  Owenii. 


diameter  at  the  larger  or  hollow  end.     A  very  perfect  specimen  of  an 
allied  genus,  from  the  Oalite  of  England,  is  shown  in  Fig.  655. 


FIG.  655. 


FIGS  652-655— 652   Belemnites  hastatus.    653.  Belemnites  unicanaliculatus.    654.  Belemnites  clavatus. 
655.  Acanthoteuthis  antiquus  (after  Man  tell). 


ANIMALS. 


425 


The  following  diagram  shows  the  order  of  succession  of  families  of 
the  class  Cephalopoda : 


PALEOZOIC. 

NEOZOIC. 

MESOZOIC.                   i    CKNOZOIC. 

Sil'n.      !     Dev'n.    i      Carb. 

Trias. 

Juras.     j      Cret      j 

Cepha 

opods. 

i 
Shelled  or   Tetrabranchs. 

Naked   or    Di  branch  s. 

j                ! 

i 

! 

She 

led. 

| 

Orthoceratites. 

1 

I    Goniatites. 

i 

. 

Ceratites. 

i 

1             i 

A  m 

m  o  n  i  t  e  s  . 

N              a               u 

« 

i                      1           |        U       8. 

Na 

ked. 

j 

Belemnites.j 

j                   J 

i  Sepia. 

FIG.  656. — Diagram  showing  Distribution  of  Cephalopoda  in  Time. 

Crustacea. — Crustacea  were  represented  in  the  Palaeozoic  first  by 
the  Trilobites ;  then  Limuloids ;  then,  in  the  last  period,  by  a  few  Macrou- 
rans.  In  the  Triassic  the  Macrourans  became  more  abundant  and  of 
more  modern  type.  In  the  Jurassic,  the  Macrourans  continue,  with  also 
many  Limuloids,  but  the  former  make  here  a  decided  approach  to  the 
Brachyourans  or  true  crabs,  by  the  shortening  of  the  tail  in  some  (Fig. 
657) ;  and  the  earliest  true  crab,  Palaeinachus — a  spider-crab — has  been 
found  in  the  Jurassic  of  England. 

Insects. — As  might  be  expected  from  the  abundant  forest  vegeta- 
tion, insects  have  been  found  in  considerable  numbers  and  variety  (Figs. 
659-663). 

Fishes. — It  will  be  remembered  that  the  Placoids  of  the  Palaeozoic 
were  nearly  all  Cestracionts,  or  crushing-toothed  sharks.  The  Hybo- 
donts,  or  sharks  with  teeth  pointed,  but  rounded  on  the  edges,  com- 
menced in  the  Carboniferous,  or  perhaps  Devonian,  and  increased  in 
the  Triassic.  Now,  in  the  Jurassic  the  Cestracionts  continue  (Fig. 


426 


MESOZOIC  ERA— AGE   OF   REPTILES. 


664),  but  in  diminished  numbers.  The  Hybodonts  culminate  (Fig. 
665),  and  the  /Squatodonts,  or  modern  sharks,  with  lancet-shaped  teeth, 
commence  in  small  numbers.  Rays  (Fig.  666),  which  may  be  regarded 


FIG.  657. 


FIGS.  657-659.— JUBASSIC  CRUSTACEANS  AND  INSECTS  :  657.  Kryon  arctifonnis,  Solenhofen.    658.  Eryon 
Barrovensis,  England.    659.  ufischna  eximia  (Hager). 

as  among  the  highest  of  Placoids,  are  found  in  considerable  numbers 
in  the  Jurassic. 

Ganoids  continue,  but  take  on  far  more  modern  forms,  and  have  now 
in  most  cases  lost  the  vertebrated  structure  of  the  tail-fin,  thus  fore- 
shadowing the  Teleosts,  which  appear  in  the  next  period.  Among  the 


ANIMALS.  427 

most  characteristic  Ganoids  of  this  period,  and,  in  fact,  of  this  age,  are 


FIG.  661. 

FIGS.  660-663.-JUEA88IC  INSECTS  :   660.  Libellula.     661.  Libellula  WestwoodlL      662.  Hemerobioides 
giganteus.    663.  Buprestidium. 


FIG.  666. 


FIGS.  664-666.— JURASSIC  FISHES— Plac,oids :   664.  Tooth  of  Acrodus  nobilis.     665.  Hybodus  reticula- 
tus,  Spine  and  Tooth.     666.  Squatina  acanthoderma. 


428 


MESOZOIC   ERA— AGE    OF   REPTILES. 


the  Pycnodonts,  a  family  characterized  by  a  broad,  flat  body,  rhom- 
boidal  enameled  scales,  pavement  palatal  teeth,  and  persistent  noto- 
chord  (Fig.  667). 


FIG.  667.—  JURASSIC  FISHES—  Ganoid:  Tetragonolepis,  restored,  and  Scales  of  the  same. 


Reptiles.  —  The  huge  reptiles  which  form  the  distinguishing  feat- 
ure of  this  age  culminate  in  the  Jurassic  period.  Their  number  and 
variety  are  so  great  that  we  can  only  select  a  few  from  each  order 


FIG.  671. 


FIGS.  668-671.— JURASSIC  EEPTILES— Ichthyosaurus  and  Pleeiosaurux :  668.  Ichthyosaurus  communis, 
x  TJ5.  (if.!*.  Plesiosaunis  dolichodoirus,  restored,  x  &,.  670.  Vertebrae  of  Ichthyosaurus  and  Sec- 
tion of  same,  showing  structure.  671.  Tooth  of  Ichthyosaurus,  natural  size. 

for  description.  They  were  emphatically  rulers  in  every  department 
of  Nature — rulers  of  the  sea,  of  the  land,  and  of  the  air.  We  shall 
treat  of  them  under  the  three  heads  thus  indicated,  viz.:  1.  Enalio- 


ANIMALS.  429 

saurs  (sea-saurians),  or  rulers  of  the  sea ;  2.  Dinosaurs  (huge  saurians), 
or  rulers  of  the  land  ;  and  3.  Pterosaurs  (winged  saurians),  or  rulers  of 
the  air.  The  first  were  wholly  swimming,  the  second  walking,  the 
third  flying,  saurians.  Intermediate  between  the  first  and  second  was  a 
fourth  order,  the  Crocodilians,  which  both  swam  and  crawled. 

1.  Enaliosaurs. — From  the  immense  variety  of  these  we  select  only 
two  for  description  as  representative  genera,  viz.,  Ichthyosaurus  and 
Plesiosaurus.  Figures  of  these  are  given  on  page  428. 

The  Ichthyosaurus  (fish-saurian}  was  a  huge  animal,  in  some  cases 
thirty  to  forty  feet  in  length,  with  a  stout  body,  short  neck,  and  enor- 
mous head,  sometimes  five  feet  long,  and  jaws  set  with  large  conical, 
striated  teeth,  sometimes  200  in  number.  The  enormous  eyes,  some- 
times fifteen  inches  in  diameter,  were  provided  with  radiating,  bony 
plates,  as  are  the  eyes  of  birds  and  some  living  reptiles,  apparently  for 
adjusting  the  eye  to  different  distances.  The  tail  was  long,  and  proba- 
bly provided  terminally  with  a  vertical,  Jin-like  expansion,  unsupported 
by  rays  (Owen).  In  addition  to  the  powerful  fin-tipped  tail,  the  locomo- 
tive organs  were  four  short,  stout  paddles,  composed  of  numerous  closely- 
united  bones,  but  without  distinct  toes.  These  paddles  were  surrounded 
by  an  expanded,  ray-supported  web  (Fig.  672),  which  greatly  increased 
its  surface,  and  therefore  its  efficiency  as  swimming-organs  (Lyell).  The 
bodies  of  the  vertebrae  were  not  united  by  ball-and-socket  joint,  as  in 
most  living  reptiles,  but  were  M-concave  (amphiccelous),  like  those  of 
fishes  (Fig.  670). 


672. — Paddle-Web  of  an  Ichthyosaurus 


That  the  habits  of  the  creature  were  predatory  and  voracious  is  suffi- 
ciently attested  by  the  teeth.  It  is  further  proved  by  the  contents  of 
the  stomach,  which  are  sometimes  partly  preserved.  These  consist 
largely  of  fish-scales. 

From  the  description  given  above  it  is  plain  that  the  Ichthyosaurus 
combined  in  a  remarkable  degree  the  characters  of  saurian  reptiles  with 
those  of  fishes.  The  vertically  expanded  tail-tip,  the  paddles,  with  sur- 


430 


MESOZOIC   ERA— AGE   OF  REPTILES. 


rounding  ray-supported  web,  and  the  bi-concave  vertebral  bodies,  are 
all  decided  fish  characters.  In  most  other  respects  it  was  reptilian. 
This  combination  is  expressed  in  the  name. 

The  Plesiosaurus  (allied  to  a  saurian)  was  a  less  heavy  and  powerful 
animal  than  the  last.  It  was  remarkable  for  its  short,  stout,  almost 
turtle-shaped  body ;  its  long,  snake-like  neck,  consisting  of  twenty  to 
forty  vertebrae ;  its  small  head ;  its  short  tail,  unadapted  for  powerful 
propulsion ;  its  long'  arid  powerful  paddles,  which  were  its  sole  swim- 
ming-organs ;  and  its  bi-concave  vertebral  bodies.  Sixteen  species  have 
been  found  in  the  Jurassic  and  Cretaceous  rocks  of  Great  Britain  alone, 
and  one,  _P.  dolichodeirus,  was  twenty-five  to  thirty  feet  long  (Fig. 
669),  with  paddles  six  to  seven  feet  long. 


FIG.  673.-  «,  Head  of  a  Pliosaurus,  greatly  reduced ;  &,  T< 


The  Pliosaurus  (more  lizard-like)  had  the  large  head  and  short 
neck  of  the  Ichthyosaurus  (Fig.  673),  with  the  powerful  paddles  of  the 
Plesiosaurus.  A  perfect  paddle  of  this  animal  has  been  found  seven  feet 
long  (Fig.  674) ;  the  animal  was  probably  at  least  forty  feet  long. 


FIG.  674. — Paddle  of  a  Pliosaurus,  x  5\j. 

Intermediate  between  this  group  and  the  next — inhabiters  both 
of  land  and  water — Crocodilians  existed  in  great  numbers,  and  of  great 
size.  Some,  like  the  Teleosaurus  (Fig.  675),  were  narrow-snouted  like 
the  Gavials  of  the  Ganges,  but  had  arnphicoelous  vertebrae  like  the 
Enaliosaurs. 


ANIMALS. 


431 


2.  Dinosaurs. — These  reptiles  were  the  most  highly  organized  in 
structure,  as  they  were  certainly  the  hugest  in  size,  which  have  ever  ex- 
isted. Though  very  decided  reptiles,  they  combined  certain  characters 
which  allied  them  strongly  with  mammals  and  especially  with  birds. 
Their  very  large,  long,  and  hollow  limb-bones,  their  strong,  massive  hip- 
bones and  sacrum,  the  latter  composed  of  four  to  five  consolidated  ver- 
tebrae, allied  them  with  both  mammals  and  birds;  while  the  great  elonga- 


FIG.  675.— Teleosaurus  brevidens:  a,  skull;  5,  side-view  of  snout  showing  the  teeth  (after  Phillips). 

tion  backward  of  the  ischium,  the  massiveness  of  the  hind-legs  as  com- 
pared with  the  fore-legs,  and  the  possession  of  only  three  functional  toes 
on  the  hind-foot,  which  therefore  formed  a  tridactyl  track,  allied  them 
still  more  strongly  with  birds.  On  account  of  this  great  likeness  to 
birds  in  the  character  of  the  hind-limbs,  they  have  been  called  by  Prof. 
Huxley  Ornithoscelida  (bird-legged).  The  following  figures  (676,  677) 
illustrate  this  bird-like  character. 


FIG.  676.— Pelvis  of  an  Iguanodon  (restored  by  Hulke). 

It  seems  certain  that  all  the  Dinosaurs  walked  with  free  step,  like 
quadrupeds,  instead  of  crawling,  like  reptiles ;  and  some  if  not  all  of 
them,  had  the  power  of  standing  and  walking  on  their  hind-legs  alone, 
like  birds.  The  backward  elongation  of  the  ischiatic  bones  seems 
evidently  connected  with  the  erection  of  the  body  on  the  hind-legs. 
We  will  briefly  describe  only  the  most  remarkable  : 

The  Iguanodon  was  a  huge  herbivorous  Dinosaur,  found  principally 
in  the  Weal  den  (Upper  Jurassic).  It  takes  its  name  from  the  form  of 


432 


MESOZOIC  ERA— AGE   OF   REPTILES. 


its  teeth,  which  are  much  like  those  of  the  Iguana,  a  living  herbivorous 
reptile,  although  in  other  respects  there  is  little  affinity.     Fig.   678 


Fig.  67T.— A,  Dromteus;  S,  Dinosaur;  C,  Crocodile. 


shows  the  tooth  pf  the  Iguanodon,  and  Fig.  679  a  section  of  the  jaw  of 
the  Iguana,  for  comparison. 

But  the  difference  in  size  between  the  living  and  the  extinct  reptile 


FIG.  678. — Tooth  of  an  Iguanodon. 


is  enormous.     The  Iguana  is  from  four  to  six  feet  long ;  the  Iguanodon 
was  certainly  thirty  feet,  perhaps  fifty  or  sixty  feet  long,  and  of  bulk 


ANIMALS. 


433 


several  times  greater  than  that  of  an  elephant.  A  thigh-bone  has  been 
found  fifty-six  inches  long,  twenty-two  inches  in  circumference  at  the 
shaft,  and  forty-two  inches  at  the  condyle.  Its  habits  are  supposed  to 
have  been  somewhat  like  those  of  a  hippopotamus.  Like  this  animal, 
it  wallowed  in  the  mud,  and  fed  on  the  rank  herbage  of  marshy  grounds. 


FIG.  679. — Section  of  Jaw  of  an  Iguana,  showing  the  teeth  (after  Buckland). 

The  Megalosaur  was  a  somewhat  smaller  but  probably  a  more  for- 
midable carnivorous  reptile,  which  lived  through  the  whole  Jurassic  pe- 
riod. Its  huge  jaws  were  armed  with  large,  curved,  flattened,  sabre-like 
teeth  (Fig.  681).  A  femur  has  been  found  forty-two  inches  long  (Phil- 
lips), and  a  tibia  thirty-six  inches.  The  animal  was  at  least  thirty 
feet  long  (Owen).  Fig.  680  is  a  restoration  of  the  head  of  this  ani- 
mal by  Phillips,  and  Fig.  681  is  a  tooth  of  natural  size. 


FIG.  680.— Head  of  Megalosaurus,  x  ^  (restored  by  Phillips). 

The  Ceteosaur  (whale-lizard)  was  probably  the  largest  reptile — in  fact, 
the  largest  land-animal — which  has  ever  existed.  It  has  been  classed 
among  the  Crocodilians,  but  Prof.  Phillips  has  shown  that  its  true  posi- 
tion is  among  the  Dinosaurs.  A  thigh-bone  has  been  found  sixty-four 
inches  long,  27.5  inches  in  circumference  at  the  shaft,  forty-six  inches 
28 


434 


MESOZOIC   ERA— AGE   OF  REPTILES. 


Fio.  681.  —  Megalosaurus 
Tooth,  natural  size  (after 
Phillips). 


FIG.  6S2. — Femur  of  Ceteosaurus, 
x  jL  (after  Phillips). 


and  44.25  inches  in  circumference  at  the  two  ends,  respectively  (Fig-. 

682).  According  to  Phillips  this 
animal  was  at  least  fifty  feet,  and 
probably  from  sixty  to  seventy 
feet  long,  ten  feet  high  when 
standing,  and  of  bulk  proportion- 
ate. It  was  probably  a  vegetable 
feeder. 

The  Hylceosaur  was  another 
huge  reptile  of  the  same  period, 
and  the  Compsognathus  a  reptile 
of  smaller  size,  but  of  most  ex- 
traordinary bird-like  character, 
viz.,  small  head,  long,  flexible 
neck,  large  and  long  hind-legs, 
and  small  and  short  fore-legs. 

Ite.  683.-Compsognathus  (restoration  by  Wagner).       From   ^   structurej  it   must  have 

walked  habitually  on  its  hind-legs  alone  (Fig.  683). 

3.  Pterosaurs. — These  flying  reptiles  were  certainly  among  the  most 
extraordinary  animals  that  have  ever  existed.     The  order  includes  sev- 


ANIMALS.  435 

eral  genera,  but  we  will  describe  only  the  best  known,  viz.,  the  Ptero- 
dactyl (wing-finger). 

The  Pterodactyl  combined  the  short,  compact  body ;  the  strong 
shoulder-girdle,  firmly  united  with  the  keeled  sternum;  the  short, 
aborted  tail ;  the  long,  flexible  neck,  and  hollow,  air-filled  limb-bones, 
characteristic  of  birds — with  the  head,  and  jaws,  and  teeth,  of  a  reptile, 


FIG.  6S4.— Pterodactylus  crassirostris. 


and  the  membranous  wings  of  a  bat.  In  the  bat,  however,  the  mem- 
brane is  supported  by  four  fingers,  enormously  elongated  for  the  pur- 
pose, and  only  one  finger  is  free  and  clawed ;  while  in  the  Pterodactyl 
there  is  only  one  finger,  which  is  enormously  elongated  and  strength- 
ened for  the  support  of  the  web,  and  all  the  others  are  free  and  clawed. 


FIG.  685.— Rhamphorhynchus  Bucklandi  (restored  by  Phillips). 

The  manner  in  which  the  web  is  supposed  to  have  been  stretched  and 
supported  is  shown  in  Fig.  684. 

Many  species  have  been  found  ranging  in  size  from  two  feet  to 
twenty  feet  in  alar  extent.  They  lived  throughout  the  whole  Jurassic 
and  into  the  Cretaceous  period.  In  one  genus  (Rhamphorliynchm, 
beak-bill)  the  anterior  portion  of  the  jaws  was  destitute  of  teeth,  and 
probably  sheathed  with  horn  like  a  bird's  beak  (Fig.  685). 


436  MESOZOIC  ERA— AGE   OF   REPTILES. 

Birds. — Until  recently,  except  the  doubtful  tracks  of  the  Connect- 
icut Valley  to  be  mentioned  further  on,  no  trace  of  birds  had  been 
found  lower  than  the  Tertiary.  But  in  1862  bird-bones  and  beautiful 
impressions  of  bird-feathers  were  found  in  the  lithographic  limestone 
(Upper  Oolite — Jurassic)  of  Solenhofen.  Still  later,  many  remains  of 
birds  were  found  by  Marsh  in  the  Cretaceous  of  the  United  States. 
These  will  be  described  in  their  proper  place. 

Thus  far  the  only  bird-bones  found  in  the  Jurassic  are  those  of  the 
Archceopteryx  (ancient  bird)  mentioned  above.  These  remains  are  the 
earliest  positive  proof  of  the  existence  of  this  class;  they. are  therefore 


FIG.  6S6.— Archseopteryx  macroura,  restored  (after  Owen). 

of  exceeding  interest  to  the  geologist.  An  examination  of  the  figures 
below  (Figs.  687,  688)  will  show  that  this  earliest  bird  was  very  differ- 
ent from  the  typical  birds  of  the  present  day ;  that  it  was,  in  fact,  won- 
derfully reptilian.  Along  with  the  distinctive  bird  characters  of  feet 
and  limb-bones  and  pelvis,  and  especially  feathers  and  feathered  wings, 
it  had  the  long  tail  and  probably  toothed  jaws  of  a  reptile.  The  differ- 
ence between  the  tail  of  a  typical  bird  and  the  tail  of  the  Archaeopteryx 
is  very  similar  to  the  difference  between  a  homocercal  and  a  hetero- 
cercal  tail  among  fishes.  In  a  typical  bird  the  tail-joints  are  greatly 
shortened  and  consolidated,  so  that  it  is  not  more  than  an  inch  long  in 
a  bird  the  size  of  a  cock ;  and  the  tail-feathers  come  out  from  these  in 
a  radiating  manner  (Fig.  687,  J)).  In  the  Archaeopteryx,  on  the  other 
hand,  the  tail  consists  of  twenty-one  long  joints ;  making  the  tail  of  the 
skeleton  eight  or  nine  inches  long,  nearly  or  quite  as  long  as  all  the 
rest  of  the  skeleton ;  and  to  these  joints  are  attached  the  feathers,  one 
on  each  side  of  each  joint  (Fig.  687,  A).  It  is  a  true  vertebrated  tail. 
Another  very  striking  reptilian  character  is  found  in  the  structure 
of  the  hand.  In  ordinary  birds  what  corresponds  to  the  hand  consists 
of  three  fingers,  two  of  which  are  united,  and  only  one  (the  thumb)  is 


ANIMALS. 


437 


FIG.  687.—^!,  Tail  of  Archseopteryx  macroura;  J3,  Vertebrae  enlarged;  (7,  a  Feather;  Z>,  Tail  of  a  Vul- 
ture ;  E,  side-view  of  the  same. 

free  ;  but  in  this  earliest  bird  the  hand  consists  of  four  fingers,  all  sepa- 
rate, and  two  of  them  terminated  with  claws. 


-A,  Fore-limb  of  Bat;  B,  Archseopteryx ;  <7,  Bird;  Z>,  Pterodactyl,  compared.     In  all— a, 
scapula;  I,  humerus ;  c,  ulna;  d,  radius ;  e,  carpus ;  /,  metacarpus ;  g,  phalanges. 


438 


MESOZOIC   ERA— AGE   OF   REPTILES. 


Mammals. — In  the  same  formation  and  nearly  the  same  horizon  in 
which  we  find  the  dirt-bed  and  stumps  mentioned  on  page  416  (Upper 
Oolite)  have  been  found  also  in  England  the  remains  of  fourteen  species 
of  small  insectivorous  Marsupial  mammals,  varying  in  size  from  that  of 
a  mole  to  that  of  a  skunk.  It  would  seem,  therefore,  that  we  have  found 
not  only  an  old  forest-ground  of  the  Jurassic  period,  but  also  the  trees 
which  grew  in,  and  the  animals  which  roamed  through,  this  old  forest. 
In  a  somewhat  lower  bed,  the  Stonefield  slate  of  England,  have  been 
found  four  more  species ;  and  still  lower  in  the  uppermost  Triassic  have 
been  found  in  all  countries  taken  together  two  or  three  other  species, 
one  or  two  in  Europe,  and  one  in  the  United  States — making  in  all  about 
twenty  species  of  mammals  known  to  have  existed  in  Jurassic  times. 


FIG.  691. 


Fro.  692.  FIG.  698. 

FIGS.  689-69S.— JUKASSIC  MAMMALS  :  689.  Amphitherium  Prevostii.    690.  Phascolotherium.    691.  Am- 
phitherium.    692.  Triconodon.     693.  Plagiaulax. 

They  were  all  small  Marsupials  /  and  with  the  exception  of  one  which, 
judging  by  its  rodent  teeth,  was  probably  a  vegetable-feeder,  they  all 
seem  to  have  been  insectivorous. 

Affinities  of  the  First  Mammals. — The  marsupials  differ  very  great- 
ly from  ordinary  typical  mammals,  in  the  fact  that  in  the  former  there 
is  no  placental  attachment  between  the  foetus  in  utero  and  the  mother. 
The  foetus,  therefore,  does  not  and  cannot  develop  before  birth  into  a 
perfect  condition  fit  for  independent  life.  In  an  imperfect  condition  it 
is  born  and  placed  in  an  abdominal  pouch  (marsupium),  permanently 
attached  to  the  teat,  and  finishes  its  embryonic  development  there. 
Thus  in  these  animals  there  are  two  periods  of  gestation,  one  intra- 
uterine,  very  short,  and  another  marsupial,  much  longer.  Marsupial 
mammals,  therefore,  are  not  truly  viviparous,  but  semi-oviparous,  in  their 


JURA-TRIAS   IN   AMERICA.  4.39 

reproduction,  and  in  this  respect  allied  to  birds  and  reptiles.  The  class 
of  Mammals  is  therefore  subdivided  into  two  sub-classes,  viz.,  Placental 
or  true  mammals  and  Non-placental  or  semi-oviparous  mammals.  The 
former  includes  all  ordinary  mammals ;  the  latter  at  present  includes 
kangaroos,  opossums,  etc.  (Marsupials),  and  Ornithorhjnchus  and 
Echidna  (Monotremes). 

Now  the  mammals  of  the  Triassio  and  Jurassic  times  were  wholly 
non-placental  or  semi-oviparous,  and  therefore  approximated  the  lower 
classes  of  Vertebrates,  especially  birds  and  reptiles.  The  non-pla- 
centals  are  now  (with  the  exception  of  a  few  species  of  opossum  found 
in  America)  wholly  confined  to  Australia  and  the  vicinity.  In  Jurassic 
times  they  were  probably  very  abundant,  and  spread  over  all  portions 
of  the  earth.  Yet  they  were  not  rulers  of  those  times  ;  for  they  were 
wholly  unable  to  contend  with  the  great  reptiles.  It  was  essentially 
an  age  of  Reptiles.  Not  only  did  this  class  greatly  predominate  in 
number  and  size,  but  the  reptilian  character  was  strongly  impressed  on 
all  the  then  existing  birds  and  mammals.  From  the  reptilian  stem  the 
bird  and  mammal  branches  had  not  yet  so  fairly  separated  that  the 
connecting  links  were  obliterated. 


SECTION  3. — JURA-TKIAS  us  AMERICA. 

We  have  already  explained  that  these  two  periods  are  not  well  sepa- 
rated in  America.  This  is  partly  on  account  of  the  poverty  of  fossils, 
and  partly  on  account  of  the  continuity  of  conditions  throughout.  It 
seems  best,  therefore,  in  the  present  state  of  knowledge  to  treat  them 
together  as  one  period.  Doubtless  they  will  be  better  separated  here- 
after. 

Distribution  of  Strata. — 1.  Atlantic  Border. — Lying  in  plication-hol- 
lows, or  denudation-hollows,  unconformably  on  the  gneiss  (metamorphic 
Laurentian  or  Silurian)  of  the  eastern  slope  of  the  Appalachian  chain, 
are  found  very  remarkable  isolated  patches  of  sandstones  or  sandstones 
and  shales,  which  are  referred  to  this  period.  These  patches  are  strung 
along  nearly  parallel  to  the  chain,  and  to  the  coast,  from  Nova  Scotia 
to  the  border  of  South  Carolina.  They  are  represented  on  the  map 
(p.  278)  by  oblique  lines.  One  of  them  is  found  in  Prince  Edward's 
Island,  another  in  Nova  Scotia ;  another  is  the  celebrated  Connecticut 
River  Valley  sandstone ;  a  fourth  commences  in  New  Jersey,  passes  as 
a  narrow  strip  through  Pennsylvania,  Maryland,  and  into  Virginia ;  a 
fifth  and  sixth  form  the  Richmond  and  Piedmont  coal-fields  of  Virginia; 
a  seventh  and  eighth,  the  Dan  River  and  Deep  River  coal-fields  of  North 
Carolina.  As  they  are  isolated,  and  without  contact  with  any  other 
formation  except  the  gneiss,  on  which  they  lie  unconformably,  their  age 
cannot  be  even  conjectured  from  their  stratigraphical  relations  ;  but  the 


440  MESOZOIC  ERA— AGE   OF  REPTILES. 

few  fossils  which  they  contain  seem  to  refer  them  either  wholly  to  the 
Triassic,  or  else,  more  probably,  their  lower  half  to  the  Triassic  and 
their  upper  half  to  the  Jurassic  of  Europe  (Hitchcock). 

In  connection  with  the  more  northern  patches  are  found  columnar 
trap  or  dolerite  ridges,  evidently  formed  by  the  fissuring  of  the  strata 
and  the  outpouring  of  igneous  matter  upon  the  surface.  Mounts  Tom 
and  Holyoke  are  examples  in  the  Connecticut  Valley,  the  Palisades  of 
the  Hudson  in  the  New  Jersey  patch  ;  similar  trap-ridges  are  also  very 
conspicuous  in  the  Nova  Scotia  patch. 

2.  Interior  Plains. — Rocks  of  this  age  seem  to  be  widely  distributed 
on  the  eastern  slopes  of  the  Rocky  Mountains,  from  the  Black  Hills 
southward,  largely  covered  in  the  northern  parts  by  Cretaceous,  but  ex- 
posed over  wide  areas  in  the  region  south  of  the  38th  parallel  and  west 
of  the  97th  meridian,  including  large  portions  of  Kansas  and  Indian 
Territory,  and  Northern  Texas. 

3.  Rocky  Mountain  Region  and  Pacific  Slope. — Portions  of  the 
Black  Hills,  of  the  Colorado  Mountains,  and  of  the  Wahsatch  range, 
consist  of  these  rocks.    Outcrops  also  occur  on  the  slopes  of  the  Uintah 
Mountains,  and  large  areas  in  the  plateau  region  north  of  Grand  Canon, 
forming  several  of  the  remarkable  cliffs  of  that  region,  and  also  large 
areas  in  the  valley  of  the  Rio  Grande,  about  Santa  Fe,  New  Mexico. 
The  auriferous  slates  of  California,  both  east  and  west  of  the  Sierra 
range,  consist  of  the  same. 

Life- System. 

The  characterization  of  the  life-system  of  the  Jura-Trias  period  in 
America  is  best  brought  out  in  connection  with  a  minuter  description  of 
some  of  the  more  interesting  localities,  and  of  the  remarkable  records 
they  contain. 

Connecticut  River  Valley  Sandstone.— The  Strata.— This  locality  has 

been  made  classic  ground  for  the  geologist  by  the  indefatigable  labors 
of  the  late  President  Hitchcock,  of  Amherst.  The  strata  border  the 
Connecticut  River,  on  both  sides,  through  the  whole  of  Massachusetts 


FIG.  694.— A  Section  across  the  Valley  of  Connecticut:  g,  gneiss;  88,  sandstone;  t,  trap  ridges. 

and  Connecticut,  forming  an  irregular  area  about  110  miles  long  and  20 
miles  wide.  They  consist  of  red  sandstones  and  shales,  dipping  somewhat 
regularly  to  the  east,  at  an  angle  of  about  20°  to  30°,  indicating  a  thick- 
ness of  at  least  5,000  feet  (Dana)  to  10,000  feet  (Hitchcock).  The  gen- 
eral relations  of  the  strata  with  the  intrusive  trap  and  the  underlying 
gneiss  are  shown  in  the  accompanying  figure  (694).  The  trap  is  seen  to 


LIFE-SYSTEM. 


441 


be  mostly  conformable  with  the  strata.  This  regular  dipping  to  the  east 
throughout  the  whole  series  can  only  be  explained  by  supposing  that 
at  the  end  of  the  Jurassic  the  whole  area  of  previously-horizontal  strata 
(Fig.  695,  A)  was  lifted  into  an  incline  of  20°  or  more,  and  afterward 
cut  away  by  denudation,  as  shown  in  the  diagram  (Fig.  695,  JB). 


The  whole  series  of  sandstone  is  very  distinctly  stratified,  and  in 
many  parts  beautifully  fissile.  When  these  parts  are  broken  open 
along  their  lines  of  lamination,  all  kinds  of  shore-marks  are  found  in  the 
greatest  perfection,  viz.,  ripple-marks,  rain-prints,  sun-cracks,  leaf- 
impressions,  and  tracks  of  animals.  It  is  evident,  therefore,  that  this 
was,  throughout,  a  littoral  or  shoal-water  deposit.  But  it  is  at  least 
5,000  feet  thick.  Therefore,  there  must  have  been  subsidence  to  that 
extent.  Here,  then,  we  have  evidence  of  rapid  deposit  (for  the  mate- 
rials are  coarse),  invasion  of  interior  heat  with  aqueo-igneous  fusion, 
subsidence,  formation  of  fissures,  and  ejection  of  lava. 

Some  identifiable  fossils,  obtained  about  the  middle  of  the  series, 
seem  to  indicate  an  horizon  similar  to  the  Lias,  lowest  Jurassic  ;  or  to 
the  Rhastic,  uppermost  Triassic  of  England.  It  is  fair  to  conclude,  there- 
fore, that  this  patch  represents  the  whole  Jura-Trias  period. 

The  Record.  —  The  general  redness  of  the  sandstone  is  sufficient  evi- 
dence that  organic  remains  are  very  a 

scarce  ;  and  so,  indeed,  we  find  it.  Two 
or  three  fishes,  a  few  leaves,  the  most 
perfect  of  which  is  a  species  of  fern^— 
Clathopteris—  and  a  fir-cone  (Fig.  696), 
and  a  few  small  fragments  of  thin,  hol- 
low bones,  which  may  have  belonged  to 
either  birds  or  reptiles,  are  all  that  have 
been  yet  found. 

But  by  far  the  most  interesting  por- 
tion of  the  record  in  this  locality  consists 
of  tracks.  These  are  partly  tracks  of 
Insects  and  Crustaceans,  and  partly  of 
Reptiles  and,  possibly,  Birds.  Some  of 


°**  ^" 


M2  MESOZOIC   ERA— AGE   OF  REPTILES. 

those  which  have  been  referred  to  Crustaceans  and  Insects  are  shown 
in  Fig.  697,  a,  b,  c.  There  lias  been  found,  also,  the  whole  form  of  one 
insect,  apparently  the  larva  of  an  Ephemera  (Fig.  698).  It  is  quite 
probable  that  many  of  the  tracks  were  formed  by  similar  larvae  inhabit- 
ing the  water. 


FIG.  698.—  Larva  of  an 
Ephemera  (after  Hitch- 
cock). 


FIG.  697.—  a,  6,  c,  Tracks  of  Insects,  Crustacea,  or  Worms  (after  Hitchcock). 


Reptilian  Tracks.—  By  far  the  larger  number  of  tracks  are  those  of 
Reptiles.  More  than  fifty  species  have  been  described  by  Hitchcock. 
These  vary  extremely,  both  in  size  and  in  character.  In  size,  they  vary 
from  the  track  of  a  living  Triton,  a  half-inch  long,  to  that  of  the  Oto- 
zoum,  twenty  inches  long,  and  with  a  stride  of  three  feet.  Some  had 
five  toes,  some  four,  and  some  only  three  functional  toes  on  the  hind- 
feet.  Again,  some  had  hind  and  fore  feet  of  nearly  equal  size,  and 
evidently  walked  or  crawled  in  true  quadrupedal  style.  Others  had 
hind-feet  much  larger  than  fore-feet,  and  were  essentially  bipedal  in 
locomotion,  only  putting  down  their  small  fore-feet  occasionally  ;  but 
walking  bird-like,  not  hopping  kangaroo-like,  on  their  hind-legs.  In 
connection  with  the  bipedal  tracks  there  have  been  found  what  seemed 
to  be  the  impression  of  a  dragging  tail  (Fig.  700)  ;  but  these  are  so 
rare  and  doubtful  that  it  is  generally  believed  the  animals  were  mostly 
long-legged  and  short-tailed. 

The  general  conclusion  from  an  attentive  study  of  these  tracks,  in 
connection  with  the  findings  elsewhere  of  bones  and  teeth,  is  that  they 
are  the  tracks  mostly  of  Amphibians  of  the  order  of  Labyrinthodonts 
(four-toed  and  five-toed,  quadruped  and  biped),  but  a  few  were  probably 
Dinosaurs  (three-toed  biped).  The  hugest  among  them,  the  Otozoum 
Moodii  (Fig.  699),  was  probably  a  long-legged,  biped  amphibian,  which 
stood  twelve  feet  high.  The  Anomcepus  (Fig.  701),  on  the  contrary, 
was  probably  a  Dinosaur,  which  walked  often  on  two  legs  only,  and  in 
so  doing  brought  the  whole  tarsus  and  heel  on  the  ground,  in  the  man- 


LIFE-SYSTEM. 


443 


ner  of  a  kangaroo.     In  Fig.  700  the  mark  of  a  supposed  dragging  tail 
is  shown. 

Bird-Tracks. — Those  which  have  been  referred  to  birds  are  :  1. 
Wholly  bipedal,  i.  e.,  there  is  no  evidence  of  fore-feet  at  all.  2.  They 
are  tridactyl.  3.  They  have  a  regular  progression  in  the  number  of 
joints  in  the  tracks,  the  inner  toe  having  two,  the  middle  toe  three, 
and  the  outer  toe  four  joints.  Now,  in  birds  the  inner  toe  has  three, 
the  middle  toe  four,  and  the  outer  toe  five  joints,  but  the  last  two  joints 


FIG.  700. 


FIGS  699-701.-REPTILE-TRACKS (after  Hitchcock):  699.  Otozoum  Moodii:  a,  hind-foot,  x  T>5;  ft,  fore-foot, 
x  ^.    701).  Gigantitherium  caudatum,  x  ,V    701.  Anounoapus  minor,  x  A:  a,  hind-foot;  &,  fore-foot. 

in  each  case  make  but  one  division  of  the  track,  so  that  the  track  is  ex- 
actly what  is  given  above.  The  discovery,  however,  that  Dinosaurs 
have  but  three  functional  toes  on  the  hind-foot,  and  that  they  also  have 
the  same  number  of  joints  as  birds,  has  greatly  shaken  confidence  in  the 
ornithic  character  of  these  tracks.  Only  the  absence  of  fore-feet  tracks, 
therefore,  remains.  But  as  many  of  these  early  reptiles  walked  occa- 
sionally on  two  legs,  it  is  not  impossible  that  some  of  them  always 
walked  thus.  It  is  quite  possible,  not  to  say  probable,  therefore,  that 
all  these  tracks  are  those  of  Reptiles.  Assuming  them  to  be  those  of 


444 


MESOZOIC   ERA— AGE   OF  REPTILES. 


Birds,  they  varied  in  size  from  those  of  a  snipe  to  those  of  the  great 
Brontozoum,  eighteen  inches  long,  and  with  a  stride  of  four  feet  (Fig. 

702).  This  huge  bird,  if  bird  it  was, 
must  have  been  at  least  fourteen  feet 
high  (Dana).  Such  a  huge  animal  must 
have  been  wingless,  like  the  ostrich,  etc., 
for  its  size  is  far  beyond  the  limit  within 
which  flight  is  possible. 

We  have  expressed  a  doubt  as  to 
whether  these  tracks  be  those  of  birds 
or  reptiles.  This  is  not  so  strange  as  it 
may  at  first  appear.  These  two  classes 
are, indeed,  now  very  widely  separated; 
but  then  they  were  very  closely  allied. 
There  were  probably  animals  then  liv- 
ing which,  even  if  we  saw  them,  might 
puzzle  us  to  decide  whether  to  call 
them  reptilian  birds  or  bird-like  rep- 
tiles. These  tico  classes  were  not  yet 
fairly  disentangled  and  separated  from 
each  other. 

We  may  easily  imagine  the  circumstances  under  which  these  tracks 
were  formed.  During  the  Jura-Trias  period  there  was  in  the  region 
of  the  Connecticut  Valley  a  shallow  inland  sea,  connected  by  a  narrow 


FIG.  702. — Track  of  Brontozoum  gigan- 
teum,  x  i  (after  Hitchcock). 


FIG.  703.— Portion  of  a  Slab  with  Tracks  of  several  Species  of  Brontozoum  (after  Hitchcock). 

outlet  with  the  ocean.     Into  this  the  tides  flowed  and  again  ebbed, 
leaving  extensive  flats  of  mud  or  sand  ribbed  with  ripple-marks.    A  pass- 


LIFE-SYSTEM. 


445 


ing  shower  pitted  the  soft  mud,  and  the  sun,  coming  out  again  from  the 
breaking  clouds,  dried  and  cracked  it.  Huge  bird-like  reptiles,  and  pos- 
sibly reptilian  birds,  sauntered  near  the  shore-margin  in  search  of  food. 
The  tide  came  in  again  with  its  freight  of  fine  sediments,  gently  cov- 
ered the  tracks,  and  preserved  them  forever.  This  occurred  constantly 
for  many  ages  about  the  end  of  the  Triassic  or  the  beginning  of  the 
Jurassic  period,  for  the  tracks  are  found  near  the  middle  of  the  series 
of  strata. 

Richmond  and  North  Carolina  Coal-Fields. — The  patches  occurring 
in  Virginia  and  North  Carolina  are  coal-bearing.  They  constitute  the 
Richmond  and  Piedmont  coal-fields  of  Virginia,  and  the  Deep  River 
and  Dan  River  coal-fields  of  North  Carolina.  Fig.  704  gives  a  general- 


FIG.  704. — Section  across  Richmond  Coal-field  (after  Lyell). 

ized  section  of  the  Richmond  coal-fields,  taken  from  Lyell.  The  strata 
of  this  field  are  sandstone  and  shales,  700  to  800  feet  thick,  lying  in 
irregular  erosion-hollows  of  the  gneiss.  All  the  phenomena  of  a  coal- 
field are  here  repeated,  viz.,  interstratified  seams  of  coal  and  beds  of 
iron-ore,  underclays  with  roots,  and  roof-shales  with  leaf-impressions. 
There  are  several  seams  of  coal,  the  lowest  of  which  is  almost  in  con- 
tact with  the  gneiss.  Some  of  the  seams  are  of  great  thickness — 
thirty  to  forty  feet — and  the  coal  is  very  pure.  It  is  probable  that 
this  coal,  like  that  of  the  Carboniferous  times,  was  formed  in  a  marsh, 
which  was  sometimes  converted  into  a  lake.  The  plants  found  are 


FIG.  705.— Dictyopyge,  a  Ganoid  (after  EmmonsX 

very  decidedly  Upper  Triassic  or  Lower  Jurassic,  viz.,  Cy cads,  Conifers, 
Equisetae,  and  Ferns.     The  animals  indicate  the  same  horizon. 

The  Deep  River  and  Dan  River  coal-fields  of  North  Carolina  are  very 
similar  to  those  in  Eastern  Virginia,  except  that  in  the  Deep  River  coal- 
fields the  coal-bearing  portion,  which  seems  to  correspond  with  the 
whole  of  the  Richmond  strata,  is  underlaid  by  3,000  feet  of  barren  sand- 
stone. If  we  call  the  coal-measures  Upper  Trias  or  Lower  Juras, 


446  MESOZOIC  ERA— AGE   OF  REPTILES. 

these  barren  sandstones  are  certainly  Triassic.     In  the  upper  portion  of 


FIG.  710.  FIG  711. 

FIGS.  706-711. — FOSSILS  OF  NORTH  CAROLINA  AND  RICHMOND  COAL-BASINS  (after  Emmonsl :  706.  Walr 
chia  diffiisus.  707.  Podozaraites  lanceolatus.  70S.  Xeuropteris  linsefolia— Richmond  Coal.  709. 
Pecopteris  falcatus.  710.  Neuropteris.  711.  Jaw  of  Dromatherium  sylvestre. 


LIFE-SYSTEM. 


447 


these  underlying  sandstones,  and  therefore  probably  in  the  Upper  Tri- 
assic,  together  with  some  reptilian  teeth,  Dr.  Emmons  found  several 
jaws  of  a  small  Marsupial,  similar  to  the  Microlestes  found  at  the  same 
horizon  in  English  rocks,  which  he  names  Dromatherium  sylvestre. 
This  is  the  only  mammal  yet  found  in  the  Jura-Trias  of  America.  We 
give  on  page  446  figures  of  the  plants  and  animals  of  these  two  basins. 
Other  Patches. — In  other  patches,  especially  in  New  Jersey,  Penn- 
sylvania, and  Nova  Scotia,  a  number  of  reptilian  bones  and  teeth  have 
been  found.  These  latter  are  those  of  Dinosaurs,  and  Crocodilians  or 
Lacertians.  The  jaw  and  teeth  of  a  huge  reptile,  called  by  Leidy  £a- 
thygnathus  (deep  jaw),  were  found  in  Nova  Scotia.  The  teeth  were 
four  inches  long.  Cope  thinks  it  may  have  been  a  Dinosaur  ;  Leidy 
regards  it  as  Amphibian,  while  Owen  now  refers  it  to  his  order  of 
Theriodonts. 


•»•  ?12.  FIG.  713.  Fio.  714: 

FIGS.  712-714.— REPTILES  :  712.  Bathygnathus  borealis.  reduced  (after  Dawson);  a,  fifth  tooth  natural 
size ;  6,  cross-section  of  a  tooth.  713.  Belodon  Carolinensis  (after  Emmons).  714  Clepsysaurus 
Pennsylvanicus  (after  Emmons). 


Interior  Plains  and  Pacific  Slope.— The  Jura-Trias  of  the  interior 
plains  are  singularly  deficient  in  remains  of  life.  The  presence  of  gyp- 
sum in  many  of  them  furnishes  the  explanation.  These  deposits  were 
probably  formed  in  interior  and  very  salt  seas,  and  these  are  usually 
deficient  in  life.  The  two  periods  are,  however,  in  some  places  at  least, 
better  separated  than  on  the  Atlantic  slope,  probably  because  the  con- 
ditions were  more  variable.  On  the  slopes  of  the  Black  Hills  and  on 
the  South  Platte  River  undoubted  fossils  of  the  Jurassic  period  are 
found,  indicating  the  existence  of  an  open  sea  at  that  place.  In  New 
Mexico  Newberry  found  many  beautiful  impressions  of  plants,  indicat- 
ing the  same  horizon  as  in  North  Carolina  and  Virginia — i.  e.,  Upper 
Triassic.  Some  of  these  are  given  below: 

On  the  Pacific  coast,  marine  life,  no  doubt,  abounded,  as  this  was 
the  margin  of  an  open  sea;  but  the  rocks  'here  are  mostly  very  highly 
metamorphic,  and  the  fossils,  therefore,  mostly  destroyed.  Wherever 


448, 


MESOZOIC   ERA— AGE   OF   REPTILES. 


FIG    720 


FIG.  722. 

is.  715-722.-Pi.ANT6  OF  THE  JuRA-TniA9  (after  Newberry) :  715.  Branch  of  Conifer  (Braehy- 
phyllutn).  716.  Branch  of  Conifer.  717.  Conifer.  Branch  and  Fruit.  718.  Zamites  oceidentalis. 
719.  Otozamites  Macoinbii.  720.  Podozamites  crassifoiia.  721.  Ta'iiiopt.  ris  rlrirans.  '•>•>.  Alethop- 
teris  Whitneyi. 


LITE-SYSTEM. 


449 


this  is  not  the  case,  the  rocks  abound  in  fossils.  In  Humboldt  County, 
Nevada,  for  example,  the  strata  in  some  places  seem  almost  wholly 
made  up  of  Ceratites  Whitneyi  (Fig.  727).  In  the  same  locality  the 


Fio.  728.  FIG.  724 

FIGS.  7-23  and  724.— JURASSIC  FOSSILS  OF  UTAH  (after  Meek) :  723.  Belemnites  densus.    724.  Gryphaea 

calceola. 

remains  of  an  Enaliosaur  (sea-saurian)  have  been  found.  On  account 
of  the  marine  conditions  prevalent,  the  two  periods  are  easily  separable 
on  the  Pacific  coast. 

Physical  Geography  of  the  American  Continent  during  the  Jura- 
Trias  Period. — During  Palaeozoic  times  the  Atlantic  shore-line  was 
certainly  farther  east  than  it  was  subsequently,  probably  farther  east 
than  it  is  now  (p.  255).  At  the  end  of  the  Palaeozoic  occurred  the  Ap- 


FIG.  725.  FIG.  727. 

FIGS.  725-727,-CALiFORNiA  JURA-TRIAS  SHELLS:  725.  Gryphsea  speciosa  (after  Gabb). 
pandicosta  (after  Gabb).    727.  Ceratites  Whitneyi  (after  Gabb). 


5.  Trigonia 


palachian  revolution.  Coincidently  with  the  up-pushing  of  the  Appa- 
lachian chain,  the  sea-border  probably  went  downward,  and  the  shore- 
line advanced  westward  on  the  land.  During  the  Jura-Trias  the 
shore-line  to  the  north  was  still  beyond  what  it  is  now,  for  no  Atlan- 
tic border  deposit  is  visible ;  and  along  the  Middle  and  Southern  States 
it  was  certainly  beyond  the  bounding-line  of  Tertiary  and  Cretaceous 
(see  map,  p.  278,)  for  all  the  Atlantic  deposits  of  this  age  have  been 
covered  by  subsequent  strata ;  and  yet,  probably  not  much  beyond,  for 
some  of  these  Jura-Trias  patches  seem  to  have  been  in  tidal  connection 
with  the  Atlantic  Ocean.  It  is.  probable,  therefore,  that  the  shore-line 
was  a  little  beyond  the  present  New  England  shore-line,  and  a  little 
beyond  the  old  Tertiary  shore-line  of  the  Middle  and  Southern  Atlantic 
States. 

29 


450  MESOZOIC   ERA— AGE   OF  REPTILES. 

A  little  back  from  this  shore-line,  and  at  the  foot  of  the  then  Appa- 
lachian chain,  there  was  a  series  of  old  erosion  or  plication  hollows 
stretching  parallel  to  the  chain.  The  northern  ones  had  been  brought 
down  to  the  sea-level,  and  the  tides  regularly  ebbed  and  flowed  there 
then  as  in  the  bay  of  San  Francisco,  or  Puget  Sound,  at  the  present 
time.  In  the  waters  of  these  bays  lived  swimming  Reptiles,  Crocodilian 
and  Lacertian,  and  on  their  flat,  muddy  shores  walked  great  bird-like 
Reptiles,  and  possibly  reptilian  Birds.  The  more  southern  hollows 
seemed  to  have  been  above  the  sea-level,  and  were  alternately  coal- 
marsh,  and  fresh-water  lake,  emptying  by  streams  into  the  Atlantic. 
Since  that  time  the  coast  has  risen  200  or  300  feet,  and  these  patches 
are  therefore  elevated  so  much  above  the  sea-level. 

During  the  same  time  the  Basin  range  region,  i.  e.,  the  region  ly- 
ing between  the  Wahsatch  and  the  Sierra  ranges,  was  land;  but  all 
between  this  and  the  Palaeozoic  area  of  North  America,  including  the 
Plateau  region,  the  eastern  Rocky  Mountain  region,  and  the  region  of 
the  Plains,  was  covered  by  a  shallow  inland  sea,  with  imperfect  con- 
nection, or  none  at  all,  with  the  ocean,  and  in  which,  therefore,  gypsum 
deposited  by  evaporation.  At  least  once  during  Jurassic  times  this 
inland  sea  became  broadly  connected  with  the  ocean,  so  that  oceanic 
conditions  prevailed.  The  place  now  occupied  by  the  Wahsatch  Moun- 
tains was  then  a  marginal  sea-bottom,  bordering  the  Basin  region  con- 
tinent. On  the  west  the  Pacific  shore-line,  and  therefore  the  coast -line 
of  the  Basin  region  continent,  was  east  of  the  Sierra  range,  the  position 
of  that  range  being  then  also  a  marginal  sea-bottom. 

Disturbances  which  closed  the  Period.— This  long  Jura-Trias  pe- 
riod was  closed,  and  the  Cretaceous  period  inaugurated,  by  the  Sierra, 
revolution,  by  which  the  sediments  accumulated  along  the  then  Pacific 
shore  bottom,  yielding  to  the  lateral  pressure,  were  mashed  together 
and  swollen  up  into  the  Sierra  and  Cascade  ranges,  and  the  coast-line 
transferred  westward  to  the  other  side  of  these  ranges.  Coincidently 
with  this  change  probably  occurred  on  the  Atlantic  slope  the  outbursts 
of  trap,  forming  the  trap-ridges  already  spoken  of  (p.  440).  Extensive 
changes  also  occurred  at  the  same  time  over  the  whole  region  of  the 
inland  seas,  by  subsidence  and  the  inauguration  of  oceanic  conditions, 
which  continued  to  prevail  during  the  Cretaceous.  There  is  reason  to 
believe  also  that  the  Wahsatch  range,  and  perhaps  also  some  of  the 
Basin  ranges,  commenced  to  rise  at  this  time.  It  was  essentially  a 
period  of  mountain-making  in  America. 

SECTION  4. — CRETACEOUS  PERIOD. 

The  most  general  characteristic  of  this  period  is  its  transitional 
character.  In  it  Mesozoic  types  are  passing  out,  and  Cenozoic  or 
modern  types  are  coming  in,  and  the  two  types  therefore  coexist  side 


CRETACEOUS   PERIOD.  451 

by  side.  Nearly  everywhere  in  America,  as  far  as  known,  the  Creta- 
ceous lie  unconformably  on  the  Jurassic  or  still  lower  rocks. 

Rock-System — Area  in  America. — On  the  Atlantic  border  going 
southward,  we  find  no  Cretaceous  rocks  until  we  reach  New  Jersey. 
Here  we  find  a  small  patch  peeping  out  from  under  the  edge  of  the 
overlying  Tertiary,  and  marked  on  the  map  (p.  278)  by  oblique  inter- 
rupted lines.  This  patch  passes  through  New  Jersey,  Delaware,  Mary- 
land, to  the  borders  of  Virginia.  Passing  south,  we  find  no  continuous 
area  until  we  reach  Georgia ;  yet  it  underlies  the  Tertiary  in  all  this 
region,  as  is  shown  by  the  fact  that  the  rivers  in  North  and  South  Caro- 
lina cut  through  the  Tertiary  and  expose  the  Cretaceous  in  many  places. 
The  ^{/"-border  Cretaceous  commences  in  Western  Middle  Georgia, 
covers  all  the  prairie  region  of  Middle  Alabama,  the  northeastern  or 
prairie  region  of  Mississippi,  then  runs  northward  as  a  narrow  strip 
through  Tennessee  nearly  to  the  mouth  of  the  Ohio.  It  then  disap- 
pears beneath  the  Tertiary  to  reappear  as  an  area  bordering  the  Gulf 
Tertiary  on  the  west  side.  On  the  interior  plains,  the  Cretaceous  con- 
necting with  the  Gulf-border  area  stretches  northwestward  to  arctic 
regions,  occupying  nearly  the  whole  of  the  great,  grassy,  level  Western 
Plains  called  Prairies — though  much  of  it  is  overlaid  by  the  subsequent 
Tertiary.  In  the  Rocky  Mountain  region  Cretaceous  strata  occupy 
also  large  areas  in  all  the  Plateau  region — i.  e.,  the  region  between 
the  Eastern  range  and  the  Wahsatch  range — although  here  also  it  is 
largely  overlaid  by  Tertiary.  Recent  investigations  in  Mexico 1  render 
it  probable  that  this  area  stretches  also  westward  through  Northern 
Mexico  to  the  Pacific.  On  the  Pacific  border.  Cretaceous  strata  form  a 
large  part  of  the  Coast  ranges,  and  also  in  places  the  lowest  western 
foot-hills  of  the  Sierra  range.  Whitney  has  estimated  the  thickness  of 
the  Cretaceous  rocks  in  portions  of  the  Coast  range  as  20,000  feet. 

Physical  Geography  in  America. — It  is  not  difficult  from  the  Creta- 
ceous area  just  given  to  reconstruct  approximately  the  physical  geog- 
raphy. At  that  time  the  Atlantic  shore-line  in  all  the  northern  portion 
of  the  continent  was  farther  out  or  east  than  now,  for  the  Cretaceous 
of  this  part  is  all  now  covered  by  sea.  From  New  Jersey  southward 
the  shore-line  was  then  farther  in  or  west  than  now.  From  Maryland 
to  Georgia  the  shore-line,  though  farther  in  than  now,  was  farther  out 
than  during  the  Tertiary,  as  the  Cretaceous  is  covered  by  the  later  de- 
posits. The  Grulf  shore-line  was  much  more  extended  both  northward 
and  westward  than  either  now  or  in  Tertiary  times.  From  the  Gulf  there 
extended  northwestward  an  immensely  wide  sea,  covering  the  Plains 
region  and  the  Rocky  Mountain  region  as  far  westward  as  the  Wah- 
satch range,  and  dividing  the  continent  into  two  continents,  an  eastern 
or  Appalachian,  and  a  western  or  Basin  region  continent.  Probably 
1  American  Journal  of  Science,  vol.  x.,  p.  386,  1875. 


452 


MESOZOIC  ERA— AGE   OF  REPTILES. 


also  this  sea  connected  across  the  region  of  Mexico  with  the  Pacific, 
thus  dividing  the  western  continent  into  two,  a  northern  and  a  southern. 
The  Pacific  Ocean  at  that  time  washed  against  the  foot-hills  of  the 
Sierra  range.  These  facts  are  represented  in  the  accompanying  map. 
The  probable  connection  of  the  Gulf  with  the  Pacific  is  also  indicated. 


FIG.  728.— Map  of  North  America  in  Cretaceous  Times. 

Rocks. — The  rocks  of  the  Cretaceous  period  consist  of  sands,  and 
clays,  and  limestones,  as  in  other  periods,  but,  as  a  whole,  are  less  gener- 
ally metamorphic  than  the  older  rocks.  There  is,  however,  one  kind  of 
rock  found  in  this  age  in  Europe  which  is  so  peculiar  and  so  interesting 
that  it  must  not  be  passed  over  in  silence.  We  refer  to  the  white 
chalk  of  England  and  France,  from  which  the  formation  and  the  period 
take  their  name,  "  Cretaceous" 

Chalk. — Chalk  is  a  soft,  white,  pure  carbonate  of  lime.  Scattered 
through  the  soft  mass  are  found  very  characteristic  nodules  of  pure 
flint.  These  nodules  are  of  various  sizes  and  shapes,  sometimes  scat- 
tered irregularly,  sometimes  arranged  in  layers.  Often  some  fossil, 
especially  a  sponge,  forms  the  nucleus  around  which  the  aggregation  of 
the  siliceous  matter  takes  place.  On  account  of  its  extreme  softness, 
chalk  is  often  sculptured  by  erosive  agencies  into  fantastic  cliffs  and 
needles  (Fig.  729). 

Examined  with  the  microscope,  chalk  is  found  to  be  composed  largely 
of  Rhizopod  shells,  and  of  Coccoliths  and  Coccospheres  (supposed  shells 
of  uni-celled  plants),  some  perfect,  more  broken,  most  of  all  completely 
disintegrated  (Fig.  730).  The  flint-nodules,  similarly  examined  by  sec- 


CRETACEOUS  PERIOD. 


453 


tion,  show  spicules  of  sponge  and  siliceous  shells  of  Diatoms.  Chalk 
such  as  described  is  found  nowhere  except  in  Europe.  Figs.  731-734 
represent  some  of  the  more  common  Rhizopods  found  in  chalk. 


FIG.  729.— Chalk-Cliffs  witli  Flint-Noduk 


Origin  of  Chalk, — A  material  so  unique  must  have  been  formed 
under  peculiar  conditions.      Recent  investigations   have   shown   that 


FIG.  734. 


FIGS.  730-734.— FORAMINIFERA  OP  CHALK:  730.  Chalk  as  seen  under  the  Microscope  (after  Nicholson). 
731.  Cuneolina  pavonia.  732.  Flabellina  rugosa.  733.  Lituola  nautiloides.  734.  Chrysalidina  gradate 
(alter  D'Orbigny). 

chalk  is  a  deep-sea  ooze.     In  all  the  deep-sea  soundings  and  dredgings 
recently  undertaken,  it  is  found  that  the  sea -bottom  between  the  depths 


454 


MESOZOIC  ERA— AGE   OF  REPTILES. 


of  3,000  and  20,000  feet,  where  not  too  cold,  is  a  white  ooze,  consisting 
wholly  of  Rhizopod  shells  (Globigerina,  Radiolaria,  etc.)  and  Coccoliths, 
Coccospheres,  etc.,  through  which  are  scattered  siliceous  shells  of  Dia- 
toms. These  shells  are  in  every  stage  of  change :  some  living,  or  at 
least  still  retaining  sarcode ;  some  perfect,  though  dead  and  empty ; 
some  broken  ;  most  completely  disintegrated  into  an  impalpable  mud. 
From  the  great  abundance  of  one  genus  of  Rhizopods,  this  calcareous 
mud  has  been  called  Globigerina  ooze.  In  deep-sea  bottoms,  therefore, 
chalk  is  now  forming.  Also,  strange  to  say,  many  Sponges,  and  Star- 
fishes, and  Echinoids,  and  Crustaceans,  very  similar  to  those  formed  in 
the  chalk  of  Cretaceous  times,  have  been  brought  up  from  present  deep- 
sea  bottoms. 


FIG.  735.— Shells  of  Living  Foraminifera :  <?,  Orbulina  universa,  in  its  perfect  condition,  showing  the 
tubular  spines  which  radiate  from  the  surface  of  the  shell ;  b,  Globigerina  bulloides,  in  its  ordinary 
condition,  the  thin  hollow  spines  which  are  attached  to  the  shell  when  perfect  having  been  broken  off; 
c.  Textularia  variabiiis ;  d.  Peneroplis  planatus ;  e,  Kotalla  concamerata ; /,  Cristellaria  subarcuatula. 
(Fig.  a  is  after  Wyville  Thomson;  the  others  are  after  Williamson.  All  the  figures  are  greatly 
enlarged.) 

There  seems  no  doubt,  therefore,  that  chalk  is  a  profound  sea-bottom 
formation.  The  flint-nodules  have  been  formed  by  a  subsequent  process 
similar  to  that  which  gives  rise  to  other  nodules  (p.  188).  The  silica, 
which  in  the  ooze  was  at  first  scattered,  is  slowly  aggregated  into  pure 
flint-nodules,  and  the  matrix  is  left  in  a  condition  of  pure  carbonate  of 
lime. 

Extent  of  Chalk  Seas  of  Cretaceous  Times  in  Europe.— Chalk  of 


CRETACEOUS  PERIOD.  455 

nearly  homogeneous  aspect  prevails  from  the  north  of  Ireland  through 
Middle  Europe  to  the  Crimea  and  Caucasus,1  a  distance  of  1,140  miles ; 
and,  in  the  other  direction,  from  the  south  of  Sweden  to  the  south  of 
Bordeaux,  a  distance  of  840  miles  (Lyell).  It  is  evident,  therefore, 
that  at  that  time  a  very  deep  sea  occupied  a  large  portion  of  Central 
Europe.  The  white  chalk  of  England  and  France  is  about  1,000  feet 
thick.  When  we  remember  the  mode  in  which  it  has  been  formed,  this 
thickness  indicates  an  almost  inconceivable  lapse  of  time. 

Cretaceous  Coal. — Coal  is  again  found  in  large  quantities  in  rocks  of 
this  period  in  the  United  States.  The  mode  of  occurrence  is  similar  to 
that  found  in  rocks  of  other  periods. 

There  has  been,  and  still  is,  much  difference  of  opinion  and  discussion 
among  the  best  observers  as  to  the  exact  position  of  the  coal  or  lignites 
of  the  Pacific  coast,  of  the  Rocky  Mountains,  and  of  the  Plains.  Some 
have  been  referred  to  the  Cretaceous,  some  to  the  Eocene,  and  some  to 
the  Miocene-Tertiary.  With  the  exception  of  the  last,  however,  most 
or  perhaps  all  the  productive  fields  seem  to  belong  to  nearly  the  same 
horizon,  which  has  been  called  the  Great  Lignitic  formation,  and  which 
by  some  geologists  is  regarded  as  uppermost  Cretaceous,  by  others  as 
lowermost  Eocene.  The  animal  fossils  seem  to  ally  the  strata  with  the 
Cretaceous,  the  plants  with  the  Eocene. 

The  truth  is,  the  Great  Lignitic  of  the  West  seems  to  be  a  transition 
between  the  Cretaceous  and  the  Eocene.  While  it  was  depositing,  the 
changes  of  physical  geography  and  climate  which  closed  the  Cretaceous 
and  inaugurated  the  Tertiary  had  already  been  accomplished ;  but  Cre- 
taceous types  still  lingered,  ready  to  disappear.  The  death-sentence 
had  been  pronounced,  but  the  execution  was  delayed.  In  this  group, 
therefore,  Cretaceous  and  Tertiary  forms  are  more  or  less  mingled. 
This  is  precisely  what  we  might  expect ;  for  in  the  drying  up,  by  up- 
heaval, of  the  Cretaceous  interior  sea,  marine  animals  would  be  gradu- 
ally changed  into  brackish-water  and  finally  into  Tertiary  fresh-water 
animals ;  the  newly-formed  land  would  be  covered  with  a  Tertiary  vege- 
tation, but  the  Cretaceous  land  animals  would  still  continue  to  hold  out 
for  a  while. 

Since  some  of  these  fields  are  undoubtedly  Cretaceous,  it  seems  best, 
in  order  to  avoid  repetition,  to  speak  of  them  all  in  this  connection ;  but 
as  the  plants  found  are  entirely  different  from  the  Cretaceous  plants 
to  be  presently  described,  and  wholly  of  Tertiary  types,  it  seems  best, 
until  the  question  is  definitely  settled,  to  speak  of  these  under  the 
Tertiary. 

First,  on  the  Plains,  just  east  of  the  Rocky  Mountains,  there  are 
several  immense  fields:  one  on  the  Upper  Missouri  and  Yellowstone, 
another  about  Denver  and  Marshall,  and  still  another  farther  south  in 
1  Favre,  "  Archives  des  Sciences,"  vol.  xxxvii.,  p.  118,  et  seq. 


456 


MESOZOIC   ERA— AGE   OF  REPTILES. 


New  Mexico.  These  coal-fields  are  supposed  to  have  an  aggregate  ex- 
tent of  at  least  15,000  square  miles.  Beyond  the  limits  of  the  United 
States,  in  the  British  possessions,  are  found  still  other  fields  (Davvson). 
Again,  in  the  Plateau  region,  between  the  eastern  Rocky  Mountain 
range  and  the  Wahsatch  Mountains,  on  the  Laramie  Plains,  is  found  a 
very  fine  field  of  5,000  square  miles.  Again,  on  the  Pacific  slope  are 
several  important  fields :  1.  Monte  Diablo  and  Corral  Hollow  coal-field, 
in  California ;  2.  Seattle  and  Bellingham  Bay  coal-field,  of  Washing- 
ton Territory ;  3.  The  Nanaimo  and  Queen  Charlotte's  Island  coal-field, 
of  British  Columbia. 

We  recapitulate  the  coal-fields  of  the  United  States,  and  present 
them  at  one  view  in  the  following  table  : 


Carboniferous . 


Jura-Trias 


Cretaceous. 


Appalachian  

60,000  } 

Central 

47  000  1 

Western  

78^000  f 

191,700  square  miles. 

Michigan         .  .      . 

6,700  J 

'  Richmond  .  .  | 

170  ) 

Piedmont  .  .  |     " 
Deep  River.  / 

500  1 

670  square  miles 

Dan  River.,  j  "  ' 

tivv/  j 

Western  Plains.  ... 
I   Rocky  Mountains.. 

-  20,000— 

20,000  square  miles. 

]   Monte  Diablo,  etc.. 
[  Washington  

unknown. 

Total 212,370  square  miles. 

Of  which  at  least  150,000  square  miles  are  workable. 

The  Cretaceous  coals  are  usually  called  lignites,  but  they  are  really 
a  very  fair  coal,  and  quite  different  from  what  usually  goes  under  that 
name. 

Subdivisions  of  the  Cretaceous. — The  Cretaceous  in  America  is  di- 
vided into  upper  and  lower ;  in  Europe  it  is  divided  into  upper,  middle, 
and  lower,  the  chalk  being  the  upper. 


Cretaceous . . . 


AMERICAN.     ENGLISH. 
f  Upper...  .j  Upper,  or  Chalk. 

Middle,  or  Greensand. 
Lower,  or  Lower  Geeensand. 


Lower. . . 


It  is  probable  that  the  lowermost  Cretaceous  of  Europe  is  unrepre- 
sented in  the  United  States.  If  so,  the  reason  is  evident.  The  Sierra 
revolution  was  a  great  event.  A  gap  in  the  record  is  the  result.  Some 
of  the  leaves  missing  here  are  recovered  in  Europe. 

Life- System  :  Plants. 

Leaf-impressions  are  very  abundant  in  the  American  Cretaceous,  and 
the  most  cursory  examination  reveals  at  once  a  type  of  plants  not  seen 


PLANTS. 


457 


in  any  lower  rocks,  viz.,  Angiosperms,  both  Dicotyls  and  Palms.     We 
have  said  that  the  Sierra  revolution  at  the  end  of  the  Jura-Trias  pro- 


FIG.  73T,  FIG.  T38.  FIG.  739. 

FIGS.  736-739.— CRETACEOUS  PLANTS  (after  Lesquereux):  T36.  Liquidamber  inteprifolium.    737.  Sassafras 
Mudgei.    738.  Lauras  Nebrascensis.    739.  Quercus  primordialis.    All  reduced. 

duced  great  change  in  America.     It  is  probable  that  a  break  occurs  in 
the  record  here.     When  the  record  commences  again  with  the  Creta- 


4:58 


MESOZOIC  ERA— AGE   OF  REPTILES. 


ceous,  we  observe  a  very  great  difference  in  the  subject  matter.  The 
whole  aspect  of  field  and  forest  must  have  been  different  and  much  more 
modern.  Nearly  all  the  genera  of  our  modern  trees  are  present,  e.  g., 
Oaks,  Maples,  Willows,  Sassafras,  Dogwood,  Hickory,  Beech,  Poplar, 


FIG.  742. 


FIGS.  740-743.— CBETACEOF  8  PLANTS  (after  Lesquereux) :  740.  Sassafras  araliopsis.   741.  Sails  protese- 
folia.    742.  Fa^us  polyclada.    743.  Protophyllum  quadratum.    All  reduced. 

Tulip-tree  (Liriodendron),  Walnut,  Sycamore,  Sweet^gum  (Liquidam- 
ber),  Laurel,  Myrtle,  Fig,  etc.  Out  of  130  species  of  plants  found  in 
the  Cretaceous  of  Nebraska,  about  110  species  are  Dicotyls,  and  at  least 
half  of  these  belong  to  living  genera  (Lesquereux).  And  if  we  include 


ANIMALS. 


459 


the  Lignitic  in  the  Cretaceous  we  may  add  200  more  species  to  the 
list,  but  these  latter  are  quite  different  and  Tertiary  in  type.  A  few 
Palms  have  also  been  found  in  Vancouver's  Island. 

It  is  a  noteworthy  fact  that  many  of  the  most  characteristic  Creta- 
ceous genera,  and  those  most  abundant  and  varied  in  species  at  that  time, 
are  now  represented  by  only  one  or  two  species.  For  example,  there 
are  now  only  two  species  of  Sassafras  ;  one  species  of  Plane-tree  ;  one 
of  Liriodendron ;  and  one  of  Liquidamber.  These  are  evidently  the 
remnants  of  an  extinct  flora. 

But  if  the  highest  plants,  the  Dicotyls,  are  abundant,  so  are  also 
the  lowest  Protophytes  or  uni-celled  plants.  Diatoms,  Desmids,  Cocco- 
spheres,  are  abundant  in  the  chalk  of  Europe.  If  they  are  not  found  in 
America,  it  is  only  because  deep-sea  deposits  have  not  yet  been  found 
there. 

Animals. 

Protozoa. — As  already  stated,  chalk  is  made  up  almost  wholly  of 
shells  of  ForaminiferEe  (Rhizopods)  and  of  certain  uni-celled  plants. 
According  to  Ehrenberg,  a  cubic  inch  often  contains  millions  of  micro- 
scopic organisms.  More  than  120  species  of  Foraminifers  have  been 


FIGS.  744,  745.— CRETACEOUS  SPONGES  :  744.  Siphonia  flcns.    745.  Ventriculites  simplex. 

found  in  the  English  chalk  alone.  Some  of  these  seem  to  be  species 
still  living  in  deep  seas.  These  a/e  all  extremely  minute,  but  some  of 
larger  size  are  found  in  the  Cretaceous  limestone  of  Texas.  Those  from 
the  chalk  have  already  been  given. 

Sponges  are  extremely  common  in  the  chalk,  as  they  are  also  in 


460 


MESOZOIC   ERA— AGE   OF  REPTILES. 


deep-sea  bottoms  of  the  present  day.     About  one  hundred  have  been 

found  in  the  chalk. 

Echinoderms. — The  free    Echinoderms    are   now  for  the  first  time 

in  excess  of  the  stemmed.     Only  very  recently,  the  first  Crinoid  yet 

found  in  the  American  Cretaceous  has  been  obtained  by  Marsh,  and 

described  by  Grinnell.  The  Tfin- 
tacrinus  socialis  (Fig.  746)  was 
a  free  Crinoid  like  the  Marsupites 
of  the  English  chalk,  or  the  Coma- 
tula  of  the  present  seas.  They 
seemed  to  have  lived  together  in 
great  numbers  in  Cretaceous  times 
in  the  region  of  the  Uintah  Moun- 
tains, then  a  Cretaceous  sea.  Fig. 
746  represents  the  body ;  the  arms 
were  exceedingly  numerous  and 
complex.  The  Echinoids  are  es- 
FIG.  746.-uintacrinus  socialis  (after  Grinneii).  pecially  abundant  and  decidedly 


Fio.  749. 

FIGS.  747-749.—  ECUINOIDB  or  THE  CRETACEOUS  OF  EUROPE:  747.  Galerites  albogalerus.    748.  Discoidea 
cylindrica.    749.  Goniopygus  major. 


ANIMALS. 


461 


modern  in  type ;  and  in  the  chalk  some  genera  are  identical  with, 
and  some  species  very  similar  to,  those  recently  gotten  from  deep-sea 
ooze.  The  above  are  from  the  European  Cretaceous. 


FIG. 

FIGS.  750-753.— CRETACEOUS  BEACHIOPODS  AND  LAMELLIBRANCHS— Brachiopods  :  750.  Terebratula  As- 
tieriana.  —  Lamellibranchs :  751.  Ostrea  Idriaensis  (after  Gabb).  752.  Inoceramus  dimidius  (after 
Meek).  753.  Exogyra  costata  (after  Owen). 

JVToUusks. — For  the  first  time  Lamdlibranchs  are  fairly  in  excess  of 
Brachlopods.  Among  the  latter  the  modern  family  of  Terebratulse  are 
especially  conspicuous  (Fig.  750).  Among  the  former  the  most  note- 
worthy fact  is  the  abundance  of  the  Oyster  family —  Ostrea,  Gryphcea, 
Exogyra,  etc.;  and  the  Avicula  family,  Avicula, Inoceramus,  etc.,  some 
of  which  are  of  great  size. 


462 


MESOZOIC  ERA— AGE   OF  REPTILES. 


Another  very  strange  and  characteristic  group  of  shells  found  here 
are  the  Rudistes  or  Hippuritidce.  In  this  family  one  valve  is  compara- 
tively small,  and  often  flat,  while  the  other  is  enormously  deep  and 
elongated  in  the  shape  of  a  covv's-horn  in  Hippurites  and  Radiolites 
(Fig.  754),  or  in  the  shape  of  a  closely-coiled  ram's-horn  in  Caprinella 
and  Caprina  (Fig.  757).  The  figures  are  taken  from  foreign  localities, 
but  similar  forms  exist  also  in  this  country. 


Fio.  754. 


FIGS.  754-757.— 754.  Hippurites  Toucasiana,  a  large  individual  with  two  small  ones  attached  fatter 
d'Orbipny).  755.  Section  of  a  Radiolites  cylindriasus,  showing  structure.  756.  Upper  Valve  of 
Eadiolites  mammelaris.  757.  Caprina  adversa  (after  Woodward). 

Among  Gasteropoda,  the  beaked  or  siphonated  kinds  are  now  for 
the  first  time  abundant,  as  in  the  present  seas  (Figs.  758-760). 

Among  Cephalopods  the  Ammonites  and  Belemnites  still  continue 
in  great  numbers  and  size,  but  they  die  out  at  the  end  of  this  period 
forever.  In  the  Cretaceous  of  the  "Western  Plains  some  Ammonites  have 


ANIMALS. 


463 


been  found  over  three  feet  in  diameter  (Dana).  This  family  seemed  to 
have  reached  its  culmination  just  before  its  extinction.  But  what  is 
still  more  remarkable  is  the  introduction  of  many  new  genera  of  very 
strange  and  unexpected  forms.  These  are  sometimes  partly  uncoiled, 


FIGS.  75S-760.  —  CRETACEOUS  GASTEROPODS  :  758.  Cyprsea  Matthewsonii  (after  Gabb).    759.  Aporrhais 
falciforinis  (after  Gabb).    760.  Scalaria  billimani  (after  Lesquereux). 

as  in  Scaphites  (boat),  Crioceras  (ram's-horn),  Toxoceras  (bow-horn), 
Ancyloceras  (hook-horn),  Hamites  (hook);  sometimes  completely  un- 
coiled, as  in  faculties  (walking-stick)  ;  sometimes  coiled  spirally,  like 
a  Gasteropod,  as  in  Turrulites  and  Helioceras.  Belemnites  also  con- 
tinue, though  in  diminishing  numbers. 


FIG.  761.— Belemnites  impressus  (after  Gabb). 


These  strange  forms  have  been  likened  by  Agassiz  to  death-contor- 
tions of  the  Ammonite  family ;  and  such  they  really  seem  to  be.  From 
the  point  of  view  of  evolution,  it  is  natural  to  suppose  that  under  the 
gradually-changing  conditions  which  evidently  prevailed  in  Cretaceous 
times,  this  vigorous  Mesozoic  typo  would  be  compelled  to  assume  a 


464 


MESOZOIC   ERA— AGE   OF   REPTILES. 


great  variety  of  forms,  in  the  vain  attempt  to  adapt  itself  to  the  new 
environment,  and  thus  to  escape  its  inevitable  destiny.    The  curve  of  its 


Fro.  768. 

Fins  762-768  —CRETACEOUS  CEPHALOPODS  :  762.  Ammonites  Chicoensis  (after  Gabb).  763.  Scaphites 
Kqualis  (after  Pictet).  764.  Crioceras,  restored  (after  Pictet).  765.  Heliooeras  Kobertinnnsjtafter 
Pictet).  766.  Ancyloceras  percostatus,  x  J  (after  Gabb).  767.  BacuUtes  anceps,  x  $  (after  Wood- 
ward). 768.  Turnilites  catenatus  (after  D'Orbigny). 


ANIMALS. 


465 


rise,  culmination,  and  decline,  reached  its  highest  point  just  before  it 
was  destroyed.  The  wave  of  its  evolution  crested  and  broke  into 
strange  forms  at  the  moment  of  its  dissolution. 

Among  Crustaceans,  the  Brachyurans,  short-tailed  Crustaceans 
(crabs),  which  were  barely  introduced  in  the  Jurassic,  are  here  repre- 
sented by  several  genera. 

Vertebrates — Fishes. — In  the  development  of  this  class  some  decided 
steps  in  advance  are  here  recorded.  Placoids  and  Ganoids  still  con- 


no.  771. 

FIGS.  769-771.— CRETACEOUS  FISHES—  Placoids:  769.    Otodus  (after  Leidy) ;   770.  Ptychodns  Mortoni 
(after  Leidy).— Teleosts:  771.  Portheus  moloseus— Tooth,  natural  size  (after  Cope). 

tinue,  but  Teleosts,  or  true  typical  modern  fishes,  are  here  introduced 
for  the  first  time,  and  in  considerable  numbers,  and  some  of  gigantic 
30 


466 


MESOZOIC  ERA— AGE  OF  REPTILES. 


size.  These  earliest  Teleosts  were  related  to  salmon,  herring,  perch, 
pike,  etc.  Beryx,  a  genus  still  found  in  open  seas,  is  found  in  the  Chalk 
of  Europe  and  Upper  Cretaceous  of  America.  Among  Placoids,  too, 
although  the  Cestracionts  and  Hybodonts  continue  (the  latter,  however, 
passing  out  with  the  Cretaceous),  the  modern  type,  the  true  sharks  or 


FIG.  772. 


FIG.  7T4. 

FIGS.  772-774.— CBETACEOITS  FISHES—  Teleosts:  772.  Portheus,  restored, 
Lewesiensis.    774.  Osmeroides  ManteUi. 


aV  (after  Cope).    773.  Beiyx 


Squalodonts,  having  lancet-shaped  teeth,  are  for  the  first  time  abundant. 
Above  we  give  figures  of  Cestraciont  and  Squalodont  teeth,  and  also  a 
tooth,  natural  size,  of  a  gigantic  pike,  eight  feet  long,  from  American 
Cretaceous,  and  a  restoration  of  the  same  by  Cope ;  also,  two  Teleosts 
from  European  Cretaceous. 

The  Hybodonts  were  essentially  a  Mesozoic  type ;  the  Squalodonts 
are  essentially  Tertiary  and  modern.    The  two  types  coexist  in  the  Cre- 


ANIMALS.  467 

taceous,  the  former  passing  out,  the  latter  increasing,  and  finally  dis- 
placing the  former. 

Cope  gives  ninety-seven  species  of  North  American  Cretaceous 
fishes  known  in  1875.  Of  these,  if  we  include  the  Chimera  family,  an 
aberrant  type  of  Placoids  very  common  in  the  Cretaceous,  forty-jive 
were  Placoids.  The  rest  are  mostly  Teleosts,  for  the  Ganoids  are  rapid- 
ly disappearing.  In  Europe,  twenty-five  genera  of  Cycloids  and  fifteen 
of  Ctenoids  are  found  in  the  Cretaceous  (Dana). 

Reptiles. — This  class  seems  to  have  culminated  about  the  end  of  the 
Jurassic  or  the  beginning  of  the  Cretaceous  period.  If  their  remains 
are  more  abundant  in  the  Jurassic  in  Europe,  they  are  far  more  abun- 
dant in  the  Cretaceous  in  America.  In  fact,  we  had  here  in  America 
during  that  time  an  extraordinary  abundance  and  variety  of  reptilian 
life,  including  all  the  principal  orders  already  mentioned,  viz.,  Enalio- 
saurs,  Dinosaurs,  Pterosaurs,  and  Crocodilians,  and  also  a  new  type,  in- 
troduced in  the  Cretaceous  for  the  first  time,  the  Mosasaurs,  wholly 
marine  in  habits,  but  of  long,  slender,  snake-like  form,  and  attaining  the 
greatest  length  yet  known  among  reptiles.  Turtles  were  also  found  in 
large  numbers  and  of  great  size.  We  can  mention  only  a  very  few 
of  the  most  remarkable  of  the  Cretaceous  reptiles. 

Among  Enaliosaurs  Leidy  describes  one  Di'scosaur  (Elasmosaur, 
Cope)  allied  to  the  Plesiosaur,  which  was  fifty  feet  long,  with  a  neck  of 


^pww^p***— 

FIG.  775.— Teeth  of  Hadrosaurus  (after  Leidy) :  a,  Pavement  of  Teeth ;  6  and  c,  Tooth  separated. 

sixty  vertebras  and  twenty-two  feet  long.  Among  Dinosaurs  the  Ha- 
drosaur  from  New  Jersey  was  twenty-eight  feet  long;  and,  judging 
from  the  huge  size  of  its  hind-legs  and  massiveness  of  its  hips  and  small 
size  of  its  fore-legs,  it  seems  to  have  been  able  to  stand  and  walk  in  the 
manner  of  birds  (Fig.  776).  This  animal  was  a  vegetable-feeder,  with 
teeth  somewhat  like  those  of  the  Iguanodon,  but  set  in  several  rows,  so 
as  to  form  a  kind  of  tessellated  pavement  (Fig.  775).  From  the  same 


468 


MESOZOIC   ERA— AGE   OF  REPTILES. 


locality  the  Dryptosaurus  (Lcelaps),  similar  to  the  Megalosaur,  and 
twenty-four  feet  long,  and  the  Ornithotarsus  (bird-shank),  thirty-five 
feet  long,  stood  twelve  to  fifteen  feet  high  when  walking 
on  their  hind-legs.  The  largest  of  the  group  is  Titanosau- 
rus,  recently  described  by  Marsh.  It  was  found  in  Colo- 
rado, and  was  at  least  fifty  feet  in  length.  Among  Ptero- 
saurs, Marsh  has  found  in  the  Western  Cretaceous  the 
remains  of  at  least  six  species;  two  of  which  were  twenty 
to  twenty-five  feet  in  alar  extent,  and  another  eighteen 
feet. 

The  American  Pterosaurs  differ  from  all  other  known 
Pterosaurs  in  the  fact,  recently  brought  to  light  by  Marsh, 
that  their  jaws  were  entirely  toothless,  and  probably  sheathed 
with  horn,  as  in  birds.  They  have  therefore  been  placed 
by  Marsh  in  a  distinct  order,  Pteranodontia,  from  the  type 
genus  Pteranodon  (winged-toothless).  Probably  all  the 

t   American  Pterosaurs  belong  to  this  order.     One  of  them, 
P.  ingens^  had  toothless  jaws  four  feet  long,  and  an  ex- 
£    panse  of  wing  of  twenty-two  feet. 

Among  the  many  Chelonians  (turtles)  found  in  the 
g.  Cretaceous  of  the  Western  Plains,  of  the  Rocky  Mountain 
I"  region,  and  of  New  Jersey,  one,  the  Atlantochelys  gigas, 
|  had  a  length  of  nearly  thirteen  feet,  and  a  breadth  across 
|  the  extended  flippers  of  fifteen  feet  (Cope).  The  structure 
I  of  this  huge  turtle  was  singularly  embryonic.  The  flattened 


FIG.  776.— Hadrosaurus  (restored  by  Hawkins). 


ANIMALS. 


469 


ribs,  which  by  their  coalescence  make  the  greater  part  of  the  shell,  of 
a  turtle,  were  in  this  species,  as  in  the  embryo  of  modern  turtles,  not 
yet  coalesced. 

But  the  most  remarkable  and  characteristic  reptiles  found  in  the  Cre- 
taceous are  the  Mosasaurs  (Pythonomorpha  of  Cope).  The  first  speci- 
men of  the  order  was  found  in  Europe,  on  the  river  Meuse,  and  hence 
the  name  Mosasaurs ;  but  they  seem  to  have  been  far  more  abundant  in 


FIG.  778. 


FIG.  780. 


FIGS.  778-781.—  778.  Head  of  a  Tylosaurus  micromus, 
x  T'5  (after  Marsh).  780.  Tooth  of  a  Mosasaurus, 
(Clidastes),  x  1  (after  Cope). 


\  (after  Marsh).    779.  Paddle  of  a  Lestosaurus, 
i  (after  Leidy).    781.  Jaw  of  an  Edestosaurus 


America.  At  least  fifty  species  (Cope)  have  been  found  in  the  Cre- 
taceous of  New  Jersey,  the  Gulf  States,  and  Kansas.  Of  these,  the 
Mosasaurus  princeps  was  sixty  to  seventy  feet  long,  and  Tylosaurus 
dyspelor,  probably  the  "  longest  known  reptile,  attained  a  length  equal 


470  MESOZOIC  ERA— AGE  OF  REPTILES. 

to  the  longest  whale"  (Cope).  These  reptiles  seem  to  have  united  the 
long,  slender  form  of  a  snake,  and  the  short,  strong,  well-fingered  pad- 
dles of  a  whale,  with  the  essential  characters  of  a  lizard.  Another 
snake-like  character  possessed  by  this  order  was  rows  of  teeth  on  the 
palatal  bones,  in  addition  to  those  in  the  jaws ;  and  a  peculiar  loose 
and  movable  articulation  of  the  lower  jaws,  by  means  of  which,  when 
aided  by  the  recurved  teeth,  the  jaws  could  act  separately  like  arms,  in 
dragging  down  their  throats  prey  which  was  too  large  to  swallow  di- 
rectly (Fig.  781).  It  is  these  snake-like  characters  which  have  pro- 
cured for  them  the  family  name  of  Pythonomorpha  (Cope). 

We  give  on  page  468  a  restoration  by  Cope  of  one  of  most  slender 
forms — Edestosaurus — and  also,  on  page  469,  head  and  tooth,  and  pad- 
dle and  jaw,  of  other  Mosasaurs. 

According  to  Cope,  147  species  of  reptiles  have  been  described  from 
the  Cretaceous  of  North  America,  of  which  fifty  are  Mosasaurs,  forty- 
eight  Testudinata  (turtles  and  tortoises),  eighteen  Dinosaurs,  four- 
teen Crocodilians,  thirteen  Sauropterygia  (Plesiosaur-like),  and  four 
Pterosaurs  At  least  two  more  Pterosaurs  have  been  found,  making 
the  whole  number  now  six  (Marsh). 

In  Europe,  Iguanodons,  Teleosaurs,  Ichthyosaurs,  Plesiosaurs,  and 
Pterosaurs  still  remain,  some  of  the  last  being  twenty-five  feet  in  ex- 
panse of  wing  ;  and  also  a  few  Mosasaurs  were  introduced. 

Birds. — The  history  of  the  discovery  of  the  earlier  fossil  birds  is  in- 
structive. Until  1858,  with  the  exception  of  the  doubtful  tracks  in 
the  Connecticut  River  sandstone,  no  birds  had  been  found  lower  than 
the  Tertiary.  In  that  year  the'  bones  of  a  bird,  probably  related  to  the 
gull,  was  found  in  the  upper  greensand  of  England.  In  1862  the  won- 
derful reptilian  bird  Archceopteryx  macroura,  already  described  (p. 
436),  was  found  in  the  Solenhofen  limestone  of  Germany  (Upper  Juras- 
sic). In  1870  and  1871  Marsh  discovered  in  the  Cretaceous  of  New 
Jersey  and  Kansas  sixteen  species  of  birds  :  five  Grallatores  (waders), 
like  the  Rail,  Snipe,  etc. ;  five  Natatores  (swimmers),  allied  to  Cormorants, 
Divers,  etc. ;  and  six  wonderful  Toothed  birds,  entirely  different  from 
any  existing  order.  These  were  the  most  extraordinary  birds  which 
have  ever  been  discovered.  Three  of  them,  belonging  to  the  two  gen- 
era Ichthyornis  and  Apatornis,  were  without  the  horny  beak  so  char- 
acteristic of  existing  birds,  but  instead  had  thin,  long,  slender  jaws, 
furnished  with  many  sharp,  conical  teeth,  set  in  sockets,  twenty  on 
each  side  below,  and  perhaps  as  many  above  (Fig.  782).  Their  verte- 
brae were  amphiccelous  or  bi-concave,  as  in  fishes  and  many  extinct  rep- 
tiles, but  in  no  modern  bird  (Fig.  783).  Like  modern  birds,  however, 
they  had  a  keel  on  the  breast-bone  for  the  attachment  of  the  powerful 
muscles  of  flight.  The  tail  has  not  been  found,  but  it  was  possibly 
vertebrated,  like  that  of  the  Jurassic  Archaeopteryx,  but  shorter  and  not 


ANIMALS. 


471 


so  reptilian  (Marsh).  These  birds  were  about  the  size  of  a  pigeon,  and 
were  evidently  capable  of  flight.  The  three  other  toothed  birds  had 
teeth  set  in  grooves  instead  of  distinct  sockets  (Fig.  784),  and  differed 
also  in  having  no  keel  and  in  having  ordinary  bird-vertebras  (Fig.  785). 


FIG.  783. 


FIGS.  792-786.— ODONTOKNirura  (after  Marsh) :  782.  Lower  Jaw  of  Ichthyornis  dispar,  x  2.  788.  Cervical 
Vertebra  of  same,  x  2.  784.  Lower  Jaw  of  Hesperornis  regalis,  x  J.  785.  Dorsal  Vertebra,  x  J. 
766.  Tooth  of  same,  x  2. 

These  were  evidently  divers,  and  incapable  of  flight.  Two  of  them — 
Hesperornis  regalis  and  Lestornis  crassipes — were  of  gigantic  size, 
being  from  five  to  six  feet  from  snout  to  toe.  Below  (Fig.  787)  we 


472 


MESOZOIC   ERA— AGE   OF  REPTILES. 


give  a  restoration  by  Marsh  of  this  remarkable  bird.  In  these  birds, 
therefore,  we  have  the  most  extraordinary  combination  of  bird  charac- 
ters with  reptilian  and  fish  characters.  So  extraordinary  and  excep- 
tional is  this  combination  of  characters,  that  Marsh  believes  he  is  jus- 
tified in  placing  them  not  only  in  new  orders —  Odontotormce  (socket- 
toothed)  and  Odontolcce  (teeth  in  groove) — but  even  in  a  new  sub-class 
—  Odontornithes  (toothed  birds).  Yet,  exceptional  as  these  characters 
may  seem,  they  are  just  what  the  law  of  evolution  would  lead  us  to 
expect  in  the  earliest  birds.  As  already  stated  (p.  439),  this  branch 


FIG.  787.— Hesperomis  regalfc  (restored  by  Marsh). 

had  not  yet  been  fairly  separated  from  the  reptilian  stem.  It  is  a  note- 
worthy fact  that  these  toothed  birds  lived  at  the  same  time  and  in  the 
same  localities  with  the  toothless  Pterosaurs  mentioned  on  page  468. 

Mammals. — It  is  a  most  remarkable  fact  that  although  Marsupial 
mammals  have  been  found  in  the  Jurassic,  and  probably  existed  in  con- 
siderable numbers  then,  yet  not  one  has  been  found  in  the  Cretaceous. 
It  is  certain,  however,  that  they  existed  at  that  time,  for  they  are  found 
in  the  Tertiary  of  Europe,  and  still  exist  in  Australia  and  elsewhere ; 
and  it  is  a  well-established  law  in  Paleontology  that  if  a  type  becomes 


CONTINUITY   OF  THE   CHALK.  4.73 

extinct  it  never  reappears:  Evolution  never  goes  backward:  Nature 
never  repeats  herself.  It  is  probable,  therefore,  that  during  the  Creta- 
ceous the  Marsupials  which  doubtless  existed  had  been  driven  to  some 
other  portion  of  the  earth,  where  we  shall  yet  find  their  remains  when 
our  knowledge  of  the  geology  of  the  globe  is  more  complete ;  and  in 
them  we  shall  also  probably  find  the  transitions  to,  or  earliest  progeni- 
tors of,  the  True  mammals  of  the  Tertiary. 

Continuity  of  the  Chalk. 

It  is  probable  that  the  deep  Atlantic  Ocean  bottom,  where  chalk  is 
now  forming,  is  continuous  with  the  chalk  of  England  and  Central 
Europe.  In  other  words,  in  Cretaceous  times  a  deep  sea  ran  from  the 
mid-Atlantic  far  into  what  is  now  Central  Europe,  and  in  the  whole  of 
this  deep  sea  chalk  was  then  formed.  At  the  end  of  the  Cretaceous 
period  the  eastern  part  was  raised  and  formed  a  portion  of  Europe,  while 
the  rest  remained  as  deep-sea  bottom,  and  continued  to  make  chalk  until 
now.  Thus  there  is  no  doubt  that  in  the  deep  Atlantic,  off  the  coast 
of  Europe,  there  has  been  an  unbroken  continuity  of  chalk-making 
from  the  Cretaceous  times  until  now.  But  we  have  seen  (p.  454)  that 
many  of  the  living  deep-sea  species  are  identical  with,  and  nearly  all  ex- 
tremely similar  to,  those  found  in  the  chalk  of  Cretaceous  times.  Thus 
there  has  been  not  only  a  continuity  of  chalk-formation,  but  also  to 
some  extent  of  the  chalk-fauna,  to  the  present  time. 

These  facts  were  certainly  unexpected,  but,  so  far  from  shaking  tha 
foundations  of  geological  science,  as  some  have  imagined,  they  are  in 
perfect  accordance  with  the  fundamental  principles  of  geological  suc- 
cession properly  understood ;  as  we  now  proceed  to  show : 

1.  The  facts  of  identity  have  been  exaggerated.  Many  of  the 
Foraminifera  only  are  identical.  Among  echinoderms  the  identity 
is  generic,  not  specific.  2.  In  comparing  higher  with  lower  species, 
we  find  that  the  lower  species  are  widely  distributed  both  in  space 
(geographically)  and  in  time  (geologically),  and  that  the  continuance 
or  range  in  time  becomes  less  and  less  in  proportion  as  we  rise  in  the 
scale.  Thus,  referring  to  diagram,  Fig.  788,  page  476  (under  Tertiary), 
constructed  to  illustrate  this  point,  we  see  that  living  species  of  mam- 
mals extend  back  only  a  little  way  into  the  Quaternary,  living  species  of 
mollusks  back  to  the  beginning  of  the  Tertiary,  while  living  species  of 
Foraminifera,  as  we  might  expect,  extend  back  into  the  Cretaceous. 
3.  There  is  a  necessary  relation  between  fauna  and  external  conditions. 
Changes  in  the  latter  determine  corresponding  changes  in  the  former. 
Now,  deep-sea  conditions  are  evidently  far  less  subject  to  change — far 
more  continuous — than  shallow-water  and  land  conditions.  For  this 
reason,  we  should  expect  deep-sea  faunae  to  change  very  slowly.  4.  But 
this  cannot  affect  the  geological  chronology,  because  this  chronology 


474  MESOZOIC  ERA— AGE  OF  REPTILES. 

rests  almost  wholly  on  the  remains  of  shallow-water  and  land  animals. 
Chalk  is  the  only  profound  sea-bottom  formation  certainly  known..  It 
is,  therefore,  wholly  exceptional.  5.  The  reason  it  is  exceptional  is 
that,  as  a  broad  general  fact,  the  present  continents  have  been,  through 
all  geological  times,  steadily  heaved  upward  out  of  the  ocean,  growing 
larger  and  higher ;  and,  therefore,  the  successive  additions  have  been 
nearly  always  shallow  marginal  bottoms  and  shallow  interior  seas. 
That  the  exception  should  occur  in  Europe  rather  than  in  America,  too, 
is  in  keeping  with  the  general  character  of  the  development  of  the 
European  as  contrasted  with  the  American  Continent.  6.  Conversely, 
the  fact  that  chalk  is  so  exceptional  is  proof  of  the  development  of 
continents  as  indicated  under  the  last  head— proof  that,  as  a  general 
fact,  the  great  inequalities  of  the  earth's  crust,  which  constitute  land- 
surfaces  and  sea-bottoms,  have  remained  substantially  unchanged  in 
position  from  the  first,  while  steadily  increasing  in  vertical  dimen- 
sions. 

General  Observations  on  the  Mesozoic. 

The  Mesozoic,  and  especially  the  Jurassic,  is  characterized  by  the  cul- 
mination of  two  great  classes  of  animals,  viz.,  Cephalopod  Mollusks  and 
Reptiles,  and  one  of  plants,  the  Cycads.  This  is  shown  in  the  diagram 
on  page  270.  The  culmination  of  reptiles  is,  of  course,  its  most  distin- 
guishing characteristic.  That  it  was  preeminently  an  age  of  Reptiles, 
may  be  shown  by  a  comparison  of  its  reptilian  fauna  with  that  of  the 
present  day.  There  are  now,  on  the  whole  face  of  the  earth,  only  six 
large  reptiles  over  fifteen  feet  long — two  in  India,  one  in  Africa,  three 
in  America — and  none  over  twenty-five  feet  long.  In  the  Wealden  and 
Lower  Cretaceous  of  Great  Britain  alone  there  were  five  or  six  great 
Dinosaurs  twenty  to  sixty  feet  long,  ten  to  twelve  Crocodilians  and 
Enaliosaurs  ten  to  fifty  feet  long,  besides  Pterodactyls,  turtles,  etc. 
(Dana).  Again,  in  the  Cretaceous  of  the  United  States  alone  the  full- 
ness of  reptilian  life  was  even  greater ;  for  147  species  of  reptiles  have 
been  found,  most  of  them  of  gigantic  size.  Among  these  were 
fifty  species  of  Mosasaurs,  some  seventy  to  eighty  feet  long;  many 
huge  Dinosaurs,  twenty  to  fifty  feet  long  ;  besides  Enaliosaurs,  Ptero- 
saurs, and  gigantic  turtles  (Cope).  These  are  preserved!  But  the 
known  fossil  fauna  of  any  period  is  but  a  fragment  of  the  actual  fauna 
of  that  period.  Not  only  did  reptiles  greatly  predominate,  but  the  age 
seemed  to  impress  its  reptilian  character  on  all  other  higher  animals 
existing  at  that  time.  The  birds  were  reptilian  birds,  the  mammals 
were  reptilian  mammals.  All  animals  as  yet  were  oviparous  (birds  and 
reptiles)  or  semi-oviparous  (marsupials).  It  is  difficult  to  imagine  the 
size  of  egg  produced  by  an  Iguanodon  or  a  Cetiosaurus. 

That  the  climate  was  then  warm  and  uniform  is  sufficiently  attested 
by  the  character  of  the  fauna  and  flora.  All  great  reptiles  and  all  Cy- 


CENOZOIC  ERA— AGE    OF  MAMMALS.  475 

cads  and  Tree-ferns  are  found  now  only  in  tropical  or  sub-tropical  re- 
gions. This  tropical  fauna  and  flora  were  substantially  similar  in  all 
latitudes  in  which  the  strata  have  been  found — even  as  far  north  as 
Spitzbergen  (Nordenskiold).1  During  the  latter  portion  of  the  Creta- 
ceous period,  as  indicated  by  the  abundance  of  deciduous  Dicotyls,  the 
climate  of  North  America  had  become  cooler,  being  about  8°  or  10° 
warmer  than  now. 

Disturbance  which  closed  the  Mesozoic. — The  disturbance  which 
in  America  closed  the  Cretaceous  period  and  the  Mesozoic  era  was 
a  bodily  upheaved  of  the  whole  western  half  of  the  continent,  by  which 
the  great  interior  Cretaceous  sea,  which  previously  divided  Amer- 
ica into  two  continents,  was  abolished,  and  the  continent  became  one. 
At  the  same  time  the  Wahsatch  and  Uintah  Mountains  were  principally 
formed,  and  the  eastern  Rocky  Mountain  range  greatly  elevated.  If 
the  end  of  the  Jurassic  was  preeminently  a  time  of  mountain-making 
(Sierra  revolution),  the  end  of  the  Cretaceous  was  preeminently  a  time 
of  continent-making.  The  disturbance,  as  usual  with  those  which  close 
an  era,  was  probably  to  some  extent  oscillatory,  i.  e.,  the  continent  was 
probably  higher  and  cooler  during  the  latter  part  of  the  Cretaceous  than 
during  the  subsequent  Eocene.  The  change  of  physical  geography  was 
enormous,  and  the  change  of  climate  was  doubtless  correspondingly 
great.  We  ought  to  be  prepared,  therefore,  to  find,  with  the  opening 
of  the  next  era,  a  very  great  change  in  the  organisms. 


CHAPTER  V. 

CENOZOIC  ERA— AGE  OF  MAMMALS. 

THIS  deserves  the  rank  of  a  distinct  era,  and  the  corresponding  rocks 
that  of  a  distinct  system,  because  there  is  here  a  great  break  in  the  rock- 
system,  and  a  still  greater  break  in  the  life-system.  Between  the  rocks  of 
the  Cretaceous  and  Tertiary  there  is,  in  Europe,  universal  unconformity. 
In  America,  on  the  contrary,  especially  on  the  Western  Plains,  there 
seems  to  be  in  some  places  a  continuous  series  of  conformable  rocks  con- 
necting the  two  eras  (Hayden).  The  record  seems  to  be  continuous. 
Yet  here,  no  less  than  in  Europe,  there  is  at  a  certain  horizon  a  rapid 
and  most  extraordinary  change  in  the  life-system.  This  it  seems  impos- 
sible to  explain  on  the  theory  of  evolution  unless  we  admit  periods  of 
rapid  evolution.  The  reason  why  there  is  no  general  unconformity  in 
America  is,  evidently,  that  the  movement  here  was  continental,  and  not 
1  Geological  Magazine,  November,  1875. 


476 


CENOZOIC   ERA— AGE   OF   MAMMALS. 


mere  mountain-making  and  strata-crushing.  Such  continental  move- 
ments, however,  would  produce  very  great  changes  in  climate,  and  there- 
fore in  organic  forms.  The  end  of  the  Jurassic  was  a  period  of  moun- 
tain-making, and  therefore  of  unconformity — the  end  of  the  Cretaceous, 
a  time  of  continent-making,  and  but  little  unconformity,  but  very  great 
change  of  climate.  Therefore,  although  the  interval  lost  in  America  is 
far  greater  at  the  end  of  the  Jurassic,  the  change  of  fauna  and  flora  was 
far  greater  at  the  end  of  the  Cretaceous. 

General  Characteristics  of  the  Cenozoic  Era. — As  indicated  by  the 
name,  modern  history  commences  here ;  modern  types  were  introduced 
or  became  predominant;  the  present  aspect  of  field  and  forest  commences. 
Then,  as  now,  the  rulers  of  the  seas  were  great  sharks  and  whales;  the 
rulers  of  the  land,  mammalian  quadrupeds;  and  the  rulers  of  the  air, 
birds  and  bats.  Many  of  the  genera  and  some  of  the  species  of  both 
animals  and  plants  were  identical  with  those  still  living.  The  dominant 
class  becomes  now  Mammals:  Reptiles,  therefore,  in  accordance  with  a 
necessary  law,  decrease  in  size  and  number,  and  thus  find  safety  in  a 
subordinate  position.  In  some  of  these  characteristics  the  Cenozoic  era 
was  anticipated  in  the  Upper  Cretaceous,  in  accordance  with  the  law  that 
the  first  beginnings  of  each  age  is  in  the  preceding  age. 

Divisions. — The  Cenozoic  era,  or  age  of  Mammals,  embraces  two 
periods,  viz. — 1.  The  Tertiary  and  2.  The  Quaternary.  In  the  Ter- 
tiary all  the  mammals  are  now  wholly  extinct,  but  the  invertebrate 
species  are  some  of  them  still  living,  and  an  increasing  percentage  of 
living  species  appears  as  time  progresses.  In  the  Quaternary  most, 
though  not  all,  of  the  mammalian  species  are  extinct,  but  most  (ninety- 
five  or  more  per  cent.)  of  the  invertebrate  species  are  living.  These 
facts  are  graphically  represented  in  the  following  diagram,  in  which 


FIG.  788.— Diagram  illustrating  the  Relative  Duration  of  Lower  and  High 


the  curved  ascending  lines  are  the  lines  of  appearance  of  living  species, 
and  of  extinction  of  extinct  species  of  Foraminifera,  of  molluscous 
shells,  and  of  mammals.  In  each  case  the  lower  shaded  space  repre- 
sents living  species  appearing  in  small  numbers,  and  increasing  with  the 
progress  of  time ;  and  the  upper  unshaded  or  less  shaded  space,  previous 
species  gradually  dying  out  and  -becoming  extinct.  It  is  seen  that 


TERTIARY  PERIOD.  477 

living  species  of  Foraminifera  commenced  in  the  Cretaceous,  and  very 
steadily  increased  in  number ;  those  of  shells  commenced  in  the  earliest 
Tertiary,  and  increased  somewhat  more  rapidly ;  while  those  of  mammals 
commenced  only  in  the  Quaternary,  and  increased  correspondingly 
rapidly.  Also  the  relative  proportion  of  living  and  extinct  at  any 
time  is  shown  by  comparing  the  amount  of  space  above  and  below  the 
line  at  that  time.  Also  the  relative  range  in  time  of  low  and  high 
species,  and  the  amount  of  overlapping  of  successive  faunse,  are  shown. 
The  mammalian  class  probably  culminated  near  the  end  of  the  Ter- 
tiary or  during  the  Quaternary  period. 

SECTION  1. — TERTIARY  PERIOD. 

Subdivisions. — We  have  already  stated  that  the  general  differential 
characteristic  of  this  period,  as  compared  with  the  next,  is  that  all  the 
mammals,  and  most  of  the  invertebrates,  are  extinct ;  but  of  the  latter 
a  percentage,  small  at  first  but  increasing  with  the  progress  of  time, 
are  still  living.  It  is  upon  this  percentage  of  living  shells  that  Lyell 
has  based  his  division  of  the  Tertiary  period  into  three  epochs — a 
Lower,  Middle,  and  Upper  Tertiary,  or  Eocene,  Miocene,  and  Pliocene. 

f  Pliocene  epoch,  or  Upper  Tertiary  =  50-90  per  cent,  living  shells. 
Tertiary  period.  -4  Miocene  epoch,  or  Middle  Tertiary  =  30  per  cent,  living  shells. 
L  Eocene  epoch,  or  Lower  Tertiary  =  5-10  per  cent,  living  shells. 

These  percentages  are  expressed  graphically  in  the  diagram,  Fig. 
788. 

Rock-System—Area  in  the  United  States.— On  the  Atlantic  border, 
going  southward,  there  is  no  Tertiary,  except  a  small  patch  on  Martha's 
Vineyard,  off  the  coast  of  Massachusetts,  until  we  reach  New  Jersey. 
From  this  point  southward  the  Tertiary  is  a  broad  strip,  about  100  miles 
wide,  bordering  the  coast,  and  shown  on  the  map  (p.  278)  by  the  space 
shaded  with  oblique  lines  running  to  the  right.  It  constitutes  the  low- 
countries  of  the  Southern  Atlantic  States.  At  its  junction  with  the 
metamorphic  region  of  the  up-countries,  there  are  in  nearly  all  the 
rivers  cascades  which  determine  the  head  of  navigation.  Here,  there- 
fore, are  situated  many  important  towns,  e.  g.,  Richmond,  Virginia  ; 
Raleigh,  North  Carolina ;  Columbia,  South  Carolina  ;  Augusta,  Milledge- 
ville,  and  Macon,  Georgia.  The  same  strip  of  flat  lands  borders  also  the 
Gulf,  expands,  in  the  region  of  the  Mississippi  River,  northward  to  the 
mouth  of  the  Ohio,  and  then  continues  around  the  western  border  of 
the  Gulf.  In  the  Gulf-border  region,  however,  the  Tertiary  is  in  con- 
tact below  with  the  Cretaceous,  instead  of  with  metamorphic  Silurian 
and  Laurentian,  as  on  the  Atlantic  border.  This  whole  Atlantic-border 
and  Gulf-border  Tertiary  is,  of  course,  a  marine  deposit. 


478  CENOZOIC  ERA— AGE   OF  MAMMALS. 

In  the  interior,  on  the  Plains  and  in  the  Rocky  Mountain  region, 
there  are  enormous  areas  of  fresh-water  deposit,  some  Eocene,  some 
Miocene,  and  some  Pliocene,  which  are  of  extreme  interest. 

Among  the  Eocene  basins  the  most  remarkable  are  :  1.  The  Green 
River  basin.  2.  The  Uintah  basins.  Both  of  these  are  on  the  east 
side  of  the  Wahsatch  Mountains,  and  separated  from  each  other  by  the 
Uintah  Mountains,  one  being  north  and  the  other  south  of  that  range. 
The  strata  of  the  Green  River  basin  are  6,000  to  8,000  feet  thick.  3. 
The  San  Juan  basin  (Cope),  in  the  region  of  the  San  Juan  River,  Colo- 
rado. Its  horizon  is  the  same  as  the  Wahsatch,  or  lowest  Green  River 
beds,  which  are  the  lowest  Eocene.  This  is  probably  an  extension  of 
the  Uintah  basin. 

Among  the  Miocene  basins  the  most  interesting  are :  1.  The  White 
River  basin,  in  Nebraska.  2.  The  John  Day  basin,  of  Oregon.  This 
latter  is  5,000  feet  thick,  but  is  largely  overlaid  by  the  great  lava- 
flood  of  the  Northwest.  3.  One  in  Montana,  recently  discovered  by 
Grinnell.1 

Of  Pliocene  basins:  1.  Niobrara  basin,  occupying  partly  the  same 
locality  as  the  Miocene  White  River  basin,  but  far  more  extensive, 
reaching  southward  far  into  Texas.  2.  In  Oregon  also  there  is  a 
Pliocene  basin,  occupying  partly  the  same  region  as  the  previous  Mio- 
cene. 3.  One  recently  discovered  by  Cope  on  the  basin  of  the  Rio 
Grande.  4.  In  connection  with  the  Miocene  deposits  of  Montana,  also 
Pliocene  deposits  are  found. 

All  these  deposits  are  imperfectly  lithified  sand  and  clays  in  nearly 
horizontal  position,  and  have  been  worn  by  erosive  agencies  in  the 
most  remarkable  way,  sometimes  into  knobs  and  buttes  like  potato- 
hills  on  a  large  scale,  sometimes  into  castellated  and  pinnacled  forms, 
which  resemble  ruined  cities.  These  are  the  "  Mauvaises  Terres  "  or 
"  Bad  Lands  "  of  the  West  (Fig.  789). 

On  the  Pacific  coast,  a  large  portion  of  the  Coast  ranges  from 
Southern  California  to  Washington  is  Tertiary,  as  are  also  in  many 
places  the  lowest  foot-hills  of  the  Sierras. 

Physical  Geography, — From  what  has  been  said  of  the  distribution 
of  the  rocks  of  this  age,  it  is  easy  to  reconstruct  in  a  general  way  the 
physical  geography  of  the  American  Continent  during  the  early  Ter- 
tiary period.  In  the  northern  part  the  ^Atlantic  shore-line  was  prob- 
ably beyond  the  present  line,  for  there  is  no  Tertiary  deposit  visible 
there.  The  shore-line  of  that  time  crossed  the  present  shore-line  in 
New  Jersey,  then  passed  along  the  line  of  junction  of  the  Tertiary  with 
the  Metamorphic,  its  waves  washing  primary  shores  all  along  the  At- 
lantic coasts,  as  it  does  now  in  the  northern  portion  only ;  then  along 
the  junction  of  the  same  with  the  Cretaceous.  The  whole  lovv-coun- 
1  American  Journal  of  Science,  1876,  vol.  xi.,  p.  126. 


TERTIARY   PERIOD.  479 

tries  of  the  Southern  Atlantic  States  and  the  whole  of  Florida  were  then 
a  sea-bottom.     The  Gulf  of  Mexico  was  far  more  extensive  than  now, 


FIG.  789.— Mauvaises  Terres,  Bad  Lands  (after  Hayden). 

and  especially  it  sent  a  wide  bay  northward  to  the  mouth  of  the  Ohio. 
The  Mississippi  River  below  that  point  did  not  then  exist. 

In  the  interior,  in  the  region  of  the  Plains  and  Rocky  Mountains, 
there  were  at  different  times  immense  fresh-water  lakes,  the  positions  of 


FIG.  TOO.— Map  of  Tertiary  Times,  showing  Outline  of  Coast  and  Places  of  Principal  Tertiary  Lakes. 


480  CENOZOIC  ERA— AGE   OF  MAMMALS. 

some  of  which  have  been  already  indicated.  These  lakes  drained  some 
of  them  into  the  Mississippi,  some  into  the  Colorado,  and  some  into  the 
Columbia  River. 

The  Pacific  shore-line  at  that  time  was  along  the  foot-hills  of  the 
Sierra  range,  and  therefore  the  whole  region  occupied  by  the  Coast 
ranges  and  the  Sacramento  and  San  Joaquin  Valleys,  and  also  portions 
of  Western  Oregon,  were  then  a  sea-bottom.  These  facts  are  roughly 
represented  on  map,  Fig.  790.  The  position  of  the  principal  mountain- 
chains,  e.  g.,  Sierra,  Wahsatch,  Uintah,  the  eastern  border  of  the  Rocky 
Mountains,  and  Appalachian,  is  represented,  in  order  the  better  to  lo- 
cate the  lakes.  It  will  be  observed  that  the  continent  is  nearly  finished. 

Europe  is  now  remarkable  for  its  inland  seas.  It  was  much  more  so 
in  Tertiary  times. 

Many  great  cities,  as,  for  example,  London,  Paris,  Vienna,  are  sit- 
uated on  Tertiary  strata,  partly  because  these  strata  are  usually  found 
on  the  borders  of  continents,  and  partly  because  they  are  often  found 
in  the  course  of  great  rivers,  which  once  drained  lake-basins. 

Character  of  the  Rocks. — The  rocks  of  this  period,  along  the  At- 
lantic border  and  in  the  interior  Plains  and  Rocky  Mountain  region,  are 
mostly  imperfectly  lithified ;  but  on  the  Pacific  coast  they  are  not  only 
of  stony  hardness,  but  in  many  cases  completely  metamorphic.  Much 
of  the  rock  in  the  Coast  Chain  is  scarcely  distinguishable  from  the  schists 
of  the  Palaeozoic  or  still  older  periods.  The  reason  is  evident — meta- 
morphism  is  closely  connected  with  mountain-making,  and  mountain- 
making  continued  until  the  Tertiary  only  on  the  Pacific  coast. 

Coal. — Again,  in  the  Tertiary  rocks  we  find  coal,  although  more 
usually  in  the  imperfect  condition  called  lignite.  We  have  already 
stated  that  the  Rocky  Mountain  coal-fields  are  by  many  referred  to  the 
Tertiary.  We  will  not  repeat  these  here.  But  there  are  others  about 
which  there  is  as  yet  no  controversy.  The  Coos  Bay  coal,  of  Oregon,  is 
probably  Miocene-Tertiary.  Again,  Mr.  Selwyn,  the  Geologist  of  Cana- 
da, has  reported  large  fields  of  coal  on  the  Qu'Appelle  and  the  North 
Saskatchewan  Rivers,  covering  an  area  of  25,000  square  miles,  a  part, 
at  least,  of  which  he  refers  to  the  Tertiary.  Much  of  this  coal  is  of 
good  quality.  It  seems  most  probable,  however,  that  this  is  of  the  same 
age  as  the  Fort  Union  coal,  concerning  the  age  of  which  there  is  so 
much  discussion. 

In  Europe  also  an  imperfect  coal  (lignite)  is  found  in  the  Miocene  in 
considerable  quantity. 

Life- System. 

General  Remarks. — We  have  already  spoken  of  the  great  and  rapid 
change  in  the  life-system  between  the  Cretaceous  and  the  Tertiary,  even 
where  the  two  series  of  rocks  are  continuous  and  conformable.  This 


PLANTS.  481 

indicates,  undoubtedly,  a  more  rapid  rate  of  evolution  at  that  time. 
But  it  also  indicates,  as  one  cause  of  this  rapid  evolution,  a  migration 
of  species  brought  about  by  changes  in  physical  geography  and  climate, 
and  the  imposition  of  one  fauna  and  flora  upon  another,  and  the  ex- 
termination of  one  by  the  other.  It  is  difficult  to  conceive  of  these 
sudden  changes  taking  place  otherwise.  We  shall  speak  more  fully 
of  this  important  point  under  the  Quaternary. 

The  general  character  of  the  life-system  of  the  Tertiary,  as  already 
said,  was  in  the  main  similar  to  the  present.  Nearly  all  the  genera  and 
many  of  the  species  of  plants  and  invertebrate  animals  were  the  same 
as  now,  and  the  difference  in  aspect  would  hardly  be  recognized  by  the 
popular  eye  ;  it  was  certainly  not  greater  than  now  exists  between  dif- 
ferent countries.  It  is  only  among  Mammals  that  the  difference  would 
be  very  conspicuous. 

Plants. 

Among  plants,  nearly  all  the  genera  q/Dicotyls,  Palms,  and  Grasses, 
were  the  same  as  now,  though  most  of  the  species  are  extinct.  The  gen- 
era were  the  same  as  now,  but  not  in  the  same  localities.  On  the  con- 
trary, the  vegetation  indicated  a  much  warmer  climate  than  exists  now 
in  the  same  localities.  For  example,  if  we  regard  the  Lignitic  as 
Eocene-Tertiary,  instead  of  Cretaceous,  as  do  paleontological  botanists 
generally,  then  of  about  250  species  of  plants  found,  a  very  large  pro- 
portion were  Palms,  and  many  of  them  of  great  size  ;  and  among  the 
Dicotyls  many,  like  Magnolias,  indicated  a  warm  climate.  Lesquereux 
thinks  the  climate  of  Fort  Union  was  then  similar  to  that  of  Florida 
and  Lower  Louisiana  now.  Again,  in  Eocene  times  there  were  fif- 
teen species  of  Palms  in  Europe ;  and  in  the  Tyrol  the  flora,  according 
to  Von  Ettingshausen,  indicated  a  temperature  of  74°  to  81°  Fahr.,  and 
many  of  the  plants  are  Australian  in  type.  In  the  Pliocene,  on  the 
contrary,  many  European  plants  were  like  those  in  America  at  the 
present  time. 

During  the,  Miocene,  Europe  was  covered  with  evergreens  such  as 
could  grow  now  only  in  the  southernmost  part ;  and  that  even  as  far  as 
Lapland,  and  Iceland,  and  Spitzbergen.  It  has  been  estimated  that  the 
Miocene  flora  indicates  a  mean  temperature  of  16°  to  20°  higher  than 
now  exists  in  Middle  Europe.  In  America,  during  the  same  epoch, 
Sequoias  almost  identical  with  the  Big  Tree  and  Redwood  of  Califor- 
nia ;  and  Libocedrus,  one  of  them  identical  with  the  L.  decurrens  of 
California  ;  and  Magnolias  similar  to  the  J/i  grandiflora  of  the  South- 
ern Atlantic  States ;  and  Taxodium  distichum,  the  cypress  of  the 
swamps  of  Carolina  and  Louisiana,  all  existed  in  Greenland,  and  most 
of  them  also  in  Northern  Europe,  and  Iceland,  and  Spitzbergen.  Heer 
estimates  the  temperature  of  Greenland  in  the  Miocene  as  30°  higher 
than  now. 

31 


482 


CENOZOIC  ERA— AGE   OF  MAMMALS. 


These  facts  show  not  only  a  warm  but  a  uniform  climate,  and  prob- 
ably also  a  connection  in  high  latitudes  between  the  American  and  Eu- 
ropean Continents.  A  similar  connection,  shown  also  by  the  vegeta- 
tion, probably  existed  between  Alaska  and  the  Asiatic  Continent  at  that 
time.  Below  we  give  figures  of  some  Dicotyls  and  Monocotyls  of  Ameri- 
can and  European  Tertiary. 


FIG.  792. 


FIG.  793. 


FIG.  794. 


FIG.  795. 


FiG8.  791-796.— AMERICAN  TERTIARY  PLANTS  (after  Safford  and  Lesquereux) :  791.  Cinnamomum  Mis- 
sissippiense.  792.  Quercus  crassinervis.  793.  Andromeda  vnecinifolite  affinis.  794.  Carpolithes 
irregularis.  795.  Fagus  ferruginea— Nut.  796.  Quercus  Saffordl. 

Another  conclusion  to  be  drawn  from  the  foregoing  facts  is  that,  in 
the  race  of  evolution,  Europe  seems  to  have  distanced  most  other  coun- 
tries. The  Australian  flora  is  now  only  where  the  European  flora  was 
in  Eocene  times,  and  the  American  flora  now  where  the  European  was 
in  the  Pliocene.  The  probable  reason  is  that,  in  Europe,  in  these  later 
geological  times,1  changes  of  physical  geography  and  climate,  and  conse- 
quent migrations  of  species,  were  more  frequent,  and  the  struggle  for  life 

1  In  Cretaceous  times  the  flora  of  America  seems  to  have  been  more  advanced  than  that 
of  Europe. 


PLANTS. 


483 


more  severe.  Australia  especially,  probably  on  account  of  its  isolation, 
has  advanced  more  slowly  than  most  other  countries.  Many  remnants 
of  extinct  faunse  and  florae  exist  there  still. 


FIG.  800. 


FIGS.  797-802.—  PLANTS  OP  EUROPEAN  TERTIARY  :  797.  Chamaerops  Helvetica.    798.  Sabal  major.    T99. 

,  Single  Fruit.   800.  Cinna 
Leaf,  6,  Flower  ;  c,  Seed 


. 

Platanusaceroides  :  a,  Leaf;  6,  Core  of  a  Cluster  of  Fruits;  c,  Single  Fruit.   800.  Cinnamomum  poly- 

802.  Podo- 


morphum  :  a.   Leaf;  6,  Flower.    801.  Acer  trilobatum  : 
gonium  Knorrii. 


Diatoms.  —  If  the  highest  of  plants  —  Dicotyls  and  Monocotyls  —  were 
abundant,  probably  more  abundant  than  now,  so  also  were  the  lowest 
order  of  uni-celled  plants  —  the  Diatoms.  Immense  deposits,  consisting 


484 


CENOZOIC  ERA— AGE   OF   MAMMALS. 


wholly  of  the  siliceous  shells  of  these  microscopic  plants,  are  found  in 
the  Tertiary.  In  Europe  the  Bohemian  deposit  is  celebrated.  It  is 
fourteen  feet  thick,  and  every  cubic  inch  of  the  material,  according  to 
Ehrenberg,  contains  40,000,000,000  shells.  The  Richmond  (Virginia) 
deposit  is  equally  well  known.  It  is  thirty  feet  thick,  and  many  miles 
in  extent.  Similar  deposits  are  peculiarly  abundant  in  California.  They 
are  found  in  at  least  a  dozen  localities  where  the  Tertiary  rocks  pre- 
vail, as,  for  example,  at  San  Pablo,  in  Shasta  County,  and  near  Mon- 
terey, the  last  deposit  being  fifty  feet  thick. 

Some  of  the  more  remarkable  forms  of  Diatoms  are  shown  below  in 
Fig.  803,  which  is  a  view  under  the  microscope  of  the  Richmond  deposit. 


FIG.  803.— Microscopic  View  of  Richmond  Infusorial  Earth  (by  Ehrenberg). 

Deposits  of  this  kind  are  usually  called  infusorial  earths.  They  may 
often  be  recognized,  even  without  microscopic  examination,  by  their 
soft,  chalky  consistence,  their  insolubility  in  acids,  and  their  extreme 
lightness. 

Origin  of  Infusorial  Earths. — It  is  well  known  that  mud  composed 
of  diatom  shells  accumulates  at  the  bottoms  of  ponds,  and  lakes,  and 
sluggish  streams.  In  the  deepest  parts  of  Lake  Tahoe,  where  sedi- 
ments do  not  reach,  the  ooze  is  composed  wholly  of  infusorial  shells.  It 


ANIMALS.  485 

has  been  shown,  also,  by  Dr.  Blake,1  that  the  deposits  from  hot  springs 
of  California  and  Nevada,  even  where  the  temperature  is  163°  to  174°, 
abound  in  Diatoms  of  the  same  species  as  those  found  in  California  in- 
fusorial earths.  It  is  probable,  therefore,  that  many  of  these  deposits 
were  made  in  hot  springs  and  hot  lakes,  which,  judging  from  the  vol- 
canic activity  of  that  time,  abounded  in  California  then  even  far  more 
than  now.  Dr.  Blake  thinks  the  infusorial  earths  of  California  are 
Miocene. 

Animals. 

As  already  stated,  among  Invertebrates  there  was  a  general  similarity 
to  the  present  fauna.  Nearly  all  the  genera,  and  many  of  the  species, 
were  identical  with  those  still  living.  The  relation  between  the  various 
orders  which  prevail  now,  commenced  then.  The  present  basis  of 
adjustment  was  then  established.  Then,  as  now,  Brachiopods  and 
Crinoids  were  nearly  all  gone,  Echinoderms  were  nearly  all  free,  and 
Bivalves  were  nearly  all  Lamellibranchs.  Then,  as  now,  naked  Ceph- 
alopods  and  short-tailed  Crustaceans  greatly  predominated.  A  glance 
at  the  following  figures  of  Tertiary  shells  will  show  the  general  resem- 
blance to  those  of  the  present  seas. 

In  regard  to  the  Invertebrates,  there  are  only  three  or  four  points  of 
sufficient  importance  to  arrest  our  attention  in  a  rapid  survey. 

Among  Rhizopods,  Nummulites  (a  foraminifer)  abounded  to  an  ex- 
traordinary degree.  Eocene  strata,  many  thousand  feet  thick,  are  formed 
of  these  shells.  The  Nummulitic  limestone  of  the  Alps  extends  east- 
ward to  the  Carpathians,  westward  to  the  Pyrenees,  and  southward  into 
Africa.  It  was  largely  quarried  to  build  the  Pyramids  of  Egypt.  It 
occurs  also  extensively  in  Asia  Minor  and  in  the  Himalayas. 


FIG.  804. — Nummulina  Isevigata. 

This  limestone  occurs  in  the  Alps  10,000  feet,  and  in  the  Himalayas 
15,000  feet,  above  the  sea-level.  We  see,  then,  the  immense  changes 
which  have  occurred  by  mountain-making  since  the  Eocene. 

Among  bivalve  shells,  common  forms  of  the  present  day,  such  as 

the  oyster,  the  clam  (  Venus),  the  scallop-shell  (Pecten),  etc.,  were  very 

numerous,  and  some  of  very  large  size.     Oysters  especially  seemed  to 

have  reached  their  maximum  development  in  the  Tertiary.     The  Ostrea 

1  American  Journal  of  Science,  III.,  vol.  iv.,  p.  148. 


486 


CENOZOIC  ERA— AGE   OF   MAMMALS. 


Georgiana  (Fig.  806)  was  ten  inches  long  and  four  wide ;  the  Ostrea 
Caroliniensis  was  of  equal  size,  but  shorter  and  broader.  A  specimen  of 
the  Ostrea  Titan  of  California  and  Oregon  now  lies  before  us,  which  by 


FIG.  808. 


Fio.  812. 


FIG.  810. 

FIGS.  805-S12.-EocE>-K  TERTIARY  SHELLS:  805.  Ostrea  selteformis  (after  Meek).  806.  Ostrea  Georfii- 
ana  (after  Meek).  807.  Pceten  nuperum  (after  Wailes).  803.  Anomalocardia  MississippiensiB  (after 
Conrad).  809.  Umbrella  planulata  (after  Wailes).  810.  Turritella  alveata  (after  Wailes).  811. 
Volutalithes  dumosa  (after  Wailes).  812.  Volutalithes  syinmetrica  (after  Wailes). 


ANIMALS.  487 

measurement  is  thirteen  inches  long,  eight  wide,  and  six  thick  (Fig.  813), 


FIG.  819.  FIG.  81T. 

FIGS.  81S-S19.-CALiFOR*iA  MIOCENE  SHELLS  (after  Gabb) :  813.  Ostrea  Titan,  x  ?T.  814.  Pecten  Cer- 
rocensis,  x  J.  S15.  Venus  pertenuis.  816.  Cardium  Meekianum.  817.  (Jancellana  vetusta.  813. 
Ficus  pyriformis.  619.  Ectdnorachnis  Breweranus. 


4:88 


CENOZOIC  ERA— AGE   OF  MAMMALS. 


and  a  specimen  of  JPecten  Cerrocensis  of  California,  nine  inches  across 
(Fig.  814).  Among  univalves  also  nearly  all  the  forms  are  familiar. 
The  illustrations  are  taken  from  the  Eocene  and  Miocene.  The  Pliocene 
shells  are  almost  undistinguishable  from  living  shells,  except  by  the 
practised  eye.  It  seems  useless  to  give  them  in  an  elementary  work. 

Insects. — There  are  several  interesting  points  connected  with  this 
class  which  must  not  be  omitted.  We  have  usually  found  insects  abun- 
dant in  connection  with  luxuriant  vegetation.  During  the  Miocene,  phe- 
nogamous  vegetation  was  even  more  abundant  than  now ;  there  was  also 
extreme  fullness  of  insect-life.  All  orders,  even  the  highest,  viz.,  Lepi- 
doptera  (butterflies — Fig.  821)  and  Hymenoptera  (bees,  ants,  etc. — 
Fig.  820),  were  represented. 

In  the  Miocene  of  Europe,  1,550  species  of  insects  have  been  found  ; 
and  of  these  more  than  900  species  at  CEningen  in  a  stratum  only  a  few 


FIG.  820.  FIG. 

FIGS.  820,  821.— INSECTS  OF  EUROPEAN  MIOCENE  :  820.  a,  Pod  of  Podogonlum  Knorrii;  Z>,  Grass-leaf;  c, 
Formica  lignitum ;  d,  Ulster  coprolithorum.    821.  Vanessa  Pluto. 

feet  thick  (Lyell).  In  places  the  stratum  is  black  with  the  remains  of 
insects.  The  same  stratum  is  also  full  of  leaves  of  Dicotyls,  of  which 
Heer  has  described  500  species.  Mammalian  remains  and  fishes  are  also 
found  in  them. 

It  is  interesting  to  inquire  the  conditions  under  which  these  strata 
were  formed  and  filled  with  these  remains.  On  Lake  Superior,  at 
Eagle  Harbor,  in  the  summer  of  1844,  we  saw  the  white  sands  of  the 
beach  blackened  with  the  bodies  of  insects  of  many  species  cast  ashore. 
As  many  species  were  here  collected  in  a  few  days,  by  Dr.  J.  L.  Le  Conte, 
as  could  have  been  collected  in  as  many  months  in  any  other  place. 
The  insects  seem  to  have  flown  over  the  surface  of  the  lake;  to  have 
been  beaten  down  by  winds  and  drowned,  and  then  slowly  carried 
shoreward  and  accumulated  in  this  harbor,  and  finally  cast  ashore  by 
winds  and  waves.  Doubtless,  at  CEningen,  in  Miocene  times,  there  was 
an  extensive  lake  surrounded  by  dense  forests  ;  and  the  insects  drowned 
in  its  waters,  and  the  leaves  strewed  by  winds  on  its  surface,  were  cast 
ashore  by  its  waves.  Heer  believes  also  that  carbonic-acid  emissions 


ANIMALS.  489 

helped  to  destroy,  and  deposits  of  carbonate  of  lime  to  preserve,  the 
insects. 

Among  the  insects  found  at  (Eningen,  Switzerland,  and  Radoboj, 
Croatia,  are  a  great  many  ants  (Fig.  820).  In  all  Europe,  there  are 
now  about  fifty  species  of  ants.  Heer  found  in  the  Miocene  of  CEnin- 
gen  and  Radoboj  more  than  100  species.1  And,  what  is  very  remark- 
able, all  of  these  are  winged  ants.  Ants  of  the  present  day  are  male, 
female,  and  neuter.  The  males  are  winged  throughout  life,  and  never 
live  in  the  nests,  but  soon  perish.  The  females  are  also  winged  until 
they  are  fertilized ;  then  they  drop  their  wings  and  live  in  communi- 
ties in  a  wingless  condition  ever  afterward.  The  neuters  are  always 
wingless,  and  therefore  always  live  in  nests  or  in  communities.  It  is 
probable  that  ants  at  first  were  only  winged  males  and  females,  living 
in  the  open  air  like  other  insects.  The  wingless  condition  and  the  neu- 
tral condition  are  both  connected  with  their  peculiar  social  habits  and 
instincts,  and  have  been  gradually  developed  along  with  the  develop- 
ment of  their  habits  and  instincts.  It  is  probable  that  all  these  remark- 
able peculiarities,  viz.,  the  wingless  condition,  the  neutral  condition,  the 
wonderful  instincts,  and  organized  social  habits,  have  been  developed 
together  since  the  Miocene  epoch. 

In  the  fresh-water  Miocene  of  Auvergne,  France,  there  is  a  remark- 
able stratum  called  indusial  limestone,  because  it  is  largely  composed 
of  the  cast-off  hollow  cases  (indusia)  of  the  caddis-worm  or  larva  of  the 
caddis-fly  (Phryganea],  cemented  together  by  carbonate  of  lime.  The 
number  of  these  cases  is  countless.  The  caddis-worm  of  the  present 
day  forms  for  itself  a  hollow  cylindrical  case,  of  bits  of  stick  or  pieces 
of  shell,  or  sometimes  of  whole  small  shells,  binding  these  together  by 
means  of  a  kind  of  web.  In  this  hollow  cylinder  it  lives,  only  put- 
ting out  the  head,  and  two  or  three  first  joints  of  the  body  to  which  the 
feet  are  attached,  in  walking.  When  they  complete  their  metamor- 
phoses, they  leave  their  shells.  Fig.  824  is  a  recent  caddis-worm  with 
its  case  of  small  shells  stuck  together ;  Fig.  823  are  indusia  of  the 
Miocene  caddis-worm ;  and  Fig.  822  is  the  limestone  in  place,  a  being 
the  indusial  layer. 

In  Auvergne,  in  Miocene  times,  there  existed  a  shallow  lake,  in 
which  carbonate  of  lime  was  depositing,  as  in  many  lakes  of  the  present 
day.  In  this  lake  lived  myriads  of  caddis-worms,  and  their  indusia  ac- 
cumulated for  countless  generations. 

In  the  Tertiary  strata,  about  the  shores  of  the  Baltic,  and  also  in 
Sicily,  in  Asia  Minor,  and  several  other  localities,  usually  associated 
with  lignite,  are  found  masses  of  amber.  This  substance  is  a  fossil 
resin  of  several  species  of  Conifer,  especially  Pinites  succinifer.  It  is 
often  quite  transparent,  and  inclosed  within  may  be  seen,  perfectly  pre- 
1  Pouchet,  Popular  Science  MontUy,  June,  18T3. 


490 


CENTOZOIC   ERA-AGE   OF   MAMMALS. 


served,  insects  of  many  kinds.  Over  800  species  of  insects,  and  frag- 
ments of  many  species  of  plants,  have  been  found  thus  inclosed.  The 
degree  of  preservation  is  marvelous ;  even  the  most  delicate  parts,  the 
slender  legs,  the  jointed  antennae,  and  the  gauzy  wings,  are  perfect. 
The  manner  in  which  these  insects  were  entangled,  inclosed,  and  pre- 
served, may  be  easily  observed  even  at  the  present  day.  The  gum 
issuing  from  Conifers  is  at  first  in  the  form  of  semi-liquid,  transparent 
tears.  Flies,  gnats,  etc.,  alighting  on  these,  stick  fast,  and  by  the  run- 


FIGS.  822-S24.— 822.  Indusial  Limestone  interstratified  with  Fresh-Water  Marls.    828.  A  Portion  (nat- 
ural size)  showing  the  Phryganea  Cases.    824.  Kecent  Larva  of  a  Phryganea,  with  its  Case. 

ning  down  of  further  exudations  are  enveloped  and  preserved  forever. 
The  legs,  both  in  the  modern  and  the  fossil  resin,  are  often  found 
broken  by  the  struggles  of  the  insects  to  extricate  themselves.  The  in- 
sects of  the  Tertiary,  like  the  plants,  show  a  decided  tropical  character. 
Fishes. — The  present  relation  between  the  three  great  orders  of 
Fishes — Teleosts,  Ganoids,  and  Placoids— was  first  fairly  established  in 
the  Tertiary.  Teleosts  were  first  introduced  in  the  Cretaceous,  but 


ANIMALS. 


491 


only  in  the  Tertiary  did  they  become  very  abundant.  Ganoids,  on  the 
contrary,  became  fewer  in  number  ;  they  sank  into  their  present  subor- 
dinate position.  Among  Placoids,  the  Hybodonts  are  gone,  the  Cestra- 
cionts  are  few  in  number,  but  the  Squalodonts  reach  their  maximum 
development,  both  in  number  and  size.  In  the  marine  Tertiary  of  the 


FIG.  829. 

FIGS.  625-S29.— TERTIAKT  Fisms-Pta<50«?* :  825.  Lamna  elegrans  (after  Atrassiz).  826.  Notidanns 
primisrenius  (after  Asrassiz).  827.  Carcharorton  augustidens  (.after  Gibbes).  823.  Carcharodon 
megalodon,  x  J  (after  Gibbes).  829.  Clupea  alta  (after  Leidy). 


492 


CENOZOIC   ERA— AGE   OF   MAMMALS. 


Atlantic  border,  both  Eocene  and  Miocene,  sharks'  teeth  are  found  in 
immense  numbers,  and  of  very  great  size.  Some  of  the  triangular 
teeth  of  the  Carcharodon  megalodon  (Fig.  828)  are  found  six  and  a 
half  inches  long  and  six  inches  broad  at  the  base.  The  owners  of  such 
teeth  must  have  been  fifty  to  seventy  feet  long.  Some  of  the  more  com- 


FIGS.  830  631.— TERTIARY  FISHES— Placoids  :  880.  Rhombus  minimus,  Lower  Eocene.    631.  Lebias 
cephalotes,  Miocene. 

mon  forms  of  sharks'  teeth  of  the  American  Tertiar}r,  and  Teleosts 
from  American  and  European  Tertiary,  are  given  above. 

Reptiles. — The  age  of  Reptiles  is  past.  The  huge  Enaliosaurs,  Dino- 
saurs, Mosasaurs,  and  Pterosaurs,  are  all  extinct.  Their  class  is  now  rep- 
resented by  Crocodiles,  Turtles,  Snakes,  and  Frogs,  though  their  place 
as  rulers  is  supplied  by  Mammals  and  Birds.  Five  species  of  Snakes, 
some  of  them  eight  feet  long,  and  nine  Crocodilians,  have  been  found  in 


ANIMALS. 


493 


the  Eocene  of  Wyoming,  and  several  also  in  Europe.  In  the  Miocene 
of  Europe,  CEningen,  a  Salamandroid  Amphibian  was  found  four  feet 
long,  and  at  first  mistaken  for  the  skeleton  of  a  man.  From  this  cir- 


Pio.  832.— Front  Portion  of  the  Skeleton  of  Andrias  Scheuchzeri.  a  Giant  Salamander  from  the  Miocene 
Tertiary  of  CEningen,  in  Switzerland,  reduced  in  size. 

cumstance  it  received  its  name,  Andrias  (Fig.  832).  The  Miocene  of  the 
Himalayas  furnishes  a  gigantic  turtle  ( Colossochelys  Atlas),  the  cara- 
pace of  which  was  twelve  feet  long  and  eight  feet  wide,  and  seven  feet 


494  CENOZOIC   ERA— AGE   OF  MAMMALS. 

high  in  the  roof,  and  the  whole  animal  was  probably  twenty  feet  long. 
Over  sixty  species  of  Tertiary  turtles,  and  eighteen  or  twenty  species 
of  crocodiles,  have  been  described  from  foreign  countries  (Dana). 

The  Crocodilians,  the  highest  order  of  reptiles,  first  appeared  in  the 
Triassic,  but  only  in  generalized  forms — Staff  onolepis,  JJelodon,  etc. — 
which  closely  connected  them  with  the  Lizards.  From  this  early  form 
Huxley  has  traced  with  consummate  skill  the  gradual  differentiation  of 
this  order,  in  the  position  of  the  posterior  nares,  the  structure  of  the 
head  and  the  form  of  the  vertebral  bodies,  step  by  step  through  the 
Jurassic,  Cretaceous,  to  the  Tertiary,  where  the  type  reached  its  per- 
fection. 

Birds. — The  class  of  Birds  in  the  Cretaceous  was  represented  only 
by  the  reptilian  birds  and  ordinary  water  birds.  Now,  in  the  Tertiary, 
however,  the  reptilian  birds — vertebrated-tailed  and  socket-toothed — 
have  disappeared.  The  bird-class  is  fairly  differentiated  from  the  rep- 
tilian class,  and  the  connecting  links  destroyed.  Birds  of  all  kinds  now 
appear — land-birds  as  well  as  water-birds.  In  America,  among  land- 
birds,  woodpeckers,  owls,  eagles,  etc.,  have  been  discovered  and  de- 
scribed by  Marsh.  The  number  of  species  found  in  Europe  is  much 
greater  than  in  America.  The  Miocene  beds  of  Central  France  alone 
have,  according  to  Milne-Edwards,  afforded  seventy  species.  The  Mio- 
cene birds,  like  the  plants  and  insects,  show  a  decided  tropical  charac- 
ter. "  Parrots  and  Trogons  inhabited  the  woods ;  Swallows  built,  in 
the  fissures  of  the  rocks,  nests  in  all  probability  like  those  now  found 
in  certain  parts  of  Asia  and  the  Indian  Archipelago  ;  a  Secretary-bird, 
nearly  allied  to  that  of  the  Cape  of  Good  Hope,  sought  in  the  plains  the 
serpents  and  reptiles  which  at  that  time,  as  now,  must  have  furnished 
its  nourishment.  Large  Adjutants,  Cranes,  Flamingoes,  Palasolodi  (birds 
of  curious  forms  intermediate  between  Flamingoes  and  ordinary  Grallas), 
and  Ibises,  frequented  the  margins  of  the  water  where  insect-larvae  and 
mollusks  abounded.  Pelicans  floated  on  the  lakes ;  and,  lastly,  Sand- 
grouse  and  numerous  Gallinaceous  birds  assisted  in  giving  to  this  or- 
nithological population  a  strange  physiognomy  which  recalls  to  mind 
the  descriptions  given  by  Livingstone  of  certain  lakes  in  Southern 
Africa." 

Recently  a  toothed  bird  has  been  found  in  the  London  clay  (Eocene), 
and  named  by  O^en  Odontopteryx  (Fig.  833).  But  this  is  not  a  true 
socket-toothed  bird.  The  so-called  teeth  are  only  dentations  of  the 
bony  edge  of  the  bill. 

During  the  present  year  (1876)  Cope  has  published  the  discovery  of 
a  gigantic  bird  from  the  lowest  Eocene  of  the  San  Juan  basin.  The 
Diatryma  gigantea,  according  to  Cope,  combines  the  characters  of 
the  Cursores  (ostrich  family)  with  those  of  the  extinct  Gastornis  of  the 
Paris  basin  (p.  496).  Judging  from  its  foot  it  was  double  the  size  of 


ANIMALS.  495 

an  ostrich.    This  is  the  first  example  of  extinct  Cursores  found  in  North 
America  (Cope). 


FIG.  833.— Skull  of  Odontopteryx  toliapicus,  restored  (after  Owen). 

Mammals — General  Remarks. — One  of  the  most  noteworthy  facts 
connected  with  the  first  mammals  is  the  apparent  suddenness  of  their 
appearance  in  great  numbers.  We  have  already  seen  small  marsupials 
quite  abundant  in  the  Mesozoic,  but  no  trace  of  true  mammals.  In 
fact,  the  existence  of  these  would  seem  to  be  incompatible  with  the 
reign  of  the  huge  reptiles.  But,  with  the  opening  of  the  Eocene,  the 
earth  seems  to  swarm  with  mammals.  And  this  is  true  not  only  in 
Europe,  where  the  unconformity  of  strata  indicates  a  lost  interval  at 
this  point  of  the  history,  but  also  on  the  Western  Plains  and  Rocky 
Mountain  region,  where  the  Cretaceous  seems  to  graduate  insensibly 
into  the  Tertiary.  Upon  any  theory  of  evolution  this  can  be  accounted 
for  only  by  supposing  the  period  between  the  Cretaceous  and  Tertiary 
to  have  been  one  of  very  great  rapidity  of  change  of  organic  forms — 
this  rapidity  of  change  being  the  result  partly  of  the  pressure  of 
changed  climate,  and  partly  of  migration  of  species  and  the  consequent 
struggle  for  life  between  different  geographical  faunae. 

True  placental  mammals  not  only  appear  suddenly  and  in  great 
numbers,  but  of  nearly  all  orders,  even  the  highest  except  man,  viz., 
monkeys.  These,  however,  are  not  typical  monkeys,  but  lemurs,  which 
may  be  regarded  as  a  generalized  form,  connecting  monkeys  with  other 
orders.  In  the  oldest  Eocene  beds  (Wahsatch  beds  of  the  Green  River 
and  San  Juan  basins),  Cope  finds  eighty-seven  species  of  vertebrates, 
two-thirds  of  which  are  mammals.  In  the  Fort  Bridger  beds  of  the 
Green  River  basin  (Middle  Eocene),  Marsh  finds  150  species  of  verte- 
brates, of  which  the  larger  number  are  mammals,  some  Herbivora,  some 
Carnivora,  and  some  Lemurine  monkeys.  The  same  species  do  not 
continue  through  the  Tertiary.  On  the  contrary,  the  mammalian  fauna 
changes  completely  several  times  in  the  course  of  that  period. 

One  general  characteristic  of  the  early  mammalian  fauna  is  the  pre- 
dominance of  Herbivora.  Especially  is  this  true  of  the  Cuvierian  order 
Pachyderms,  an  order  which  now  includes  such  diverse  forms  as  ele- 


496  CENOZOIC   ERA— AGE   OF  MAMMALS. 

phant,  rhinoceros,  hippopotamus,  tapir,  hog,  horse  ;  and  still  more 
especially  is  this  true  of  tapir-like  Pachyderms.  But  there  is  much 
reason  to  believe  that  the  very  first  Tertiary  mammals  were  far  more 
generalized  in  structure  than  any  family  of  mammals  now  living. 

The  Tertiary  mammals  are  of  so  great  interest  from  the  evolution 
point  of  view,  that  we  must  dwell  upon  them  somewhat  in  detail.     But 


FIG.  834.— Tapirus  Indicus. 

it  seems  impossible  to  present  selections  from  the  immense  mass  of 
material  at  hand  in  an  interesting  manner,  except  by  taking  a  few  clas- 
sic localities  from  different  epochs  and  different  countries,  and  briefly 
describing  what  has  been  found  in  each.  We  will  commence  with 
some  foreign  localities,  because  these  were  first  discovered : 

1.  Eocene  Basin  Of  Paris. — This  basin  has  been  made  celebrated 
by  the  immortal  labors  of  Cuvier.  The  discovery  in  the  early  portion 
of  the  present  century  of  the  rich  treasures  imbedded  in  the  strata 
of  this  basin,  and  the  consummate  skill  with  which  they  were  worked 
up  by  Cuvier,  gave  an  incredible  impulse  to  geology.  Fifty  species 
of  mammals,  of  which  forty  species  were  tapir-like;  ten  species  of 
birds,  among  which  one,  the  Gastornis,  was  a  huge  wader  as  large  as  an 
ostrich  ;  besides  reptiles,  fishes,  and  shells  in  abundance,  were  discovered. 
In  Eocene  times  the  Paris  basin  seems  to  have  been  an  estuary  full 
of  shells  and  fishes,  etc.,  into  which  the  bodies  of  birds  and  mammals 
were  drifted.  Among  the  many  remarkable  mammals  we  will  select 
two  as  types,  viz.,  the  Palceothere  and  the  AnoplotTiere. 

The  Palseothere,  like  the  Rhinoceros  and  like  some  of  the  earlier 
representatives  of  the  horse  family,  had  three  hoofed  toes  on  all  the 
feet.  It  is  usually  supposed  to  have  had  also  the  general  form  and  the 
short  flexible  snout  of  a  tapir,1  and  it  is  with  this  family  that  Cuvier 

1  The  tapir  has  three  toes  on  the  hind-foot,  and  four  on  the  fore-foot,  but  the  outer 
one  is  small  and  not  functional. 


ANIMALS.  497- 

supposed  it  has  its  nearest  alliance.    The  figure  below  is  Cuvier's  resto- 
ration, and  until  recently  subsequent  discoveries  seemed  to  confirm  its 


FIG.  835. — Restoration  of  Pateotheriuin  magnum  (after  Owen). 

general  truthfulness.1     In  1874,  however,  the  discovery  of  a  complete 
skeleton  showed  that  the  restoration  of  Cuvier  is  far  from  correct,  and 


FIG.  836.— Palaeotherium  magnum  (recently-discovered  skeleton). 

that  the  neck  and  limbs  were  much  longer  than  had  been  supposed.    In 

1  Owen,  "Paleontology,"  p.  365. 
32 


498  CENOZOIC  ERA— AGE  OF  MAMMALS. 

general  form  it  seems  to  have  been  more  like  the  horse  family  than  the 
tapirs. 

The  Anoplothere  was  a  slender  and  graceful  animal  without  snout, 
and  possessing  only  two  toes,  like  ruminants.     Most  of  its  characters, 


FIG.  637.  —  Anoplotherium  commune,  restored. 

however,  allied  it  to  the  tapirs.     It  was,  therefore,  a  remarkable  con- 
necting link  between  the  tapirs  and  ruminants. 

2.  Siwalik  Hills,  India—  Miocene.  —  Near  the  base  of  the  Himalayas 
occurs  a  range  of  hills  which  are  composed 
of  fresh-water  Miocene  strata.  They  are 

jf^^.  Hfifl^k     extremely  rich  in  vertebrate  and  especially 

^jSgJfej^sji  BSk      in  mammalian  remains,  which  have  been 

^•BlUH^A          thoroughly    studied   by   Falconer.     More 

AtfMttttdH  Wgk        than  forty  species   of  mammals   are   de- 

^^HH^^H^        scribed  from  this  locality.     They  are  of 

{&  great  variety  of  forms,  both  Carnivora  and 

\.J  Herbivora,  but  the  latter  are  most  abun- 

yL  dant.     Among   these,   perhaps    the    two 

^^  most    remarkable   are    Dinotherium  and 


FIG.  838.—  Head  of  Dinotherinm  gigan- 

The  Dinothere  has  been  found  also  in 

the  European  Miocene.  It  was  a  huge  animal,  with  a  skull  three  feet 
long,  to  which  was  attached  a  proboscis.  The  lower  jaw  was  bent 
downward,  and  carried  two  long,  tusk  -like  teeth,  projecting  also  down- 
ward. The  whole  height  of  the  head,  from  the  points  of  these  lower 
teeth  to  the  top  of  the  cranium,  was  five  feet. 

Recently  a  perfect  pelvis  has  been  found,  showing  the  great  mas- 
siveness  of  these  bones,  and  showing  also,  in  these  huge  animals,  the 
existence  of  marsupial  bones.1  This  strange  animal  combined,  in  the 
structure  of  its  head,  the  characters  of  Elephant,  Hippopotamus,  Tapir, 
and  Dugong;  but  it  also  had  affinities  with  marsupials.  It  was  the 
earliest  of  Proboscidians. 

1  American  Journal  of  Science,  II.,  vol.  xxxviii.,  p.  427. 


ANIMALS.  499 

The  Sivathere  was  a  four-horned  antelope,  of  elephantine  size  and 
some  elephantine  characters.  The  four-horned  antelope  of  the  present 
day  lives  in  the  same  locality,  but  is  a  comparatively  small  animal,  with 
two  short  conical  horns  from  the  front  part  of  the  frontal  bone,  and  two 
somewhat  longer  ones  in  the  usual  place  on  the  back  part  of  the  same 
bone.  The  Sivathere,  on  the  other  hand,  was  of  elephantine  height, 
though  of  slenderer  form,  with  two  short  conical  horns  in  front,  and  two 
large,  palmately  branching  ones  behind.  The  form  of  the  nose-bones 
suggests  the  existence  of  a  snout.  The  feet  and  legs  were  clearly  those 


. — Head  of  a  Sivatherium  giganteum,  greatly  reduced. 


of  a  Ruminant. .  It  seems  to  have  combined  the  characters  of  a  Rumi- 
nant and  a  Pachyderm.  The  JZramathere  was  a. similar  animal,  of 
equally  gigantic  size,  found  in  strata  of  the  same  age. 

In  the  same  locality  were  found  also  three  species  of  Mastodons, 
seven  species  of  Elephants,  one  of  them  E.  ganesu,  remarkable  for  the 
prodigious  length  and  size  of  its  tusks ;  three  species  of  the  Horse  fam- 
ily ;  five  species  of  Rhinoceros ;  four  to  seven  species  of  Hippopotamus, 
and  three  species  of  hog ;  also,  Anoplotheres,  Camels,  Camelopards, 
Oxen,  Sheep,  Antelope,  Musk-ox,  Monkeys,  etc ;  also,  many  Reptiles, 
among  which  were  narrow-nosed  Crocodiles,  like  the  Gavials  now  liv- 
ing in  the  Ganges,  and  the  huge  Turtle,  Colossochelys,  already  men- 
tioned (p.  493). 

In  the  Miocene  and  Pliocene  of  Europe  are  first  found  remains  of 
that  most  destructive  of  carnivores,  the  sabre-toothed  tiger — Machai- 


500 


CENOZOIC   ERA— AGE   OF  MAMMALS. 


rodus  (Fig.  840).     In  the  Miocene  of  Europe,  also,  the  first  true  Mon- 
keys were  introduced  (Flower). 

Perhaps  it  is  well  to  call  attention  now  to  the  fact  that,  while  the 
tapir-like  Pachyderms  predominate  in  the  Eocene,  the  huge  forms,  e.  g., 


FIG.  840.— A,  Skull  of  Machairodus  cultridens,  without  the  lower  }aw,  reduced  in  size ;  £,  Canine  Tooth 
of  the  same,  one-half  the  natural  size.     Pliocene,  France. 

Rhinoceros,  Hippopotamus,  and  Proboscidians,  were  first  introduced 

and  immediately  became  abundant  in  the  Miocene. 

American  Localities.— 3.  Marine  Eocene  of  Alabama.— We  select 
this  as  an  example  of  American  marine  Eocene. 
At  Claiborne,  Alabama,  according  to  Lyell,  there 
occur  no  less  than  400  species  of  shells,  besides 
many  JEchinoderms,  and  abundance  of  sharks' 
teeth.  But  the  most  remarkable  remains  found 
there  are  those  of  an  extinct  whale — Zeuglodon 
cetoides — so  called  from  the  yoke-like  form  of 
the  double-fanged  molar  teeth,  which  were  six 
inches  in  length  (Fig.  841).  The  skull  was  long 
and  pointed  (Fig.  842),  and  set  with  the  double- 
fanged  teeth  behind  and  conical  in  front.  The 
vertebrae,  which  are  in  such  abundance  that  they 
are  used  for  making  fences  and  even  burned  by 
farmers  to  rid  the  fields  of  them,  are,  some  of 
them,  eighteen  inches  long  and  twelve  inches  in 
diameter  (Dana),  and  the  vertebral  column  has 

FIG.  841.-Tooth  of  a  Zeuglo-    be6n    f°Und    in    PkC6    "^    S6Venty   feet    lonS 

don  cetoides,  x  f  (Lyell).     The  animal  must  have  been  more  than 


ANIMALS.  501 

seventy  feet  long,  and  the  remains  of  at  least  forty  individuals  have 
been  found  (Lyell). 

This  animal  is  peculiarly  interesting  as  the  first  appearance  of  the 
very  distinct  order  Cetacea.  No  intermediate  links  have  yet  been 
found  connecting  this  with  other  orders  of  mammals,  or  with  the  great 


FIG.  S42.-Skull  of  Zeuglodon  hydrarchus, 


reptiles.  And  yet,  from  their  large  size  and  marine  habits,  they  are 
more  likely  than  land  mammals  to  have  been  found,  if  they  existed  in 
earlier  or  Cretaceous  times. 

The  Atlantic  and  Gulf  border  strata  are  of  course  all  marine,  and 
therefore  contain  very  few  land-animals.  It  is  to  the  fresh-water  ba- 
sins of  the  interior  that  we  must  look  for  a  full  record  of  the  mam- 


A 

FIG.  843.— Vertebra  and  Tooth  of  Zeuglodon  cetoides,  reduced. 

malian  fauna  of  America  in  Tertiary  times.  These  basins  furnish  the 
fullest  and  most  continuous  record  of  the  whole  Tertiary  which  has  ever 
yet  been  found.  It  will  be  best  to  take  them  in  the  order  of  their  age, 
as  we  can  thus  best  show  the  evidences,  if  any,  of  derivation  of  the 
later  from  the  earlier  faunae. 

4.  Green-River  Basin— Wahsatch  Beds— Lower  Eocene.— About  eigh- 
ty-seven species  of  vertebrates  have  been  found  by  Cope  in  the  San 
Juan  basin,  of  which  fifty-four  are  mammals,  one  bird  (Diatryma\ 
twenty-four  reptiles,  and  eight  fishes.  A  large  number  of  mammals 
have  also  been  found  in  beds  of  the  same  horizon  in  the  Green  River 
basin.  These  beds  have  been  shown  by  Marsh  to  be  the  equivalent  of 
the  lowest  Eocene  of  England  and  France,  and  therefore  contain  the 


502 


CEXOZOIC  ERA— AGE  OF  MAMMALS. 


very  earliest  known  true  mammalian  fauna.  In  both  countries  they  are 
characterized  by  the  occurrence  of  the  remains  of  animals  of  the  genus 
Coryphodon  (peak -tooth),  one  of  the  most  generalized  forms  of  mammals 
both  in  tooth-structure  and  in  foot-structure  yet  known.  They  were 
five-toed  Ungulates,  having  the  full  number  of  foot-bones  unmodified, 
and  a  general  structure  connecting  the  more  generalized  forms  of  Her- 
bivores, such  as  tapirs,  with  the  more  generalized  Carnivores,  such  as 
bears  (Cope).  The  genus  Coryphodon  includes  seven  or  eight  Ameri- 


FIG.  844.— Coryphodon  Hamatus  (after  Marsh) :   A,  Head,  showing  form  of  the  brain, 
foot;  C,  Fore-foot,  x  i. 


J;   .B,  Hind- 


Can  species.  The  average  size  was  about  that  of  a  tapir;  some  were 
smaller,  and  some  twice  as  large  (Marsh).  These  generalized  forms 
have  been  put  into  a  distinct  family  called  Coryphodontidce  by  Marsh. 
5.  Green  River  Basin — Bridger  Beds— Middle  Eocene. — From  this 
wonderful  fresh-water  deposit  there  have  been  described  by  Marsh, 
Cope,  and  Leidy,  150  species  of  vertebrates,  of  which  the  larger  number 
are  mammals.  This  shows  a  marvelous  abundance  of  mammalian  life 
in  this  early  Tertiary  time.  The  most  numerous  of  these  are  tapir-like 
animals,  such  as  JHyrachyus,  Limnohyus  (Palceosyops — Fig.  846),  etc. ; 


ANIMALS. 


503 


but  the  most  formidable  are  the  Dinocerata^  discovered  by  Marsh,  and 
placed  by  him  in  a  new  order.  The  Dinoceras,  the  type  of  this  family, 
was  an  animal  of  elephantine  size,  and  armed  with  both  horns  and 
tusks.  Of  horns  there  were  three  pairs — one  pair  of  small  ones  far  in 
front  on  the  nasal  bones ;  another  pair  of  larger  ones  on  the  maxillary 


FIG.  845.— Dinoceras  mirabilis,  x  |  (after  Marsh) :  A,  Skull ;  £,  Hind-foot,  x  J ;  C,  Fore-foot,  x  J. 

bones,  immediately  above  the  canines  ;  and  a  third  and  much  larger  pair 
farther  back  on  the  parietals.  Besides  these  formidable  weapons,  it 
was  furnished  also  with  powerful  pointed  tusks  eight  inches  long. 
This  order  includes,  according  to  Marsh,  many  species  belonging  to 
the  genera  Dinoceras,  Tinoceras,  and  Uintatherium. 

Another  extraordinary  group  of  animals  discovered  by  Marsh  in  the 


504 


CENOZOIC  ERA— AGE   OP  MAMMALS. 


same  beds  has  been  placed  by  him  in  a  new  order  (called  Tillodontia). 
These  animals  combine  the  head  of  a  bear  with  the  incisors  of  a  Ro- 
dent and  the  general  characters  of  Ungulates.  The  order  must  be  re- 


FIG.  846.— Limnohyus  (Pateo?yops)  (after  Leidy). 

garded,  therefore,  as  a  remarkable  generalized  type.     The  head  and 
brain  of  the  Tillotherium  are  given  in  Fig.  847. 

The  first  appearance  of  the  horse  family  (Equidce)  is  in  the  Eocene. 
First  of  all  in  the  Lower  Green  River  or  Coryphodon  beds  appears  the 


FIG.  847.— Skull  and  Brain  of  Tillotherium  fodiens,  +  J  (after  Marsh). 

Eohippus  (earliest  horse),  a  small  animal  no  bigger  than  a  fox,  having 
three  toes  on  the  hind-foot,  and  four  perfect  ones  on  the  fore-foot,  like  the 
tapir,  and  a  rudimentary  fifth  toe  ;  then  in  Green  River  Bridger  beds, 
the  Orohippus  (mountain-horse),  similar  to  the  last  in  size,  but  wanting 
the  fifth  toe. 


ANIMALS.  505 

Although  the  Herbivores  predominated,  there  were  many  mammals 
belonging  to  other  orders.  For  example,  there  were  species  allied  to 
the  Cat,  Wolf,  and  Fox  ;  also,  Bats,  Squirrels,  Moles,  and  Marsupials  ; 
also  many  Monkeys  allied  to  the  Lemurs,  Marmosets,  etc.,  but  more 
generalized  than  any  living  Lemur. 

6.  Mauvaises  Terres  of  Nebraska — White  River  Basin — Miocene. — 
From  this,  the  first  discovered  of  the  fresh-water  basins  of  the  West, 
have  been  collected  by  Hayden,  and  described  by  Leidy,  at  least 
forty  different  species  of  mammals,  among  which  twenty-five  are  Ungu- 
lates, eight  Carnivores,  and  most  of  the  remainder  Rodents.  All  of  the 
species,  and  many  of  the  families,  are  entirely  different  from  those 
found  in  the  preceding  epoch.  Although  the  tapir-like  animals  still 
prevail,  the  deer,  camel,  and  horse  family  are  also  abundant,  as  seen  in 

the  following  schedule : 

f  Hyena.       ] 

I      lTrrt|f 

Panther.    J 
Rhinoceros  family. 
Brontotheridae. 
Tapir-like  animals. 
Deer  family. 
Camel   " 
Horse    " 
Rodents. 
Turtles. 

Among  the  most  remarkable  ungulates  of  this  time  were  the  Bron- 
totheridce.  This  family,  according  to  Marsh,  includes  the  Brontotherium, 
Menodus  (Titanotherium),  and  several  other  genera.  They  were  ani- 
mals of  elephantine  size,  and  armed  with  at  least  two  horns  on  the 


Carnivores J  - Allies. 


FIG.  843.— Skull  of  Brontotherium  Ingens. 


maxillaries.     Their  nearest  allies  were  the  Rhinoceros  and  the  Tapir, 
but  they  had  affinities  also  with  the  Dinocerata  of  the  Eocene. 

The  Oreodon  is  another  very  remarkable  animal,  intermediate  be- 
tween the  hog,  the  deer,  and  the  camel,  which  at  this  time  inhabited 


506 


CEXOZOIC   ERA— AGE   OF  MAMilALS. 


the  whole  continent  from  Nebraska  to  Oregon.     A  head  of  one  is 
shown  below. 


Rhinoceros. 

Elephant. 

Mastodon. 

Three  of  the  Camel  family. 

Five  of  the  Horse         " 

Oreodon. 

Deer. 

Fox. 

Wolf. 

Tiger. 

Beaver. 

Porcupine. 


FIG.  649.— Oreodon  major,  x  J  (after  Leidy). 

7.  Mauvaises  Terres— Niobrara  Basin— Pliocene.— Nearly  in  the 

same  locality  as  the  last,  but  extending  much  farther  south,  occur  lake- 
deposits  of  the  Pliocene  epoch  full  of  mammalian  remains ;  but  these 
mammals,  though  occurring  in  the  same  locality,  belong  to  species 
entirely  different  from  those  of  the  Miocene.  Among  the  Ungulates 
there  is  a  Rhinoceros  as  large  as  the  Indian 
species;  an  Elephant  (E.  Americanus)  the 
same  as  lived  in  Quaternary  times,  as  large 
as  any  living  ;  a  Mastodon,  but  much  smaller 
than  the  great  mastodon  of  later  times ;  and 
a  large  number  of  species  of  the  Horse  and 
Camel  families,  besides  other  families  of  Un- 
gulates, Carnivores,  Rodents,  etc.,  as  shown 
in  the  accompanying  schedule.  Among  the 
Pliocene  horses  was  one  (Protohippus  par- 
vulus),  discovered  by  Marsh  in  the  Upper 
Pliocene  of  Nebraska,  only  two  feet  high.  "The  large  number  of 
camels  and  horses  gives  a  decided  Oriental  character  to  the  fauna  " 
(Dana).  Both  the  horse  and  the  camel  seem  to  have  originated  on  this, 
instead  of  on  the  Eastern,  continent ;  at  least  the  several  steps  of  their 
derivation  are  more  abundant  and  distinct  here. 

Some  General  Observations  on  the  Tertiary  Mammalian  Fauna.— 1. 
Lartet  has  shown  that  the  brain-cavity  of  some  of  the  Tertiary  ani- 
mals is  decidedly  smaller  relatively  than  that  of  their  living  congeners. 
Marsh  has,  moreover,  traced  a  gradual  increase  in  the  relative  size  of  thq 
brain  from  the  earliest  Eocene  to  the  present  time.  The  brain  of  the 
Coryphodon,  Lower  Eocene,  is  not  only  extremely  small  in  proportion 
to  the  size  of  the  animal,  but  the  higher  portion  of  the  brain — the  cere- 
bral lobes — is  very  small  in  proportion  to  the  cerebellum.  The  brain 
of  the  Middle  Eocene  Dinoceras  is  only  about  one-eighth  the  size  of  a 
living  Rhinoceros  of  equal  bulk.  The  brain  of  the  Miocene  Brontothere  is 
larger  than  that  of  the  Eocene  Dinoceras,  but  much  smaller  than  that  of 


ANIMALS.  507 

the  Pliocene  Mastodon  of  nearly  the  same  size.     Through  the  whole  line 


FIG.  852. 

FIGS.  850-852. — BRAINS  OF  CORYPHODON,  DINOCERAS,  AUD  BRONTOTHERTITM,  COMPARED  (after  Marsh) : 
850.  Corvphodrm.  skull  and  Brain,  x  i.  851.  Dinoceras,  Skull  and  Brain,  x  £.  852.  Brontothen- 
um,  Skull  and  Brain,  x  T\. 


508 


CENOZOIC  ERA— AGE   OF   MAMMALS. 


of  ancestry  of  the  horse  the  gradually-increasing  size  of  the  brain  may 
be  traced  step  by  step. 

2.  The  animals  of  the  Tertiary  are  nearly  all  connecting  types.  As 
the  Ungulates  are  the  most  largely  represented,  we  can  best  illustrate 
the  gradual  differentiation  of  modern  types  in  this  order. 

Cuvier  divided  all  Ungulates  (hoofed  animals)  into  two  orders,  viz., 
Pachyderms  and  Ruminants.  The  Pachyderms  have  always  been 
acknowledged  to  be  a  heterogeneous  order,  but  the  Ruminants  have 
been  regarded  as  one  of  the  most  distinct  of  all  mammalian  orders. 
Their  frontal  horns  in  pairs,  their  hoofs  in  pairs,  the  absence  of  upper 
front-teeth,  their  complex  stomachs,  and  habit  of  rumination,  differ- 
entiated them  widely  from  all  other  animals.  But  Prof.  Owen  showed 
that  this  distinction,  so  clear  in  zoology,  was  untenable  in  paleontology. 
He  found,  in  studying  extinct  Ungulates,  that  another  distinction,  viz., 
the  foot-structure,  was  more  fundamental  and  persistent.  He  therefore 
divided  all  Ungulates  into  Perissodactyls  (odd-toed)  and  Artiodactyls 
(even-toed).  A  Perissodactyl  may  have  five  toes,  as  in  the  Corypho- 
don  and  the  Elephant ;  or  three  toes,  as  in  the  Palseothere,  the  Rhi- 
noceros, and  the  Tapir;  or  one  toe,  as  in  the  Horse.  The  Artiodactyls 
always  have  their  toes  in  pairs :  there  may  be  only  two  toes,  as  in 
Anoplothere  and  in  Ruminants ;  or  four,  as  in  the  Hog  and  the  Hippo- 
potamus. Owen,  indeed,  made  the  Elephant,  Mastodon,  etc.,  a  distinct 


CORYPHOOONTIDAE 


PRIMAL  •  UNGULATE 
FIG.  853.— Diagram  illustrating  the  Differentiatiro  of  the  Different  Families  of  Ungulates. 

order,  under  the  name  of  Proboscidians,  but  these  are  probably  best 
regarded  as  a  very  distinct  offshoot  or  sub-order  of  the  Perissodactyls. 

Now,  in  earliest  Tertiary  times  the  Perissodactyls  and  Artiodac- 
tyls already  had  diverged  from  a  common  stock,  probably  something 
like  the  Coryphodontidao,  although  these  were  doubtless  more  nearly 


ANIMALS.  509 

allied  to  the  Perissodactyls.  Each  of  the  primary  branches  then 
divided  and  again  divided,  until  the  extreme  branch  in  one  direction 
became  the  Horse,  and  the  extreme  branch  in  the  other  direction  the 
Ox.  In  the  tree  above  we  have  attempted,  in  a  general  way,  to  repre- 
sent the  differentiation  of  the  several  orders  of  Ungulates.  The  Cu- 
vierian  orders,  Pachyderms  and  Ruminants,  are  indicated  by  a  vinculum. 
It  is  seen  at  a  glance  why,  by  studying  living  animals  alone,  the  Rumi- 
nants seem  so  distinct. 

Genesis  of  the  Horse. — In  conclusion,  it  will  be  interesting  and  in- 
structive to  run  out  one  of  these  branches  and  show  in  more  detail  the 
genesis  of  one  of  the  extreme  forms.  For  this  purpose  we  select  the 
Horse,  because  it  has  been  somewhat  accurately  traced  by  Huxley  and 
by  Marsh.  About  thirty-five  or  forty  species  of  this  family,  ranging 
from  the  earliest  Eocene  to  the  Quaternary,  are  known  in  the  United 
States.  The  steps  of  evolution  may  therefore  be  clearly  traced. 

In  the  lowest  part  of  the  Eocene  basin  ( Coryphodon  beds)  of  Green 
River  is  found  the  earliest  known  animal  which  is  clearly  referable  to 
the  horse  family,  viz.,  the  recently-described  Eohippus  of  Marsh.  This 
animal  had  three  toes  on  the  hind-foot  and  four  perfect,  serviceable  toes 
on  the  fore-foot;  but,  in  addition,  on  the  fore-foot  an  imperfect  fifth 
metacarpal  (splint),  and  possibly  a  corresponding  rudimentary  fifth  toe 
(the  thumb),  like  a  dew-claw.  Also,  the  two  bones  of  the  leg  and  fore- 
arm were  yet  entirely  distinct.  This  animal  was  no  larger  than  a  fox. 
Next,  in  the  Middle  Eocene  (Bridger  beds),  came  the  Orohippus  of 
Marsh,  an  animal  of  similar  size,  and  having  similar  structure,  except 
that  the  rudimentary  thumb  or  dew-claw  is  dropped,  leaving  only  four 
toes  on  the  fore-foot.  Next  came,  in  the  Lower  Miocene,  the  Mesohip- 
pus,  in  which  the  fourth  toe  has  become  a  rudimentary  and  useless  splint. 
Next  came,  still  in  the  Miocene,  the  Miohippus  of  the  United  States 
and  nearly-allied  Anchithere  of  Europe,  more  horse-like  than  the  pre- 
ceding. The  rudimentary  fourth  splint  is  now  almost  gone,  and  the 
middle  hoof  has  become  larger ;  nevertheless,  the  two  side-hoofs  are 
still  serviceable.  The  two  bones  of  the  leg  have  also  become  united, 
though  still  quite  distinct.  This  animal  was  about  the  size  of  a  sheep. 
Next  came,  in  the  Upper  Miocene  and  Lower  Pliocene,  the  Protohip- 
pus  of  the  United  States  and  allied  Hipparion  of  Europe,  an  animal 
still  more  horse-like  than  the  preceding,  both  in  structure  and  size. 
Every  remnant  of  the  fourth  splint  is  now  gone  ;  the  middle  hoof  has 
become  still  larger,  and  the  two  side-hoofs  smaller  and  shorter,  and  no 
longer  serviceable,  except  in  marshy  ground.  It  was  about  the  size  of 
the  ass.  Next  came,  in  the  Pliocene,  the  Pliohippus,  almost  a  complete 
horse.  The  hoofs  are  reduced  to  one,  but  the  splints  of  the  two  side- 
toes'  remain  to  attest  the  line  of  descent.  It  differs  from  the  true  horse 
in  the  skull,  shape  of  the  hoof,  the  less  length  of  the  molars,  and  some 


510  CEXOZOIC   EKA— AGE   OF  MAMMALS. 

other  less  important  details.     Last  comes,  in  the  Quaternary,  the  mod- 

a  b  c          d  e  f 


Pliohippus:  Pliocene. 


Protohippns :  Lower  Pliocene. 


Miohippus:  Miocene. 


Mesohippus:  Lower  Miocene. 


Orohippus:  Eocene. 


FIG.  R54.— Diagram  illustrating  Gradual  Changes  in  the  Horse  Family.    Throughout  a  is  fore-foot ; 
hind-foot :  c,  fore-arm ;  rf.  shank  ;  e,  molar  on  side-view ;  /  and  g,  grinding  surface  of  upper  at 
lower  molars.    (Aafter  Marsh.) 


GENERAL  OBSERVATIONS  ON  THE  TERTIARY  PERIOD.  5H 

ern  horse — Equus,  The  hoof  becomes  rounder,  the  splint-bones  shorter, 
the  molars  longer,  the  second  bone  of  the  leg  more  rudimentary,  and 
the  evolutionary  change  is  complete. 

Similar  gradual  changes,  becoming  more  and  more  horse-like,  may 
be  traced  in  the  shape  of  the  head  and  neck,  and  especially  in  the  grad- 
ually-increasing length  and  complexity  of  structure  of  the  grinding- 
teeth.  All  these  changes  are  shown  in  Fig.  854,  for  which  we  are  in- 
debted to  the  kindness  of  Prof.  Marsh.  The  Eohippus  is  omitted,  as 
no  figures  of  this  have  yet  been  published. 

There  can  be  no  doubt  that  if  we  could  trace  the  line  of  descent  still 
farther  back  we  would  find  a  perfect  five-toed  ancestor.  From  this 
normal  number  of  five,  the  toes  have  been  successively  dropped,  ac- 
cording to  a  regular  law :  first,  the  thumb,  No.  1 ;  then  the  little  finger, 
No.  5 ;  then  the  index,  No.  2  ;  and  last  the  ring-finger,  No.  4;  and  the 
middle  finger,  No.  3,  only  remains.  Nos.  2  and  4  are,  however,  usually 
dropped  together. 

A  somewhat  similar  line  of  descent  has  been  traced  by  Cope  from 
the  Miocene  Poebrotherium  through  the  Pliocene  Procamelus  to  the 
modern  camel.  It  is  remarkable  that  both  the  horse  and  the  camel  seem 
to  have  originated  on  this  continent. 

From  the  earliest  and  most  generalized  types,  therefore,  to  the  present 
specialized  types,  the  principal  changes  have  been,  first,  from  planti- 
grade to  digitigrade ;  second,  from  short-footed  digitigrade  to  long-footed 
digitigrade,  i.  e.,  increasing  elevation  of  the  heel /  third,  from  five  toes 
to  one  toe  in  the  Horse,  or  two  toes  in  Ruminants  ;  and,  fourth,  from 
simple  omnivorous  molars  to  the  complex  herbivorous  millstones  of  the 
Horse  and  the  Ox. 

The  change  from  plantigrade  to  digitigrade,  with  increasing  eleva- 
tion of  the  heel,  when  taken  in  connection  with  increasing  size  of  the 
brain,  and  therefore  presumably  with  increasing  brain-power,  shows  a 
gradual  improvement  of  structure  adapted  for  speed  and  activity,  and  a 
pari-passu  increase  of  nervous  and  muscular  energy,  necessary  to  work 
the  improved  structure. 

3.  Not  only  does  the  mammalian  fauna  of  the  Miocene  differ  com- 
pletely from  that  of  the  Eocene,  which  precedes,  and  from  the  Pliocene, 
which  succeeds  it,  but  there  seem  to  have  been  at  least  two  distinct 
Eocene  and  two  distinct  Miocene  faunae.  Thus  there  have  been  many 
complete  changes  in  the  mammalian  fauna  in  Tertiary  times. 

General  Observations  on  the  Tertiary  Period. 

We  have  already  seen  (p.  452)  that  during  Cretaceous  times  a  wide 
sea,  occupying  the  position  of  the  Western  Plains  and  Plateau  region, 
divided  America  into  two  continents,  an  Eastern  and  a  Western.  We 
have  also  seen  (p.  475)  that  at  the  end  of  the  Cretaceous  this  sea  was 


512  CENOZOIC  ERA— AGE  OF  MAMMALS. 

obliterated  by  continental  upheaval,  and  the  continent  became  one. 
During  the  Eocene,  the  eastern  portion  of  the  place  formerly  occu- 
pied by  this  sea  was  probably  dry  land,  but  in  the  Plateau  region 
there  were  great  fresh- water  lakes,  one  north  of  the  Uintah  Moun- 
tains, Green  River  basin,  and  one  south  of  the  same,  and  possibly  one 
in  Oregon.  There  were  possibly  others  yet  unknown.  At  the  end  of 
the  Eocene,  there  was  a  rise  in  the  Plateau  region,  which  drained  the 
Eocene  lakes,  and  a  corresponding  depression  in  the  region  of  the  Plains, 
not  sufficient  to  form  a  sea  again,  but  sufficient  to  form  great  Miocene 
lakes  there.  At  the  end  of  the  Miocene  occurred  the  greatest  event  of 
the  Tertiary  period,  one  of  the  greatest  in  the  history  of  the  American 
Continent.  At  that  time  the  sea-bottom  off  the  then  Pacific  coast  was 
crushed  together  into  the  most  complicated  folds  (pp.  242,  256),  and 
swollen  up  into  the  Coast  Chain,  and  at  the  same  time  fissures  were 
formed  in  the  Cascade  range,  with  the  outpouring  of  the  great  lava- 
flood  of  the  Northwest,  already  spoken  of  (p.  259).  Coincidently  with 
this  there  was  a  further  letting  down  of  the  region  of  the  Plains,  and 
an  extension  of  the  Pliocene  lakes  southward  almost  to  the  Gulf  (at- 
tended probably  with  a  further  rise  of  the  Plateau  region).  At  the 
end  of  the  Tertiary,  these  lakes  were  in  their  turn  obliterated  by  the 
further  upheaval  of  the  continent,  which  inaugurated  the  Quaternary. 

While  this  was  going  on  in  the  western  portion  of  the  continent,  on 
the  southeastern  and  southern  border  the  continent  gained,  by  gradual 
rise,  nearly  all  the  area  shaded  as  Tertiary.  In  this  direction  the  con- 
tinent was  finished  with  the  exception  of  the  larger  portion  of  Florida 
and  the  sea-islands  and  alluvial  flats 1  about  the  shores  of  the  Southern 
Atlantic  and  Gulf  States.  These  belong  to  a  still  later  period. 

Thus  we  see  that  from  the  end  of  the  Cretaceous  to  the  end  of  the 
Tertiary  there  was  a  gradual  upheaval  of  the  whole  western  half  of  the 
continent,  by  which  the  axis,  or  lowest  line,  of  the  great  interior  con- 
tinental basin  was  transferred  more  and  more  eastward  to  its  present 
position,  the  Mississippi  River.  Probably  correlative  with  this  up- 
heaval of  the  western  half  of  the  continent  was  the  down-sinking  of 
the  mid-Pacific  bottom,  indicated  by  coral-reefs  (p.  144).  Also  as  a 
consequence  of  the  same  upheaval  the  erosive  power  of  the  rivers  was 
greatly  increased,  and  thus  were  formed  those  deep  canons  in  the 
regions  (New  Mexico,  Colorado,  and  Arizona)  where  the  elevation  was 
greatest.  Thus  the  down-sinking  of  the  mid-Pacific  bottom,  the  bodily 
upheaval  of  the  Pacific  side  of  the  continent,  and  the  down-cutting  of 
the  river-channels  into  those  wonderful  canons,  are  closely  connected 
with  each  other. 

«  J  In  some  places  about  the  shores  of  the  Gulf,  for  reasons  which  will  be  explained  here- 
after, the  Quaternary  deposits  are  considerably  elevated  above  the  sea-level 


QUATERNARY   PERIOD.  513 

SECTION  2. — QUATERNARY  PERIOD. 

Characteristics. — The  chief  characteristic  of  the  Quaternary  is  that 
it  is  a  period  of  great  and  widely-extended  oscillations  of  the  earth's 
crust  in  high-latitude  regions,  attended  with  great  changes  of  climate. 
During  this  period  the  class  of  mammals  seem  to  have  culminated. 
During  this  period  also  man  seems  to  have  appeared  on  the  scene.  We 
do  not  call  it  the  age  of  Man,  however,  because  he  had  not  yet  estab- 
lished his  reign.  His  appearance  here  is  rather  in  accordance  with  the 
law  of  anticipation.  As  already  stated,  the  invertebrate  fauna  was 
almost  identical  with  that  still  living,  but  the  mammalian  fauna  was 
almost  wholly  peculiar,  differing  both  from  the  Tertiary  which  preceded 
and  from  the  present  which  followed  it. 

Subdivisions. — The  Quaternary  period  is  divided  into  three  epochs, 
viz. :  I.  Glacial;  II.  Champlain;  III.  Terrace.  These  epochs  are  char- 
acterized by  the  direction  of  the  crust-movement,  and  of  the  change  of 
climate.  The  Glacial  epoch  is  characterized  by  an  upward  movement 
of  the  crust  in  high-latitude  regions,  until  the  continents  in  those 
regions  stood  1,000  to  2,000  feet  above  their  present  height.  Large 
portions  of  these  regions  seem  to  have  been  sheeted  with  ice,  and  an 
arctic  rigor  of  climate  extended  far  into  now  temperate  regions. 

The  Champlain  epoch,  on  the  contrary,  is  characterized  by  a  down- 
ward motion  of  land-surfaces  in  the  same  region  until  the  sea  stood 
relatively  500  to  1,000  feet  above  its  present  level,  covering,  of  course, 
much  that  is  now  land-surface.  It  was,  therefore,  a  period  of  inland 
seas.  Coincident  with  this  sinking  was  a  moderation  of  climate,  and  a 
melting  of  the  ice.  It  was,  therefore,  also  a  period  of  great  lakes  and 
flooded  rivers.  Over  the  inland  seas  and  great  lakes,  loosened  masses 
of  ice  floated.  It  was,  therefore,  also  a  period  of  icebergs. 

The  Terrace  epoch  is  characterized  by  the  gradual  rising  again  to 
the  present  condition  of  the  continents,  and  the  establishment  of  the 
present  condition  of  climate.  It  is,  in  fact,  a  transition  to  the  pres- 
ent era. 

Although  we  call  these  divisions  epochs,  yet  we  must  not  suppose 
that  they  are  equal  in  length  to  the  epochs  of  earlier  times.  As  we 
approach  the  present  time,  and  the  number  and  interest  of  events  in- 
crease, our  divisions  of  time  become  shorter  and  shorter. 

It  is  so  difficult  to  separate  these  epochs  sharply  from  each  other  in 
all  countries,  and  to  synchronize  them,  that  it  seems  best  to  treat  of  the 
whole  Quaternary  period,  taking  up  the  epochs  successively — first  in 
Eastern  North  America,  as  the  type  or  term  of  comparison,  then  of  the 
same  on  the  Pacific  coast,  and  last  of  the  same  in  Europe. 
33 


514 


CEXOZOIC   ERA— AGE   OF  MAMMALS. 


Quaternary  Period  in  Eastern  North  America. 
I.   Glacial  Epoch. 

The  Materials — Drift. — Strewed  all  over  the  northern  part  of  North 
America,  over  hill  and  dale,  over  mountain  and  valley,  covering  alike, 
in  places,  all  the  country  rock,  Archaean,  Palaeozoic,  Mesozoic,  and  Ter- 
tiary, to  a  depth  of  30  to  300  feet,  and  thus  largely  concealing  them 
from  view,  is  found  a  peculiar  surface  soil  or  deposit.  It  consists  of  a 
heterogeneous  mixture  of  clay,  sand,  gravel,  pebbles,  subangular  stones 
of  all  sizes,  unsorted,  unstratified,  unfossiliferous.  The  lowest  part, 
lying  in  immediate  contact  with  the  subjacent  country  rock,  is  often  a 
stiff  clay  inclosing  subangular  stones — i.  e.,  rock-fragments  with  the 
corners  and  edges  rubbed  off.  This  we  will  call  the  "  Stony  clay  "  or 
^bowlder-clay."  It  is  precisely  like  the  moraine  prof onde  of  a  glacier 
(p.  53).  Lying  on  the  surface  of  this  drift-soil  are  found  many  bowl- 


FIG.  855.— Subangular  Stone  (after  Geikie). 

ders  of  all  sizes,  often  of  huge  dimensions,  sometimes  even  100  tons  or 
more.      The  imbedded  subangular   stones   are   usually  marked  with 

parallel  scratches  (Fig.  855), 

and  the  large  surface-bowlders 
are  usually  angular  and  un- 
scratched.  The  depth  of  this 
material  is  greatest  in  the  val- 
leys and  least  on  hill  and  moun- 

Fio.  856.— Section  on  Rush  Creek,  near  Mono  Lake,        tain  tops. 

It  is  difficult,  nay,  impossi- 
ble, to  give  a  description  of  this  peculiar  deposit,  which  will  apply  in 
all  cases.  Sometimes  scattered  about  irregularly  through  the  unstrati- 


GLACIAL   EPOCH. 


515 


fied  mass  are  portions  which  are  roughly  and  irregularly  stratified,  the 
laminaa  being  often  contorted  in  the  most  fantastic  way  (Figs.  856- 
858).  Sometimes  the  true  stony  clay  is  covered  with  a  more  reg- 
ularly stratified  material,  consisting  of  sand  and  gravel,  apparently  sub- 


FIG.  857.— Section  of  Orange  Sand,  Mississippi  (after  HUgard). 


sequently  deposited  from  water.  This  is  particularly  the  case  in  the 
basin  of  the  Mississippi,  as,  e.  g.,  in  Ohio,  Illinois,  and  Iowa.  It  is  prob- 
able, however,  that  this  belongs  to  the  next  epoch,  Champlain. 

We  have  said  that  the  deposit  is  peculiar.     Nothing  resembling  it 
is  found  anywhere  in  tropical  or  low-latitude  countries.     In  the  South- 


Fio.  853.— Section  of  Orange  Sand,  Mississippi  (after  Hilgard). 

era  Atlantic  States,  for  instance,  the  soil  is  mostly  either  the  insoluble 
residue  of  rocks  decomposed  in  situ,  or  else  consists  of  neatly-stratified 
sands  and  clays. 

Drift-material  is  not  usually  represented  on  geological  maps,  since 
it  covers  all  kinds  of  country  rock ;  or  else  the  colors  representing  the 


FIG.  859.— Outcropping— Eroded  Country  Eock  overlaid  by  Drift. 


various  kinds  and  ages  of  country  rock  are  simply  dotted  to  indicate  the 
presence  of  this  surface-material.  In  sections,  of  course,  it  is  easily 
represented,  as  in  Fig.  859. 

The  Bowlders.— The  most  casual  examination  of  the  great  bowlders 


516  CENOZOIC   ERA— AGE   OF   MAMMALS. 

is  sufficient  in  many  cases  to  show  that  they  do  not  belong  to  the  coun- 
try where  they  now  lie,  for  they  are  of  entirely  different  material  from 
the  country  rock.  For  example,  blocks  of  granite  are  found  where 
there  is  no  granite  within  many  miles,  blocks  of  sandstone  on  a  country 
rock  of  limestone,  or  vice  versa.  In  many  cases  it  is  easy  to  find  the 
parent  ledge  from  which  these  great  fragments  were  torn,  and  thus  to 
trace  the  direction  of  their  transportation.  From  many  observations 
of  this  kind  it  has  been  determined  that  in  New  England  the  bowlders 
have  come  usually  from  the  northwest,  in  Ohio  from  the  north,  and  in 
Iowa  from  the  northeast.  In  other  words,  from  the  highlands  of 
Canada  and  a  ridge  running  thence  northwestward  (Archaean  area),  the 
general  direction  of  travel  has  been  southeast,  south,  and  southwest. 
The  distance  carried  may  be  only  a  few  miles,  or  may  be  ten,  fifty,  one 
hundred,  or  even  several  hundred  miles.  In  many  cases  they  must  have 
been  carried  across  valleys  1,000  or  2,000  feet  deep,  and  lodged  high 
up  on  the  mountain  beyond.  In  many  portions  of  New  England  and 
about  Lake  Superior  the  number  of  fragments,  small  and  great,  is  so 
large  as  seriously  to  encumber  the  soil.  Not  only  the  large  bowlders, 
however,  but  the  whole  mass  of  the  material  we  have  been  describing, 
seem  to  have  been  shifted  to  a  greater  or  less  extent.  It  is  for  this 
reason  that  the  material  has  been  called  Drift. 

Surface-Rock  underlying  Drift— On  removing  the  drift-covering 
the  underlying  rock  is  every  where  polished  and  planed  and  scored  with 
parallel  lines,  and  moutonn'e,  precisely  like  rocks  over  which  a  glacier 
has  passed.  We  will,  therefore,  call  this  surface-appearance  "  glacia- 
tion."  We  reproduce  here  from  page  52  the  roches  moutonn'ees  of  an 
ancient  glacier  in  Colorado  (Fig.  860).  Examinations  of  the  scorings 
show  that  they  often  pass  straight  up  inclines  for  considerable  distances, 
i.  e.,  up  one  side  of  a  hill,  over  the  top,  and  down  the  other  side.  Their 
direction  is  uninfluenced  by  smaller  inequalities  of  surface,  though  they 
are  thus  influenced  by  the  great  valleys  and  mountain-ridges. 

The  general  direction  of  the  scorings  corresponds  with  that  of 
transportation  of  the  bowlders,  showing  that  they  are  due  to  the  same 
cause.  Perfect  soil  on  perfect  sound  rock  always  shows  that  the  soil 
has  not  been  formed  in  situ,  but  has  been  shifted:  the  polishing,  plan- 
ing, scoring,  etc.,  of  the  rock  show  that  the  agent  of  the  shifting  has 
been  ice. 

Extent. — The  general  extent  of  these  more  conspicuous  and  char- 
acteristic phenomena,  viz.,  the  glaciation,  the  stony  clay,  and  the  great 
bowlders,  is  down  to  about  40°  north  latitude.  The  line  of  southern 
limit  cuts  the  Atlantic  coast  about  40°,  near  New  York  ;  it  then  bends 
a  little  southward  to  38°  in  Southern  Ohio,  and  then  turns  a  little  north- 
ward again  as  it  passes  west,  and  finally  southward  again,  along  the 
Rocky  Mountains.  Stretching  southward  of  this  general  limit  are 


THEORY   OF  THE   ORIGIN  OF  THE   DRIFT. 


517 


local  extensions,  usually  down  valleys.  Beyond  this  the  characteristic 
phenomena  mentioned  above  are  not  found,  but  in  the  valley  of  the 
Mississippi,  and  on  each  side  to  a  considerable  distance,  a  superficial 
gravel  and  pebble  deposit,  containing  northern  bowlders — called  by 


FIG.  860.— Roches  Moutonnees  of  an  Ancient  Glacier,  Colorado  (after  Hayden). 

Prof.   Hilgard  "  Orange   Sand  " — extends  to  the   shores  of  the   Gulf. 
This  deposit,  however,  probably  belongs  to  the  early  Champlain  epoch. 

Marine  Deposits. — Along  the  northern  Atlantic  coasts  we  find  no 
marine  deposits  of  this  time,  for  the  obvious  reason  that  the  continent, 
in  that  part,  was  then  more  elevated  than  now ;  whatever  marine  de- 
posits were  then  formed  are  now  covered  by  the  sea.  But  along  the 
Southern  Atlantic  States,  coast-deposits  of  the  ordinary  kind  seem  to 
have  been  made  continuously,  and  are  still  exposed.  This  shows  that 
the  peculiar  and  violent  phenomena  of  the  North  did  not  reach  so  far, 
and  therefore  the  epochs  of  the  Quarternary  period  are  undistinguish- 
able  there.  The  formation  of  the  Peninsula  and  Keys  of  Florida, 
already  explained  (p.  149),  probably  belongs  to  the  Quaternary  and  the 
present. 

Theory  of  the  Origin  of  the  Drift. 

When  the  phenomena  of  the  Drift  were  first  observed,  they  were 
supposed  to  indicate  the  agency  of  powerful  currents,  such  as  could  be 
produced  only  by  the  most  violent  and  instantaneous  convulsions.  A 
sudden  upheaval  of  the  ocean-bed  in  northern  regions  was  supposed 
to  have  precipitated  the  sea  upon  the  land,  as  a  huge  wave  of  trans- 


518  CENOZOIC  ERA— AGE  OF  MAMMALS. 

I 

lation,  which  swept  from  north  toward  the  south,  carrying  death  and 
ruin  in  its  course.  Hence  the  deposit  was  often  called  Diluvium 
(deluge-deposit).  Now,  however,  they  are  universally  ascribed  to  the 
agency  of  ice  acting  slowly  through  great  periods  of  time.  Hence  the 
name  Glacial  epoch. 

As  to  the  manner  in  which  the  ice  acted,  however,  opinions  have 
been  more  or  less  divided,  some  attributing  the  phenomena  to  the 
agency  of  land-ice — glaciers — others  to  that  of  drifting  icebergs.  Ac- 
cording to  the  one,  the  land  during  this  epoch  was  greatly  raised  and 
covered  with  glaciers  ;  according  to  the  other,  the  same  area  was  sunk 
several  thousand  feet  and  swept  by  drifting  icebergs,  carried  south- 
ward by  currents,  and  dropping  their  load  of  earth  and  stones.  The 
one  is  called  the  glacier  theory,  the  other  the  iceberg  theory. 

It  is  probable  that  both  these  agencies  were  at  work,  either  at  the 
same  time  or  consecutively ;  but  the  decided  tendency  of  science  is 
toward  the  recognition  of  glaciers  as  the  principal  agent  during  this 
earliest  epoch  of  the  Quaternary.  The  more  the  phenomena  are  stud- 
ied, and  the  more  glaciers  are  studied,  especially  in  polar  regions,  the 
larger  is  the  share  attributed  to  this  agency.  We  will  not  discuss  this 
question,  but  simply  give  the  present  condition  of  science  on  the 
subject. 

Statement  of  the  most  Probable  View. — The  most  probable  view 
for  America,  and  also  for  other  countries,  is,  that  the  Drift,  or  at  least 
the  most  characteristic  phenomena  of  the  Drift,  viz.,  the  glaciation,  the 
unsorted  bowlder-clay,  and  in  many  cases  also  the  great  traveled 
bowlders,  are  due  to  the  action  of  glaciers.  They  are  therefore  a  land- 
deposit,  and  not  a  sub-aque.ous  deposit.  For  general  proof  of  this,  let 
any  one  study  the  phenomena  of  living  glaciers,  in  the  Alps  and  else- 
where ;  then  let  him  study  the  appearances  left  by  the  recently  dead 
glaciers  of  the  Sierra ;  and  then  let  him  study  the  phenomena  of  the 
Drift,  especially  the  stony  clay  and  the  underlying  glaciated  surfaces. 
It  will  be  impossible  for  him  to  come  to  any  other  conclusion  than  that 
the  same  agent  has  been  at  work  in  all  these.  In  some  cases,  viz.,  in 
the  valley-extensions  of  the  Drift  area,  still  more  conclusive  evidence  is 
found  in  the  existence  of  distinct  terminal  moraines. 

Objections  answered. — Many  objections  have  been  brought  against 
this  view,  which  may  be  compendiously  stated  as  follows  :  1.  In  glacial 
regions,  like  Switzerland,  the  Himalayas,  etc.,  the  glaciers  run  in  all 
directions  ;  but  the  Drift  was  carried  over  wide  areas,  in  a  general  direc- 
tion. Such  a  general  direction  is  easily  accounted  for  by  the  action  of 
icebergs  carried  by  marine  currents.  2.  The  agent  of  the  Drift  seems 
to  have  been  often  uninfluenced  by  the  direction  of  valleys  and  ridges 
even  of  considerable  size ;  thus,  for  instance,  bowlders  are  carried 
across  valleys  500  or  1,000  feet  deep,  and  lodged  as  high  up  on  the 


THEORY  OF  THE  ORIGIN  OF  THE  DRIFT.  519 

mountain-slope  on  the  other  side.  This  is  perfectly  consistent  with  the 
action  of  icebergs  drifting  over  an  uneven  sea-bottom,  but  inconsistent 
with  our  usual  notions  of  glacial  action.  3.  The  great  distance  carried, 
sometimes  one  hundred  miles  or  more,  is  precisely  what  we  might 
expect  of  icebergs,  but  difficult  to  reconcile  with  our  usual  notions  of 
glaciers.  4.  Alpine  glaciers  will  not  move  on  a  slope  of  less  than  2° 
or  3°,  but  such  a  slope,  carried  several  hundred  miles,  would  produce 
an  incredible  elevation  of  land.  A  slope  of  2|°  for  200  miles  would 
produce  an  elevation  of  nearly  nine  miles  ! 

These  were  unanswerable  objections  so  long  as  our  ideas  of  glaciers 
were  confined  to  those  of  temperate  climates ;  but  they  all  find  their 
complete  answer  in  the  phenomena  of  the  polar  ice-sheet.  Greenland 
is  1,200  miles  long  and  400  or  500  miles  wide.  This  whole  area  of 
over  a  half-million  of  square  miles  is  covered  3,000  feet  deep  with 
ice.  This  ice-mantle  moves  en  masse  seaward,  moulding  itself  on 
the  surface  inequalities  of  the  country,  and  moulding  that  surface  be- 
neath itself,  producing  universal  glaciation,  and  only  separating  into 
distinct  glaciers  at  its  margin.  In  antarctic  regions  the  general  ice- 
sheet  is  even  still  more  extensive  and  thick.  Now,  it  is  to  such  an  ice- 
mantle  that  the  Drift  is  to  be  ascribed,  for  it  moves  irrespective  of 
smaller  valleys,  in  one  general  direction  over  great  areas,  to  great  dis- 
tances, and  over  a  slope  of  only  1°  or  even  £°. 

Probable  Condition  during  Glacial  Times  in  America.— During 
Glacial  times  the  Archaean  region  of  Canada  seems  to  have  been  elevated 
1,000  to  2,000  feet  above  its  present  level,  and  covered  with  'a  general 
ice-mantle  3,000  feet  to  6,000  feet  thick.  This  ice-sheet  moved  with 
slow  glacier  motion  southeastward,  southward,  and  southwestward,  over 
New  England,  New  York,  Ohio,  Illinois,  Iowa,  etc.,  regardless  of 
smaller  valleys,  glaciating  the  whole  surface,  and  gouging  out  lakes  in 
its  course.  Northward  the  ice-sheet  probably  extended  to  the  poles ;  it 
was  an  extension  of  the  polar  ice-cap,  but  its  southern  limit  was  about 
38°  to  40°  north  latitude,  except  in  the  Rocky  Mountain  region,  where, 
favored  by  the  elevation,  it  probably  extended  considerably  farther 
south.  From  its  southern  margin  the  ice-sheet  stretched  out  icy  fin- 
gers, as  separate  glaciers,  down  some  of  the  principal  valleys.  For 
example  :  one  great  extension  stretched  southward  as  the  Hudson  River 
glacier,  and  its  bed  may  still  be  traced  far  out  to  sea.  Another  was 
the  Susquehanna  glacier.  Those  along  the  eastern  coast  ran  into  the 
sea  and  produced  icebergs ;  but  westward  over  Ohio.  Illinois,  etc.,  where 
the  glaciers  did  not  run  into  the  sea,  these  separate  glaciers  must  have 
produced  terminal  moraines  ;  but  these  have  been  mostly  washed  away 
by  the  floods  of  the  Champlain  epoch.  Some  evidences  of  them,  how- 
ever, have  been  observed.  Along  the  eastern  slopes  of  the  Rocky 
Mountains  the  evidences  of  these  separate  glaciers  are  very  abundant, 


520 


CENOZOIC  ERA— AGE   OF  MAMMALS. 


and  their  lateral  and  terminal  moraines  very  distinct  (Fig.  861).  Some 
evidences  of  glaciers  have  also  been  detected  in  the  mountains  of  Vir- 
ginia.1 

It  is  probable  that  in  the  valley  of  the  Mississippi  the  northern  ele- 
vation extended  even  to  the  shores  of  the  Gulf.  Prof.  Hilgard  finds 
the  evidence  of  this  in  the  Orange,  sand  which  belongs  to  this  epoch, 


FIG.  861.— Moraines  of  Grape  Creek,  Sangre  del  Cristo  Mou 


lo  (after  Stevenson). 


or  the  beginning  of  the  Champlain,  and  which  indicates  torrential  cur- 
rents,  and  must  have  been  therefore  deposited  above  sea-level,  and  yet 
in  the  region  of  the  Mississippi  Delta  is  now  several  hundred  (400)  feet 
below  that  level.  The  evidence  is  made  still  more  conclusive  by  the 
discovery  above  the  Orange  sand  of  a  stump-layer,  or  old  forest-ground, 
also  several  hundred  feet  below  present  tide-level. 

II.    Champlain  Epoch. 

During  the  Glacial  epoch,  as  just  seen,  the  whole  northern  portion 
of  the  continent  was  elevated  1,000  to  2,000  feet  above  the  present  con- 
dition ;  the  polar  ice-cap  had  advanced  southward  to  40°  latitude,  with 
still  farther  southward  projections  favored  by  local  conditions;  and  an 
arctic  rigor  of  climate  prevailed  over  the  United  States  even  to  the 
shores  of  the  Gulf.  At  the  end  of  this  epoch  an  opposite  or  downward 
movement  of  land-surface  over  the  same  region  commenced,  and  con- 

1  American  Journal  of  Science,  vol.  vi.,  p.  371  (Stevens). 


CHAMPLAIN  EPOCH.  521 

tinued  until  a  depression  of  500  to  1,000  feet  below  the  present  level 
was  attained.  This  downward  movement  marks  the  beginning  of  the 
Champlain  epoch.  As  a  necessary  consequence,  large  portions  of  the 
now  land  were  submerged  ;  it  was  therefore  a  time  of  inland  seas. 
Another  result,  or  at  least  a  concomitant,  was  a  moderation  of  the 
climate,  a  melting  of  the  glaciers  and  a  retreat  of  the  margin  of  the 
ice-cap  northward.  It  was  therefore  a  time  of flooded  lakes  and  rivers. 
Lastly,  over  these  inland  seas  and  great  lakes  loosened  masses  of  ice 
floated  as  icebergs.  It  was  therefore  preeminently  a  time  of  iceberg 
action. 

Evidences  Of  Subsidence. — The  evidences  of  the  condition  of  things 
described  above  are  found  in  old  sea-margins,  old  lake-margins,  old 
river-terraces,  and  old  flood-plain  deposits. 

Sea-Margins. — Old  sea-margins,  containing  shells  and  other  remains 
of  living  species,  are  found  all  along  the  Northern  Atlantic  coast,  be- 
coming higher  as  we  pass  northward.  In  Southern  New  England  the 
highest  beaches  are  40  to  50  feet ;  about  Boston  they  are  75  to  100 
feet  ;  in  Maine  they  are  200  feet  and  upward  ;  on  the  Gulf  of  St. 
Lawrence  they  are  470  feet  ;  in  Labrador  500  feet.  In  arctic  regions 
they  are  in  some  places  1,000  feet  (Dana).  The  beaches  may  be  traced 
up  both  sides  of  the  St.  Lawrence  River,  and  thence  around  Lake  Cham- 
plain,  where  the  highest  is  393  above  tide-level.1  Upon  the  beaches 
about  Lake  Champlain  have  been  found  abundance  of  marine  shells, 
and  also  the  skeleton  of  a  stranded  ichale.  Evidently  there  was  here 
a  great  inland  sea  connected  with  the  ocean  through  the  Gulf  of  St. 
Lawrence ;  and  over  this  sea  icebergs  must  have  floated.  This  condi- 
tion of  things  has  given  name  to  the  epoch.  In  the  subsequent  reele- 
vation  of  the  continent,  this  salt  lake  (as  it  must  have  been  at  first) 
was  gradually  rinsed  out  and  freshened  by  river-water  discharged 
through  the  lake  and  into  the  St.  Lawrence  River,  as  already  explained 
on  a  previous  page  (p.  74). 

Flooded  Lakes.— All  the  lakes  in  the  region  affected  by  drift  show 
unmistakable  evidences  of  a  far  more  extended  and  higher  condition 
of  the  waters  than  now  exists.  About  all  these  lakes  is  found  a 
succession  of  terraces  or  old  lake-margins.  The  highest  of  these  marks 
the  highest  water-level,  and  is  the  oldest ;  the  lower  ones  mark  succes- 
sive steps  in  the  draining  away  or  drying  awav  of  the  waters. 

For  example,  about  Lake  Ontario  successive  margins  are  found  up 
to  500  feet  above  the  present  lake-level ;  about  Lake  Erie,  up  to  250 
feet ;  about  Lake  Superior,  up  to  330  feet ;  and  similar  margins  are 
found  about  Lakes  Michigan  and  Huron.  There  can  be  no  doubt  that 
at  this  time  these  lakes  ran  together  to  form  an  immense  sheet  of  fresh 
water  covering  the  larger  portion  of  Ohio,  which  according  to  New- 
1  Dana,  "  Manual,"  p.  550. 


522  CENOZOIC  ERA— AGE   OF  MAMMALS. 

berry1  drained  southward  into  the  Ohio  and  Mississippi  Rivers,  and 
on  which  floated  many  icebergs  loosened  from  the  Canadian  glaciers, 
and  dropping  earth  and  bowlders  over  Ohio.  Hilgard  *  thinks  that  the 
bursting  of  this  great  lake  over  a  barrier  across  Southern  Ohio  and  Illi- 
nois, discharging  its  waters  southward,  carried  the  Orange  sand  over 
that  region. 

G.  M.  Dawson  3  finds  abundant  evidence  of  a  prodigious  lake  or  sea 
in  British  America,  extending  from  the  Laurentian  axis  to  the  Rocky 
Mountains  (doubtless  connected  with  the  lake  previously  mentioned), 
into  which  ran  glaciers  from  the  Laurentian  axis  on  the  one  side,  and 
the  Rocky  Mountains  on  the  other,  forming  icebergs  which  dropped 
their  debris  over  the  whole  area. 

Both  the  elevation  of  the  previous  epoch  and  the  subsidence  of 
this  seem  to  have  been  greater  along  the  axis  of  the  continent,  the  val- 
ley of  the  Mississi2)pi,  than  on  the  coasts.  Hilgard  finds  evidence  in 
the  Orange-sand  deposit,  and  in  the  thickness  of  the  subsequent  Cham- 
plain  deposit,  of  an  elevation  of  450  feet  above  the  present  level,  and 
a  depression  of  450  feet  (for  this  is  the  maximum  elevation  of  the 
Champlain  deposit  above  the  same  level),  or  an  oscillation  of  900  feet, 
in  Louisiana.  Farther  north  it  is  probably  much  greater. 

River  Terraces  and  Old  Flood-Plain  Deposits. — Nearly  all  the  rivers 
in  the  eastern  portion  of  the  continent,  over  the  Drift  region,  are  bor- 
dered with  high  terraces,  which  have  been  cut  wholly  out  of  an  old 
flood-plain  deposit  belonging  to  the  Champlain  epoch.  In  fact,  these 
rivers  show  first  an  elevation,  then  a  depression,  and  finally  a  partial  re- 
elevation  ;  in  other  words,  all  the  oscillations  of  the  Quaternary  period 
are  recorded  by  them. 

An  examination  of  the  rivers  north  of  the  fortieth  parallel  shows:  1. 
An  old  river-led  far  deeper  and  broader  than  the  present ;  2.  This  deep 
and  broad  river-bed  is  filled  up,  often  several  hundred  feet  deep,  by  old 
river-deposit;  3.  Into  this  old  river-deposit  the  shrunken  stream  is 
again  cutting,  but  is  still  far  above  the  bottom  of  the  old  river-bed. 
This  cutting  into  the  old  river-deposit  produces  bluffs  and  terraces  on 
each  side.  It  is  evident  that  the  great  river-bed  was  gouged  out  during 
the  Glacial  epoch  ;  the  filling  up  took  place  during  the  Champlain,  and 
the  cutting  and  terracing  during  the  Terrace  epoch. 

Fig.  862  is  an  ideal  section  across  a  river-bed  in  the  Drift  region,  in 
which  b  b  is  the  old  river-bed,  scooped  out  during  the  epoch  of  eleva- 
tion; the  dotted  line  represents  the  highest  level  to  which  the  old 
river-deposit  accumulated,  and  the  shaded  portion  that  part  of  such 

1  Newberry,  "  Surface  Geology." 

8  American  Journal  of  Science  and  Arts,  December,  1871. 

3  Quarterly  Journal  of  the  Geological  Society,  vol.  xxxi.,  pp.  620,  et  seq.,  and  "  Geology 
of  the  Forty-ninth  Parallel,"  chaps,  ix.,  x. 


CHAMPLAIN  EPOCH.  523 

deposit  which  still  remains.  The  upper  terraces,  t  £,  are  of  course  the 
oldest,  the  lower  ones  being  made  as  the  shrunken  stream  cut  deeper 
and  deeper. 

These  phenomena  are  shown  in  all  the  river-beds  of  the  Drift  region, 
but  especially  by  those  of  the  Mississippi  basin.     Sometimes  there  is 


FIG.  862.— Ideal  Section  across  a  River-bed  in  Drift  Region :  b  b  6,  old  river-bed  ;  R,  the  present  river ,  1 1, 
upper  or  older  terraces ;  t'  t',  lower  terraces. 

only  one  terrace  or  bluff ;  sometimes  there  are  several,  on  each  side. 
The  Connecticut  River  is  a  good  example  of  the  latter,  the  Mississippi 
River  of  the  former. 

The  Connecticut  River  is  bordered  on  each  side  by  a  succession  of 
terraces  rising  one  above  and  beyond  the  other,  composed  wholly  of 
old  river-deposit.  Beyond  this,  of  course,  is  the  country  rock  of  Jura- 
Trias  sandstone,  covered  more  or  less  with  drift. 

The  Mississippi  River  is  bordered  on  each  side  by  its  present  flood- 
plain  deposit,  or  river-swamps.  This,  as  already  said  (p.  23),  extends 
from  the  mouth  of  the  Ohio  River  to  the  head  of  the  delta,  a  distance  of 
800  miles,  and  has  an  average  width  of  20  miles.  This,  its  present 
flood-plain  deposit,  is  limited  on  the  eastern  side  by  bluffs  in  some  places 
200  to  400  feet  high,  composed  of  Tertiary  strata,  capped  with  an  old 
river-silt,  or  Loess,  50  to  70  feet  thick,  and  this,  again,  covered  by  a 
yellow  loam,  which  extends  beyond  the  limits  of  the  Loess.  A  layer 
of  Orange  sand  separates  the  Loess  from  the  Tertiary.  Patches  of  the 
Loess  or  bluff-deposit  are  found  also  on  the  western  side,  showing  that 
the  old  flood-plain  extended  beyond  the  present  flood-plain  on  both 
sides ;  but  on  the  west  side  it  has  been  mostly  removed  by  subsequent 
erosion.  Beneath  the  present  river  swamp-deposit  isfotmd,  by  borings, 
a  deposit  belonging,  like  the  Loess,  to  the  Champlairi  epoch,  but  to  an 
earlier  period,  probably  an  estuary  deposit,  and  called  by  Hilgard 
"Port  Hudson"  varying  in  thickness  from  30  feet  at  Memphis  to  sev- 
eral hundred  feet  in  the  delta.  Beneath  this  is  first  the  Orange  sand 
and  then  the  Tertiary. 

All  these  facts  are  represented  in  the  ideal  section  of  the  river  and 
the  strata  in  its  vicinity,  given  below,  constructed  from  the  investiga- 
tions of  Prof.  Hilgard.  It  is  evident  that  a  great  trough  was  hollowed 
out  in  the  Tertiary  strata  during  the  Glacial  epoch,  filled  with  deposit 
to  the  level  1 1  during  the  Champlain,  and  again  partly  cut  out  during 
the  Terrace. 


524  CENOZOIC  ERA— AGE   OF  MAMMALS. 

The  cause  of  the  flooded  condition  of  the  rivers  and  lakes  was  part- 
ly the  depression  of  the  land,  by  which  the  sea  entered  into  the  old 
glacial  beds,  forming  estuaries;  partly  the  smaller  angle  of  slope  of  the 
rivers,  by  reason  of  which  the  waters  in  their  lower  parts  ran  off  less 


FIG.  863.— Ideal  Section  across  Mississippi  below  Vicksburpr :  OS,  Orange  sand ;  PH.  Port  Hudson, 
'       te,  Loess  or  old  flood-plain  d. 
>•«,  river-swamp  deposit,  moder 


estuary  deposit.  Champlain;  Is,  Loess'or  old  flood-plain  deposit,  Champlain;  t,  loam  covering  the 
Loess,  but  more  extensive ;  r 


rapidly,  and  therefore  were  more  swollen,  and  therefore  also  deposited 
more  sediment ;  and  partly  the  greater  abundance  of  the  water-supply, 
from  the  melting  of  the  glaciers. 

III.   Terrace  Epoch. 

At  the  end  of  the  epoch  of  subsidence,  when  the  condition  of  sea 
and  lakes  and  rivers  was  what  we  have  described,  there  commenced  a 
movement  again  in  an  opposite  direction,  by  which  the  lands  were 
slowly  brought  upward  to  their  present  condition — a  condition,  how- 
ever, far  less  elevated  than  during  the  Glacial  epoch. 

Evidences. — Sea. — The  reelevation  was  not  perfectly  steady  and  uni- 
form, but  stopped,  from  time  to  time,  sufficiently  long  for  the  sea  to 
make  distinct  beaches.  Below  the  highest  beach,  which  marks  the 
maximum  depression  of  the  Champlain  epoch,  and  which  has  already 
been  described,  several  other  beaches  are  traceable,  which  evidently 
mark  the  successive  steps  of  reelevation. 

Lakes. — Also,  the  reelevation  of  the  land  would  bring  down  the  level 
of  the  lakes,  partly  by  change  of  climate  diminishing  the  water-supply, 
and  partly  by  increasing  the  slope,  and  thereby  increasing  the  erosive 
power,  of  the  discharge-rivers,  and  thus  draining  off  the  lake- waters. 
This  is  well  shown  on  the  Canadian  lakes,  where,  in  addition  to  the 
highest  terrace,  already  mentioned,  which  marks  the  highest  flood-level 
of  the  Champlain  epoch,  are  found  several  lower  terraces,  which  mark 
the  successive  stages  of  the  subsequent  depression  of  the  lake-surface. 
These  distinct  beaches  would  seem  to  indicate  that  the  rate  of  draining 
away  and  letting  down  of  the  water  was  not  uniform,  but  had  periods 
of  greater  and  periods  of  less  rapidity. 

Rivers. — It  is  hardly  necessary  to  say  that  the  reelevation  would  lay 
bare  the  old  flood  and  estuary  deposits  of  the  rivers,  and  the  rivers 
would  immediately  commence  cutting  into  these  deposits,  forming  ter- 


TERRACE  EPOCH.  525 

races  and  bluffs,  in  number  and  height  depending  upon  the  depth  of 
the  cutting.  The  Connecticut  River  has  made  many  of  these  terraces, 
the  highest,  of  course,  being  the  oldest.  The  Mississippi  has  apparent- 
ly made  but  one,  but  this  one  is  very  high  (Fig.  863).  The  highest 
point  of  this  Champlain  deposit,  according  to  Hilgard,  is  at  least  450 
feet  above  tide-level,  showing  a  reelevation  and  a  cutting  to  that  extent 
during  the  Terrace  epoch. 

History  of  the  Mississippi  River. — It  may  be  interesting  to  stop  a 
moment,  and  trace,  briefly,  the  history  of  this  great  river.  During  the 
Cretaceous  period,  the  Ohio  probably  ran  into  the  embayment  of  the 
Gulf,  represented  in  Fig.  728  (p.  452)  ;  but  the  Mississippi  did  not  yet 
exist.  The  drainage  of  all  that  part  of  the  continent  was,  doubtless, 
into  the  great  Cretaceous  inter-continental  sea.  At  the  beginning  of 
the  Tertiary  period,  the  Mississippi  probably  commenced  to  run  into 
the  Tertiary  embayment,  shown  in  Fig.  790  (p.  479).  The  Red  and  Ar- 
kansas, if  they  then  existed,  were  not  tributaries,  but  separate  rivers, 
emptying  into  the  same  embayment.  The  Ohio  was  almost,  if  not  quite, 
a  separate  river  also.  During  the  Glacial  epoch,  the  whole  embayment 
of  the  Gulf  was  abolished  by  elevation.  This  is  clearly  demonstrated 
by  the  torrential  pebble-deposit  (Orange  sand),  and  by  the  stump-layer 
(old  forest-ground),  found  by  Hilgard  beneath  the  Port  Hudson  (Cham- 
plain)  deposit,  on  the  shores  of  the  Gulf.  During  the  same  epoch,  bv 
reason  of  this  elevation,  the  great  trough,  represented  in  Fig.  863, 
was  scooped  out  of  the  Tertiary  strata,  200  to  500  feet  deep,  either 
by  a  tongue-like  extension  of  the  northern  ice-sheet,  or  else,  more  prob- 
ably, by  the  erosive  power  of  water,  favored  by  the  greater  slope  of  the 
country  southward  at  that  time,  and  also  by  the  greater  water-supply. 
During  the  Champlain  epoch,  by  subsidence  this  great  trough  became 
an  arm  of  the  Gulf,  or  an  estuary,  fifty  to  one  hundred  miles  wide,  and 
reaching  up  to  the  mouth  of  the  Ohio,  with  extensions  up  the  tribu- 
taries ;  and  this  estuary  became  filled,  200  to  500  feet  deep,  with  sedi- 
ments. This  deposit  was  at  first  estuarian  (Port  Hudson),  and  after- 
ward river-silt  (Loess).  During  the  Terrace  epoch,  this  silt  was  laid 
bare,  and  the  river  commenced,  and  continued  to  cut,  until  the  bluffs  be- 
came 200  to  400  feet  high.  Finally,  during  the  Recent  epoch,  the  river 
has  again  commenced  building  up  by  sedimentation,  showing  thus  a 
slight  depression  again,  or,  at  least,  a  cessation,  of  the  reelevation  of 
the  Terrace  epoch.  This  up-building  by  sedimentation  has  continued 
up  to  the  present  moment,  and  the  deposit  (river-swamp  and  delta  de- 
posit) has  reached,  according  to  Hilgard,  a  thickness  of  forty  to  fifty 
feet.  Thus  the  phenomena  of  the  Mississippi  distinctly  separate  the 
Terrace  from  the  Recent  epoch. 


526  CENOZOIC  ERA— AGE  OF  MAMMALS. 

Quaternary  Period  on  the  Western  Side  of  the   Continent. 

All  the  most  characteristic  phenomena  of  this  period,  such  as  gen- 
eral glaciation,  raised  sea-margins,  flooded  lakes,  and  flooded  rivers, 
are  abundant  and  conspicuous  on  the  Pacific  side  of  the  continent. 
Especially  are  the  evidences  of  separate  ancient  glaciers  far  more  per- 
fect than  on  the  eastern  coast,  in  fact  as  perfect  as  in  any  part  of  the 
world. 

Glaciers.1 — There  seems  little  doubt  that  during  the  fullness  of 
Glacial  times  an  extension  of  the  northern  ice-sheet  covered  the  Sierra 
Nevada,  even  to  Southern  California.  From  the  margins  of  this  ice- 
sheet,  doubtless,  stretched  valley-extensions  in  the  form  of  separate 
glaciers  into  the  plains  east  and  west.  The  direction  of  motion,  and 
therefore  of  transportation,  was  mainly  eastward  and  westward  from 
the  crest,  determined  by  the  mountain-slope;  but  also  partly  southward, 
determined  by  northern  elevation.  The  evidence  of  this  condition  of 
things  is  yet  imperfect.  It  consists  mainly  in  the  general  contour- 
forms  of  the  surface  of  the  whole  higher  or  granite  region  of  the  Sierra 
— a  rounded,  billowy  appearance,  like  moutonne  rocks  on  a  huge  scale. 

Following  this  ice-sheeted  condition,  we  have  in  the  same  region 
the  most  perfect  and  abundant  evidences  of  an  epoch  of  great  separate 
glaciers,  and  associated  with  these  are  evidences  of  flooded  lakes,  into 
which  the  glaciers  ran  and  formed  icebergs,  and  of  flooded  rivers, 
whose  swollen  currents  carried  away  and  redeposited  the  glacial  debris. 
This  time  of  great  glaciers  in  California  probably  corresponds  with  the 
Champlain  epoch. 

It  is  impossible  to  describe  all  the  great  ancient  glaciers  whose 
tracks  have  been  traced.  They  filled  all  the  larger  canons,  and  their 
tributaries  all  the  higher  and  smaller  valleys  and  meadows.  Their 
tracks  are  everywhere  marked  by  glaciation  and  strewed  bowlders,  and 
their  terminus  at  different  times  by  a  succession  of  terminal  moraines 
and  lakelets.  We  will  mention  three  or  four  as  examples : 

1.  During  the  epoch  spoken  of  a  great  glacier,  receiving  tributaries 
from  Mount  Hoffman,  Cathedral  Peaks,  Mount  Lyell  and  Mount  Clark 
groups,  filled  Yosemite  Valley,  and  passed  down  Merced  Canon.     The 
evidences  are  clear  everywhere,  but  especially  in  the  upper  valleys, 
where  the  ice-action  lingered  longest. 

2.  At  the  same  time  tributaries  from  Mount  Dana,  Mono  Pass,  and 
Mount   Lyell,  met  at  the  Tuolumne  meadows  to   form  an  immense 
glacier,  which,  overflowing  its  bounds  a  little  below  Soda  Springs,  sent 
a  branch  down  the  Tenaya  Canon  to  join  the  Yosemite  glacier,  while  the 
main  current  flowed  down  the  Tuolumne  Canon  and  through  Hetch- 

1  For  a  fuller  account  of  the  glaciers  of  the  Sierra,  and  the  condition  of  things  dur- 
ing the  Glacial  epoch,  see  American  Journal  of  Science,  vol.  iii.,  p.  325,  and  vol.  x.,  p.  26. 


QUATERNARY   PERIOD  ON   WESTERN   SIDE  OF   CONTINENT.        527 

hetchy  Valley.  Knobs  of  granite,  500  to  800  feet  high,  standing  in  its 
pathway,  were  enveloped  and  swept  over,  and  are  now  left  round,  and 
polished,  and  scored,  in  the  most  perfect  manner.  This  glacier  was  at 
least  forty  miles  long  and  1,000  feet  thick,  for  its  stranded  lateral  mo- 
raine may  be  traced  so  high  along  the  slopes  of  the  bounding  moun- 
tain. 

3.  The  Sierra  range  on  its  western  side  slopes  gradually  for  fifty  or 
sixty  miles ;  but  on  the  eastern  side  it  is  very  precipitous,  so  that  the 
plains  5,000  to  7,000  feet  below  the  crest  are  reached  in  two  or  three 
miles.    In  glacial  times  long  and  complicated  Glaciers  with  many  tribu- 
tuaries  occupied  the  western  slope,  while  on  the  eastern  slope  innumer- 
able short,  simple  glaciers  flowed  in  parallel  streams  down  the  steep  in- 
cline and  out  for  several  miles  on  the  level  plain,  or  even  into  the  waters 
of  Lake  Mono.     One  of  the  largest  of  these  took  its  rise  in  the  snow- 
fields  about  M*mo  Pass,  flowed  down  Bloody  Cation,  and  six  to  seven 
miles  out  on  the  plain,  and  evidently  into  the  waters  of  Lake  Mono, 
which  was  then  far  more  extensive  and  higher  than  now.     Parallel  mo- 
raines, 300  feet  high,  formed  by  the  dropping  of  glacial  dkbris  oh  each 
side  of  the  icy  tongue,  as  it  ran  out  on  the  plain  or  on  the  bottom  of 
the  shallow  lake,  are  very  conspicuous,  as.  are  also  the  successive  ter- 
minal moraines  left  in  the  subsequent  retreat.     Behind  these  moraines 
water  has  accumulated,  forming  lakelets. 

4.  In  the  fullness  of  Glacial  times  Lake  Tahoe  basin  was  wholly 
occupied  by  ice,  which  probably  ran  out  upon  the  plains  of  the  Basin 
region.      But  in  the  epoch  of  great  glaciers,  of  which  we  are  now 
speaking,  its  basin  was  filled  with  water,  and  to  a  somewhat  higher 
level  than  at   present  ;    and  into  the  lake  ran  many  glaciers,  whose 
tracks  are  still  perfectly  distinct.     The  lakelets  and  lake-like  bay  seen 
about  the  southern  end  of  the  great  lake,  and  which  form  so  conspicu- 
ous a  feature  of  its  scenery,  were  scooped  out  by  these  steeply-de- 
scending glaciers;    and  the  long   parallel  debris-ridges  bordering  the 
lakelets,  and  stretching  down  to  the  shores  of  the  great  lake,  have 
been  deposited  on  each  side  of  the  glaciers  as  they  ran  out  into  the  lake, 
doubtless  to  form  icebergs.1    (See  Fig.  864). 

During  the  Terrace  epoch  these  great  glaciers  of  the  Sierra  retreat- 
ed, but  not  at  uniform  rate,  leaving  verv  distinct  terminal  moraines  at 
the  places  where  their  points  rested  awhile,  until  they  have  mostly 
retired  within  the  snow-fields  which  gave  them  birth.  The  feeble  re- 
mains of  some  of  them  may  still  be  found  hidden  away  among  the 
coolest  and  shadiest  hollows  of  the  high  Sierra  region. 

Lake-Margins. — About  all  the  great  lakes  there  are  terraces  or  other 
evidences  of  a  higher  and  more  extensive  condition  of  their  waters. 
About  Lake  Mono  there  are  five  or  six  very  distinct  terraces,  the  high- 
est of  which  is  600  to  700  feet  above  the  present  lake  level.  This 
1  American  Journal  of  Science,  vol.  x.,  p.  126,  1875. 


528 


CENOZOIC   ERA— AGE   OF   MAMMALS. 


would  carry  the  lake-waters  to  the  base  of  the  Sierra,  and  necessitate 
the  flow  of  glaciers  into  them,  and  the  formation  of  icebergs. 

About  Great  Salt  Lake  successive  terraces  have  been  traced  up  to 
more  than  900  feet  above  the  present  lake-level.  At  that  time  it  is 
estimated  to  have  contained  400  times  its  present  volume  of  water ; 
and  there  are  some  reasons  for  thinking  that  it  probably  once  dis- 


FIG.  864.— Diagram  Mnp,  showing  the  Southern  End  of  Lake  Tahoe  with  its  Lakelets  and  Lateral 
Moraines. 

charged  into  the  Pacific  through  the  Snake  and  Columbia  Rivers,  for 
the  divide  between  the  Salt  Lake  basin  and  the  Columbia  River  is  only 
about  600  feet  above  the  present  lake-level.1  If  so,  it  was  then  a 
fresh,  or,  at  least,  a  brackish-water  lake.  About  other  salt  lakes  in 
this  region  the  same  phenomenon  is  observed.  In  fact,  in  all  the  Basin 

1  Dr.  Blake,  "  Proceedings  of  California  Academy  of  Science,"  vol.  iv.,  p.  276. 


QUATERNARY  PERIOD   ON  WESTERN  SIDE   OF   CONTINENT.          529 

region,  the  valleys  between  the  parallel  ranges  were  then  filled  with 
water  (Gilbert). 

During  the  Terrace  epoch  these  lakes  were  partly  drained  away, 
but  still  more  dried  away  to  lower  and  lower  levels,  marked  now  by 
successive  terraces.  For,  if  in  the  East  the  lakes  were  mostly  drained 
away  by  change  of  level,  in  the  West  they  were  mostly  dried  away  bj' 
change  of  climate. 

Rivers. — The  rivers,  especially  in  California,  mark  very  distinctly 
all  the  stages  of  the  Quaternary.  There  are  in  many  parts  of  Califor- 
nia two  systems  of  river-beds,  an  old  and  a  new.  The  old  belongs  to 
the  later  Tertiary  and  earliest  Quaternary ;  the  new,  to  the  later  Qua- 
ternary and  the  present.  The  change  took  place  during  the  oscillations 
of  the  Quaternary.  The  old  or  dead  river-system  runs  across  the  pres- 
ent drainage-system  in  a  direction  far  more  southerly.  This  is  espe- 
cially true  of  the  more  northern  members  of  the  system.  Farther  south 
the  two  systems  are  more  nearly  parallel,  showing  less  movement  in 
that  region.  These  old  river-beds  are  filled  with  Drift-gravel,  and  often 
covered  with  lava-streams.  They  will  be  again  referred  to  and  de- 
scribed in  connection  with  gold  (p.  555).  These  Drift-gravels  probably 
represent  the  Glacial  epoch,  though  Whitney  thinks  an  earlier  or  Plio- 
cene epoch.  The  present  river-system  sometimes  cuts  across,  some- 
times runs  parallel  to,  the  lava-filled  beds  of  the  old  river-system,  and 
the  beds  of  the  former  have  in  their  turn  been  eroded  2,000  to  3,000 
feet  in  solid  rock.  In  these  also  have  been  accumulated  immense  quan- 
tities of  gravel  and  bowlder  Drift,  evidently  brought  down  from  the 


FIG.  865.— Lava-Stream  cut  through  by  Elvers  :  a,  a,  basalt;  b,  &,  volcanic  ashes  ;  c,  c,  Tertiary;  d,  d, 
Cretaceous  rocks;  R  R,  direction  of  the  old  river-bed;  K',  B',  sections  of  the  present  river-beds  (from 
Whitney). 

glacial  moraines  by  the  swollen  rivers  of  the  Champlain  and  early  Ter- 
race epochs.  These  facts  are  illustrated  by  Figs.  865  and  866,  in  which 
R'  represents  the  present  river-system,  in  Fig.  865,  cutting  across,  and 
in  Fig.  866  running  parallel  to,  the  old  system  It. 


FIG.  866.— Section  across  Table  Mountain,  Tuolumne  County,  California:   Z,  lava;  G,  gravel;  S,  slate  ; 
R,  old  river-bed ;  R',  present  river-bed. 

34 


530  CENOZOIC  ERA— AGE   OF  MAMMALS. 

Although  it  is  impossible  to  synchronize  with  certainty  these  events 
with  the  changes  in  the  eastern  portion  of  the  continent,  yet  the  order 
of  sequence  is  evident ;  and  that  the  greater  part,  if  not  all,  occurred 
in  the  Quaternary,  is  also  evident. 

Seas. — The  boldness  of  the  whole  Pacific  coast,  especially  in  high 
latitudes,  indicates,  as  will  be  more  fully  shown  hereafter  (p.  534),  a 
previous  more  elevated  condition  of  the  land-surface  than  now  exists. 
Demonstrative  evidence  of  the  same  is  also  found  in  elephant-bones 
recently  discovered  on  the  small  island  of  Santa  Rosa,  which  must  then 
have  been  connected  with  the  mainland.1  This  was  during  the  Glacial 
epoch.  Again,  elevated  terraces  found  in  many  places  along  the  Cali- 
fornia coast  belong  undoubtedly  to  the  Champlain  epoch.  At  San 


FIG.  867.— Sea-Terraces  at  San  Pedro,  California  (after  Davidson). 

Pedro,  Lieutenant  Davidson  finds  ten  of  these,  most  of  them  well  marked, 
rising  one  above  another  from  sixty-five  to  1,200  feet  above  present  sea- 
level.1  At  that  time  the  sea  not  only  occupied  the  bay  of  San  Fran- 
cisco, but  covered  all  the  flat  lands  about  the  bay,  including  the  valleys 
of  Santa  Clara,  Napa,  and  Sonoma  ;  and  thence  extending  inward, 
covered  also  the  whole  Sacramento  and  San  Joaquin  plains,  forming 
thus  an  immense  sound  300  miles  long  and  fifty  miles  wide.  In  Oregon 
Mr.  Condon  has  traced  an  old  sea-margin  from  the  coast  up  the  Colum- 
bia River  to  and  beyond  the  Cascade  range.  At  that  time,  according  to 
him,  the  sea  entered  the  Columbia  as  a  great  estuary,  spread  out  over , 
the  Willamette  Valley  as  a  great  sound,  and  thence  up  the  river. 
Puget  Sound,  with  its  deep,  narrow,  complicated  channels,  was  prob- 
ably produced  by  subaerial  erosion,  at  a  time  of  greater  land-elevation. 
Again,  the  complicated  system  of  prairies  which  surrounds  its  southern 
end  is  evidence  of  former  extension  of  the  sound,  and  therefore  of 
an  epoch  of  subsidence,  from  which  reelevation  has  brought  the  waters 
to  their  present  condition. 

On  the  Thompson  River,  a  branch  of  Fraser  River,  British  Colum- 
bia, beautiful  terraces  are  seen  from  50  to  500  feet  above  the  present 
level  of  the  river  (Lord  Milton). 

The  Quaternary  Period  in  Europe. 

In  Europe  the  phenomena  were   more   irregular,  the  oscillations 
more  numerous,  and  perhaps  more  local,  than  in  America.     This  is  in 
1  "  Proc.  California  Academy  of  Science,"  vol.  v.,  p.  152.        2  Ibid.,  Tol.  v.,  p.  90. 


THE   QUATERNARY  PERIOD   IN  EUROPE. 


531 


accordance  with  the  general  difference  in  the  geological  history  of  the 
two  continents.  Again,  in  America  elevation  predominated  ;  in  Europe, 
subsidence.  Therefore,  in  America  true  glacial  phenomena  predomi- 
nated ;  in  Europe,  iceberg  phenomena.  Nevertheless,  the  general  char- 
acter of  the  phenomena  was  similar  in  the  two  countries.  The  most 
conspicuous  and  universal  effects  reach,  in  Europe,  as  far  as  about  50° 
north  latitude. 

1.  Epoch  of  Elevation— First  Glacial  Epoch.— The  Quaternary  was 
inaugurated  in  Europe,  as  in  America,  by  an  epoch  of  elevation,  when 
the  northern  portions  of  that  continent  stood  1,000  feet  or  more  above 


FIG.  868. -Map  of  Outline  of  Coast  of  Western  Europe,  if  elevated  600  Feet  (after  Lyell). 


its  present  level.     The  whole  of  Scandinavia,  the  whole  of  Scotland, 
and  the  northern  and  mountainous  portions  of  England,  were  ice-sheeted 


532 


CEXOZOIC  ERA— AGE   OF  MAMMALS. 


— the  ice  moving  from  these  regions  southward,  southeastward,  and 
eastward,  producing  universal  glaciation.  The  Baltic  Sea,  the  North 
Sea,  and  a  wide  border  about  the  British  Isles,  were  then  land,  and  swept 
over  by  glaciers.  Above  we  give  a  map  (Fig.  668),  from  Lyell,  showing 
what  would  be  the  outline  of  Northwestern  Europe,  if  raised  only  600  feet. 
Switzerland,  at  this  time,  though  not  ice-sheeted,  developed  glaciers 
on  a  prodigious  scale.  Some  of  these  have  been  traced  out  with  great 
care  and  skill.  Especially  has  this  been  done  for  the  great  Rhone 
glacier,  by  Guyot.  At  that  time  a  great  glacier  came  down  the  valley 
of  the  Rhone,  emerged  on  the  plains,  and  filled  the  whole  valley  of 
Switzerland,  fifty  miles  wide,  between  the  Alps  and  the  Jura,  forming 
a  great  mer  de  glace  50  miles  wide,  150  miles  long,  and  2,000  feet  deep. 
A  figure  is  given  below  of  this  great  glacier.  The  dotted  lines  show 
the  direction  of  motion  as  determined  by  bowlders  left  in  the  valley  or 
stranded  high  up  on  the  slopes  of  the  Jura. 


Fio.  869.— Map  showing  the  Outline  and  Course  of 
Flow  of  the  Great  Rhone  Glacier  (after  Lyell). 


FIG.  870.— Map  showing  the  Lines  of  DiM*  ex- 
tending from  the  Alps  into  the  Plains  of  the  Po 
(after  Lyell). 


Lakes  Geneva  and  Neufchatel  were  probably  scooped  out  by  this 
great  glacier. 

At  the  same  time,  also,  on  the  southern  slopes  of  the  Alps,  long 
glaciers  stretched  out  on  the  plains  of  Lombardy,  as  shown  by  the  pro- 
digious piles  of  debris  (moraines)  still  left.  Some  of  these  moraines 
are  1,500  feet  high.  Fig.  870  is  a  map  of  these  lines  of  debris. 

Evidences  of  glaciers  of  this  time  are  also  found  in  the  Vosges,  in 
the  Pyrenees,  and  other  high  mountains  of  Central  Europe. 

2.  Epoch  of  Submergence— Champlain.— Following  the  epoch  of 
elevation  was  an  epoch  of  subsidence,  during  which  the  same  regions 


THE  QUATERNARY  PERIOD  IN  EUROPE. 


533 


which  were  before  most  elevated  became  now  most  depressed.  It  is 
believed  that  in  Scotland  the  land  was  at  least  2,000  feet  below  the 
present  level.  By  this  depression  a  great  part  of  Northern  Europe  was 
submerged,  and  Great  Britain  was  reduced  to  an  archipelago  of  small 
islets.  Over  the  area  thus  submerged  drifted  icebergs  loosened  from 
the  Scandinavian  ice-field. 


FIG.  871.— Map  of  the  British  Isles  and  Norway,  if  subsided  1.200  to  2,000  Feet  (after  Lyell).    The  lower 
shaded  portion  was  not  touched  by  Drift. 

At  the  same  time,  partly  by  subsidence,  and  therefore  slackened 
water-currents,  and  partly  by  moderated  climate  and  melting  of  glaciers, 
there  was  a  flooded  condition  of  rivers  and  lakes  in  Middle  Europe, 
France,  Germany,  and  Switzerland.  At  the  same  time,  also,  the  north- 
ern portion  of  Asia  and  the  lake-region  of  that  continent  were  submerged. 
The  Caspian  Sea,  Lake  Aral,  and  other  lakes  in  that  region,  were  prob- 
ably then  united  into  one  great  inland  sea,  connected  either  with  the 


534:  CENOZOIC  ERA— AGE   OF   MAMMALS. 

Black  Sea  or  the  then  greatly-extended  Arctic  Ocean,  or  with  both.1 
Either  at  this  time,  or  more  probably  during  the  Glacial  epoch,  the 
Desert  of  Sahara  was  submerged. 

Evidences  of  this  condition  of  things  are  found  in  old  sea-margins, 
lake-margins,  river-terraces,  and  flood-plain  deposits. 

3.  Epoch,  of  Reelevation — Second  Glacial  Epoch — Terrace  Epoch. — 
The  period  of  submergence  was  followed,  as  in  America,  by  another  of 
reelevation,  as  shown  by  the  successive  beaches  and  terraces  on  sea- 
shores, about  lakes,  and  on  rivers.     But  in  Europe  the  reelevation  went 
much  beyond  the  present  level,  and  brought  on  a  second  Glacial  epoch, 
not,  indeed,  equal  to  the  first — not  an  ice-sheeted  epoch — but  a  reign  of 
great  separate   glaciers.     During  this  time  Great  Britain  was  again 
connected  with  the  continent. 

4.  Modern  Epoch. — Afterward  the  continent  again  came  down  to  its 
present  condition,  and  thus  inaugurated  the  Modern  epoch.     In  Europe, 
therefore,  the  Terrace  is  more  distinctly  separated   from  the  present 
epoch  than  in  America. 

Some  General  Results  of  Glacial  Erosion. 

1.  Fiords, — We  have  seen  that  the  phenomena  of  rivers,  in  the 
region  affected  by  the  Drift,  show  elevation,  then  subsidence,  and  then 
reelevation  to  a  less  height  than  the  first.  The  first  elevation  is  shown 
in  their  deep,  ancient  beds;  the  subsidence,  in  the  filling  up  of  these 
with  deposit ;  the  reelevation,  in  the  cutting  down  into  the  deposit,  and 
forming  terraces.  Now,  all  these  changes  are  also  shown  in  the  phe- 
nomena of  fiords  (Dana). 

It  will  be  remembered  (p.  35)  that  the  Norway  coast  is  wonderfully 
bold  and  deeply  dissected,  consisting  of  high,  rocky  headlands,  separated 
by  deep  inlets  running  50  to  100  miles  into  the  country ;  and  off  shore 
there  is  a  line  of  high,  rocky  isles,  evidently  the  remnants  of  an  old 
shore-line.  These  deep  inlets  are  called  in  Norway  Fiords;  and  the 
name  is  now  used  for  all  such  deep  inlets  separating  high  headlands. 
The  coast  of  Greenland  has  a  precisely  similar  structure.  It,  also,  con- 
sists of  bold,  rocky  headlands,  separated  by  fiords  running  far  into  the 
country ;  and  off  shore  a  line  of  rocky  isles  2,000  feet  high.  In  Green- 
land these  fiords  are  now  occupied  by  glacial  extensions  of  the  general 
ice-mantle.  The  same  coast-structure  is  found  on  the  western  side  in 
high  latitudes.  The  coast  of  British  America  and  Alaska  is  also  bold 
and  deeply  dissected  by  fiords;  and  in  Alaska  these  fiords  are  now 
occupied  by  great  glaciers  running  down  to  the  sea  (Fig.  872). 

Now,  it  seems  certain  that  fiords  are  deeply-eroded  valleys,  which 

have  become  half  submerged ;  and  as  glaciers  are  the  most  powerful  of 

erosive  agents,  they  are  usually  half-submerged  glacial  valleys.     These 

1  Nature,  vol.  xiii.,  p.  74  ;  Natural  History  Magazine,  vol.  xvii.,  p.  1*76 ;  "  Archives  des 

Sciences,"  vol.  liv.,  p.  427. 


LIFE   OF  THE   QUATERNARY  PERIOD.  535 

valleys  can  in  most  cases  be  traced  as  submarine  troughs,  far  out  to  sea. 
In  Greenland,  for  instance,  the  extension  of  these  troughs,  deep  below 
the  present  sea-level  and  far  out  beyond  the  reach  of  the  present  glaciers, 
shows  a  former  more  elevated  condition;  and  terraces  and  recent  deposits 
up  to  500  feet  show  a  subsidence  below,  and  a  reelevation  to,  the  present 
level.  Also,  Puget  Sound,  as  already  stated,  shows  the  same  succession 
of  changes. 

All  shores  in  northern  regions  are  bold  and  rocky  and  deeply  dis- 
sected, and  have  rocky  islets  off  shore ;  in  other  words,  are  more  or  less 


PIG.  872.— Ideal  Section  through  a  Fiord. 

affected  with  fiord-structure.  They  have  been  elevated,  glacially 
eroded,  and  subsided.  It  is  probable  that  during  the  epoch  of  greatest 
elevation  a  broad  continental  connection  existed  between  America  and 
Asia,  including  the  whole  area  between  the  Aleutian  Isles  and  Behring 
Straits. 

2.  Glacial  Lakes. — Lakes  are  found  in  all  parts  of  the  earth,  and  are 
doubtless  due  to  different  agencies,  but  there  can  be  little  doubt  that 
most  of  those  found  in  the  Drift  region  are  formed  by  glacial  agency. 
The  whole  region  which  has  been  affected  by  glacial  agency  is  thickly 
dotted  over  with  lakes,  while  south  of  this  region  there  is  a  comparative 
absence  of  them.  In  the  glacial  region  of  the  Sierra  Nevada,  glacial 
lakes  are  evidently  formed  in  two  ways :  They  are  either  rock-basins 
scooped  out  by  a  glacier  at  some  point  of  its  path  where  the  rock  is 
softer,  or  where  the  angle  of  slope  becomes  suddenly  less ;  or  else  they 
are  formed  by  the  damming  up  of  waters  behind  the  terminal  moraines 
left  by  a  retiring  glacier.  Both  of  these  kinds  are  very  abundant  in 
the  Sierra  and  other  mountain  regions.  The  former  are  usually  high 
up  the  valleys,  the  latter  somewhat  lower  down.  The  marshes  and 
meadows  so  common  in  old  glacial  regions  are  also  often  traceable  to 
the  filling  up  of  glacial  lakes. 

Life  of  the  Quaternary  Period. 

Plants  and  Invertebrates. — Remains  of  the  life  of  the  Quaternary, 
both  animal  and  vegetable,  are  very  numerous,  and  often  very  well  pre- 


536  CENOZOIC  ERA-AGE   OF  MAMMALS. 

served.  Both  the  plants  and  the  invertebrate  animals  are  almost  wholly 
identical  with  those  now  living  on  the  earth.  We  will  therefore  dis- 
miss these  with  one  important  remark :  The  plants  and  the  marine 
shells  show  an  arctic  climate  in  now  temperate  regions.  The  species 
found  are  still  living,  but  living  farther  north.  There  has  been  a  mi- 
gration of  species  northward  since  Glacial  times. 

Mammals. — But  the  mammalian  fauna  of  the  Quaternary  is  almost 
wholly  peculiar.  It  differs  greatly  from  the  Tertiary  fauna  preceding, 
and  the  present  fauna  succeeding.  The  species  are,  moreover,  very 
numerous,  and  many  of  them  of  extraordinary  size ;  for  it  is  the  culmi- 
nation of  the  mammalian  age.  It  is  necessary,  therefore,  to  describe 
some  of  them,  and  the  conditions  under  which  they  were  preserved,  and 
thus  to  realize  in  some  degree  the  conditions  under  which  they  lived. 
We  will  take  our  first  illustrations  from  Europe,  because  the  remains  are 
more  numerous  and  have  been  more  thoroughly  studied  there. 

Mammalian  remains  of  this  time  are  found  in  Europe — 1.  In  caverns, 
where  in  great  numbers  they  have  become  entombed ;  2.  On  beaches 
and  terraces,  where  their  floating  carcasses  have  become  stranded;  3. 
In  marshes  and  peat-bogs,  where,  venturing  in  search  of  food,  they  have 
mired  and  perished ;  4.  In  ice-cliffs  and  frozen  soils,  where  they  have 
been  hermetically  sealed  and  preserved  to  the  present  time. 

1.  Bone-Caverns. — The  richest  sources  of  Quaternary  mammalian 
remains  are  undoubtedly  bone-caverns.  These  occur  in  nearly  all  coun- 
tries, often  along  the  course  of  streams,  but  high  above  the  present 
stream-level.  Their  formation  and  their  filling  are  in  some  way  con- 
nected with  the  floods  of  the  Champlain  epoch.  They  are  rich  in  or- 
ganic remains,  to  a  degree  which  is  almost  incredible.  One  of  the 
most  striking  peculiarities  of  these  remains  is,  that  they  often  consist 
of  a  heterogeneous  mixture  of  all  kinds,  carnivorous  and  herbivorous, 
and  all  sizes,  from  the  Elephant  and  Cave-bear  on  the  one  hand  down  to 
Rats  and  Weasels  on  the  other  ;  sometimes  perfect,  more  often  broken, 
mingled  with  earth  and  gravel,  forming  unstratified  lone-rubbish.  An- 
other peculiarity  of  these  deposits  is  that  they  are  often  covered 
and,  as  it  were,  sealed  by  a  stalagmitic  crust  formed  by  subsequent 
drippings  from  the  roof,  and  thus  preserved  against  even  the  suspicion 
of  disturbance  to  the  present  time.  We  give  (p.  537)  a  section  of  the 
cave  of  Gailenreuth,  with  its  bone-rubbish  and  stalagmitic  crust. 

Among  the  remains  of  Herbivores  found  in  bone-caverns,  the  most 
remarkable  are  those  of  the  Elephant,  Rhinoceros,  Hippopotamus,  the 
great  Irish  Elk,  besides  Horses  and  Oxen.  Among  Carnivores  are  the 
Cave-bear  ( Ursus  spelceus),  larger  than,  the  Grizzly,  the  Cave-hyena,1 
the  Cave-lion,1  the  Sabre-toothed  Tiger  (Machairodus  latidens},  with  its 

1  These  are  supposed  to  be  the  same  species  as  the  African  lion  and  hyena  of  the 
present  day,  but  much  larger. 


LIFE   OF  THE   QUATERNARY  PERIOD. 


537 


sabre-like  tusks,  ten  inches  long,  besides  smaller  animals  of  the  same 
order.  The  remains  of  the  larger  Carnivora,  especially  the  Cave-bear 
and  the  Cave-hyena,  are  the  most  abundant.  The  bones  of  the  smaller 


Herbivores  bear  the  marks  of  teeth,  as  if  they  had  been  gnawed.  The 
skeletons  of  the  large  Pachyderms  are  usually  more  perfect.  In  the 
Kirkdale  Cave,  England,  the  teeth  and  other  parts  of  300  individuals  of 
the  Cave-hyena  were  found.  In  the  Gailenreuth  Cave,  Franconia,  the 


538 


CENOZOIC   ERA— AGE   OF  MAMMALS. 


remains  of  800  Cave-bears  were  obtained.  In  many  bone-caves  are 
found  also  the  bones  and  rude  implements  of  primeval  man.  Of  these 
we  will  speak  more  fully  hereafter. 


FIG.  874.-- Skull  of  Ursus  spelaeus,  x  J. 


FIG.  875.—  Skull  of  Hyena  spelaea,  x  J. 


Origin  of  Cave  Bone-Rubbish.  —  When  it  was  supposed  that  the  Drift 
was  caused  by  a  great  wave  of  translation  sweeping  across  the  conti- 
nent and  carrying  ruin  in  its  course,  the  phenomena  of  bone-caves  were 
supposed  to  give  countenance  to  this  view.  Animals  of  all  sizes  and 
kinds  were  supposed  to  have  huddled  together  in  these  caves,  forgetting 
their  mutual  hostility  in  the  sense  of  a  common  danger,  and  perished 
miserably  together  there. 

But  at  present  it  is  usually  believed  :  1.  That  these  caves  were  the 
dens  of  the  larger  Carnivores,  especially  the  Cave-bear  and  Cave-hyena, 
which  dragged  their  prey  there  to  devour  them,  and  also  later  the 
abodes  of  men  ;  2.  That  also  the  floating  bodies  of  large  Herbivores,  such 
as  the  Elephant,  Rhinoceros,  etc.,  were  carried  into  them  by  the  flooded 
rivers  which  then  ran  at  that  level  ;  and  3.  That  during  the  Champlain 
epoch,  when  water  ran  through  these  caves  in  large  quantities,  bones. 
and  earth  were  drifted  in  from  above,  through  fissures  and  subterranean 


LIFE   OF   THE   QUATERNARY  PERIOD.  539 

passages,  and  thus  found  their  lodgment  in  the  caves.     This  last  was 
probably  the  principal  source  of  the  bone-rubbish  in  most  cases. 

Origin  of  Bone-Caverns. — In  limestone  regions  caverns  are  very 
abundant  everywhere.  They  do  not  seem  to  be  enlarging  now ;  but 
on  the  contrary  to  be  in  most  cases  filling  up  either  with  rubbish  or 
with  stalactitic  and  stalagmitic  deposit.  In  some  cases  streams  still  run 
through  them.  It  seems  probable  that  they  are  mostly  due  to  the  ac- 
tion of  subterranean  waters  in  Champlain  times.  At  that  time  full 
streams  ran  through  and  excavated  them,  partly  by  erosion,  partly  by 
solution.  Gradually,  as  the  Terrace  elevation  came  on,  the  great 
streams  into  which  these  cavern  tributaries  ran  cut  down  their  beds  to 
lower  levels,  the  subterranean  waters  sought  lower  levels,  and  the  part 
running  through  the  caverns  was  reduced  to  drippings;  and  stalag- 
mitic crusts  covered  the  Champlain  rubbish  and  preserved  them.  Thus, 
then,  the  date  of  the  eaves  is  Champlain ;  of  the  bone-rubbish  is  Cham- 
plain  and  early  Terrace ;  of  the  stalagmitic  crust  is  later  Terrace  and 
Recent. 

2.  Beaches  and  Terraces. — On  these  are  found  the  remains  of  bodies 
which  have  floated  and  become  stranded.     The  most  abundant  of  these 
are  remains  of  ElepJias  primigenius  or  Mammoth.     It  is  believed  that 
the  bones  of  500  individuals  have  been  found  on  the  coast  of  Norfolk 
and  Suffolk,  and  over  2,000  grinders  have  been  dredged  up  by  the 
fishermen  of  the  little  village  of  Happesburgh  (Woodward).     On  river- 
terraces  associated  with  bones  of  Quaternary  animals  have  been  found 
also  the  rude  implements  of  primeval  man.     We  speak  of  these  more 
particularly  hereafter. 

3.  Marshes  and  Bogs. — As  might  have  been  anticipated,  the  re- 
mains found  in  these  are  mainly  those  of  the  larger  Herbivores — ele- 
phants, oxen,  stags,  etc.     It  is  in  these  that  were  found  most  of  the 
fine  skeletons  of  the  gigantic  Irish  elk  ( Gervus  megaceros).     This  mag- 
nificent elk  was  ten  to  eleven  feet  in  height  to  the  top  of  its  palmate 
antlers,  and  ten  to  twelve  feet  between  the  antler-tips  (Fig.  876). 

4.  Frozen  Soils  and  Ice  Cliffs. — As  in  these  have  been  found  the 
most  perfect  specimens  of  the  Mammoth  (Elephas primigenius),  this 
seems  to  be  the  proper  place  to  describe  the  animal. 

The  genus  Elephas  ranges  in  time  from  about  the  latter  part  of 
the  Miocene  to  the  present.  There  are  about  twenty  fossil  species 
known.  The  genus  seems  to  have  reached  its  maximum  development 
in  the  Quaternary.  During  that  period  three  species  inhabited  Europe, 
viz. :  E.  antiquus,  E.  meridionalis,  E.  primigenius  (Lyell),  besides 
two  dwarf  species,  E.  Melitensis,  four  and  a  half  feet  high,  and  E.  Fal- 
coneri,  three  feet  high,  found  in  the  Quaternary  of  Malta.  Of  these, 
the  largest,  the  most  numerous,  and  the  latest,  was  the  primigenius  or 
Mammoth.  This  species  roamed  in  immense  herds  all  over  Europe, 


540 


CENOZOIC   ERA— AGE   OF  MAMMALS. 


from  the  shores  of  the  Mediterranean  to  Siberia,  and  extended  also 
over  the  northern  portions  of  North  America.  In  Siberia  the  tusks 
are  so  abundant  and  so  well  preserved  that  much  of  the  ivory  of  com- 
merce is  gotten  from  this  source. 

The  Mammoth  (Fig.  877)  was  over  twice  the  bulk  and  weight  of  the 
largest  modern  species,  and  nearly  one-third  taller.     It  was  thickly  cov- 


Fia.  876.— Skeleton  of  the  Irish  Elk  (Cervus  megaceros),  Post-Pliocene,  Britain. 

ered  with  a  brownish  wool,  and  in  parts  with  long  hair ;  and  was  there- 
fore well  adapted  to  endure  cold.  It  may  seem  strange  that  we  should 
speak  of  the  hair  and  wool  and  the  color  of  an  extinct  animal ;  but 
perfectly-preserved  specimens  have  been  found  sealed  in  the  ice  in 
Siberia — so  perfectly  preserved  that,  when  first  exposed,  wolves  and 
dogs  of  the  present  epoch  fed  on  the  flesh  of  this  animal  belonging  to 
an  extinct  fauna.  The  whole  skeleton,  with  portions  of  the  skin,  hair, 
wool,  hoofs,  and  eyes  of  this  animal,  is  now  to  be  found  in  the  museum 
at  St.  Petersburg.  The  existence  of  elephants  so  far  north  does  not 
indicate  a  warm  climate,  although  the  Champlain  epoch  was  doubtless 


LIFE  OF   THE   QUATERNARY  PERIOD. 


541 


far  less  rigorous  than  the  Glacial.     These  elephants  were  covered  with 
thick  wool,  as  was  also  the  rhinoceros  of  Europe. 

Quaternary  Mammalian  Fauna  of  England. — In  England  alone  there 
were,  in  Quaternary  times,  of  Carnivora,  the  great  Cave-bear,  the  Cave- 
hyena,  a  tiger  larger  than  the  Bengal,  the  Sabre-toothed  tiger,  as  large, 


with  its  flat,  curved  tusks,  eight  inches  beyond  the  gums,  besides  wolves 
and  lesser  Carnivores.  Of  Herbivores,  there  were  the  Mammoth  in 
herds,  two  species  of  rhinoceros,  one  hippopotamus,  the  great  Irish  elk, 
three  species  of  oxen,  two  of  them  of  gigantic  size,  besides  horses, 
deer,  and  other  smaller  species.  Surely  this  was  the  culmination  of 
the  Mammalian  age  in  England. 


542  CENOZOIC  ERA— AGE   OF  MAMMALS. 


Mammalian  Fauna  in  North  America. 

The  animals  of  North  America,  in  Quaternary  times,  were  equallv 
abundant ;  but  the  country  has  been  less  perfectly  explored,  and  the 
collections,  therefore,  less  complete.  Bone-caverns,  the  richest  sources 
of  European  collections,  are  also  far  more  rare. 

Among  Herbivores^  the  most  remarkable  were  the  great  Mastodon 
(M.  Americanus) ;  two  species  of  elephants,  the  E.  Americanus  and  the 
E.  primigenius  ;  at  least  two  gigantic  bisons,  one  of  which  was  prob- 
ably ten  feet  between  the  horn-tips  ; 1  gigantic  horses;  gigantic  beavers, 
one  five  feet  long ;  a  gigantic  stag  (  Cervus  Americanus},  fully  as  large 
as  the  Irish  elk ;  tapirs,  peccaries,  and  a  large  number  of  Edentates,  an 
order  now  mostly  confined  to  South  America,  to  which  belong  the  sloths 
and  armadillos.  Many  of  these  were  also  of  gigantic  size.  Carnivores 
were  not  so  abundant  as  in  Europe.  The  most  remarkable  were  a  lion 
(Felis  atrox),as  large  as  the  European,  and  two  species  of  bear  ( Ursus 
pristinus  and  amplidens). 

Bone-Caves. — Caves  are  found  in  limestone  regions  in  America 
as  elsewhere,  but  they  do  not  seem  to  have  been  to  the  same  extent 
the  dens  of  Carnivores.  In  a  vertical  opening  in  limestone  strata  in 
Pennsylvania,  a  kind  of  cave,  mammalian  remains  have  been  found  be- 
longing to  thirty-four  species,  among  which  were  six  Edentates,  eight 
Ungulates,  and  twelve  Rodents.  A  number  have  also  been  found  in 
the  caves  of  Virginia,  and  a  few  in  those  of  Illinois  (Cope). 

Marshes  and  Bogs. — Most  of  the  remains  of  large  Herbivores  have 
been  found  in  marshes  and  bogs.  In  the  Big  Bone  Lick,  Kentucky, 
the  remains  of  one  hundred  mastodons  and  twenty  elephants  are  said 
to  have  been  dug  up.  Many  very  perfect  skeletons  of  the  great  masto- 
don have  been  obtained  from  marshes  in  New  York,  New  Jersey,  In- 
diana, and  Missouri.  One  magnificent  specimen  was  found  in  a  marsh 
near  Newburg,  New  York,  with  its  legs  bent  under  the  body  and  the  head 
thrown  up,  evidently  in  the  very  position  in  which  it  mired.  The  teeth 
were  still  filled  with  the  half-chewed  remnants  of  its  food,  which  con- 
sisted of  twigs  of  spruce,  fir,  and  other  trees  ;  and  within  the  ribs,  in 
the  place  where  the  stomach  had  been,  a  large  quantity  of  similar  mate- 
rial was  found.  In  1866  a  very  perfect  skeleton  was  found  in  a  bog  at 
Cohoes,  New  York. 

The  Mastodon  Americanus  (Fig.  878)  is  probably  the  largest  land- 
mammal  known.  It  was  twelve  to  thirteen  feet  high,  and,  including 
the  tusks,  twenty-four  to  twenty-five  feet  long.  It  differed  from  the 

1  A  specimen  of  -Bos  latifrotis  has  recently  been  found  in  Ohio,  the  horn-cores  of  which 
were  twenty  inches  around  the  base,  and  more  than  seven  feet  between  the  points.  Be- 
tween the  horn-tips  must  have  been  at  least  ten  feet. 


MAMMALIAN  FAUNA  IN  NORTH  AMERICA. 


543 


elephant  chiefly  in  the  character  of  its  teeth.  The  difference  is  seen  in 
Figs.  879,  880,  881.  The  elephant's  tooth,  given  below  (Fig.  880),  is 
sixteen  inches  long,  and  the  grinding  surface  eight  inches  by  four 
inches. 


FIG.  8T8. — Mastodon  Americanus  (after  Owen). 


The  two  genera  of  Proboscidians,  Elephas  and  Mastodon,  appeared 
together,  or,  more  probably,  the  mastodon  a  little  the  earlier,  in  the 


FIG.  879.— Tooth  of  Mastodon  Americanus. 


FIG.  880.— Perfect  Tooth  of  an  Elephas, 
found  in  Stanislaus  County,  Cali- 
fornia, £  natural  size. 


Miocene  epoch ;  they  ranged  together  through  the  rest  of  the  Tertiary, 
the  species,  of  course,  changing  several  times.  At  the  end  of  the  Ter- 
tiary, the  mastodon  became  extinct  on  the  Eastern  Continent,  but  con- 


544  CENOZOIC  ERA— AGE   OF  MAMMALS. 

tinued  through  the  Quaternary,  with  its  companion,  the  elephant,  in 
America.  At  the  end  of  the  Quaternary,  the  mastodon  became  extinct 
wholly,  and  the  elephant  in  America  and  Europe,  though  it  still  con- 
tinues in  Asia  and  Africa.  During  the  Quaternary,  therefore,  one  spe- 
cies of  mastodon  and  two  species  of  elephant  roamed  in  herds  over 
North  America  from  the  Gulf  to  arctic  regions.  Of  the  two  species  of 


Fia.  881.— Molar  Tooth  of  Mammoth  (Elephas  primigenius) :  a,  grinding  surface ;  b,  side-Tiew. 

elephant,  however,  the  primigenius  was  mostly  confined  to  the  higher 
latitudes,  and  the  Americanus  to  the  southern  portions.  The  latter  is 
distinguished  from  the  former  by  less  crowded  enamel  plates  in  the 
grinders  and  less  curved  tusks.  Of  the  three  genera  of  Proboscidians 
known,  the  Dinotherium  appeared  first,  then  the  Mastodon,  and  last  the 
Elephant.  This  is  also  the  order  of  specialization  of  teeth-structure. 

Among  Edentates,  a  Megatherium,  a  Megalonyx,  and  several  Mylo- 
dons,  have  been  found  in  North  America ;  but  as  their  principal  home 
was  in  South  America,  we  will  describe  them 
under  that  head. 

River-Gravels. — In  many  portions  of  the  United 
States,  but  especially  in  California,  remains  of 
mastodon  and  elephant,  and  bison,  etc.,  are  found 
in  great  numbers  in  river-gravels.  The  river-grav- 
els of  California  are  spoken  of  again  further  on. 

Quaternary  in  South  America. — A  large  num- 
ber (more  than  100)  of  species  of  mammals  have 
been  found  in  the  soil  of  the  pampas  and  in  the 
caves  of  Brazil.     They  are  mastodons  (different 
species    from    the    North    American),     llamas, 
FIG.  682.-Tooth  of  Machatro-  horses,  tapirs,  rodents,  many  species  of  panther- 
*  (drawn  like    carnivores,    a    large    sabre-toothed    tiger 


MAMMALIAN  FAUNA   IN  SOUTH  AMERICA. 


545 


(Machairodus  neogceus),  with  curved,  sabre-like  tusks  twelve  inches 
long  and  eight  inches  beyond  the  gums  (Fig.  882),  and  especially  a 
large  number  of  Edentates  allied  to  the  sloths  and  armadillos,  but  of 
gigantic  size. 

Of  the  Edentates,  the  most  remarkable,  in  fact,  one  of  the  most 
remarkable  animals  which  have  ever  existed,  is  the  Megatherium  (great 
beast)  Guvieri.  The  genus  Megatherium  ranged  in  Quaternary  times 
through  South  America,  and  into  North  America  as  far  as  the  shores 


FIG.  833.— Megatherium  Cuvieri. 


of  Georgia  and  South  Carolina.  At  the  mouth  of  the  Savannah  River 
the  remains  of  several  individuals  of  a  species  of  this  genus  (M.  mira- 
bilis)  have  been  found.  But  the  largest  species  and  the  most  perfect 
specimens  have  been  found  in  South  America. 

The  Magatherium  Ouvieri,  of  which  we  give  a  figure  above,  was 
larger  than  a  rhinoceros,  but  was  still  more  remarkable  for  the  clumsy 


Fro.  8S4.— Lower  Jaw  of  a  Megatherium,  showing  the  Gradual  Surface  of  the  Teeth  (after  Owen). 

massiveness  of  its  skeleton  than  for  its  size.     This  is  especially  true  of 
its  hind-legs,  hip-bones,  and  tail.     For  this  reason,  it  is  supposed  to 
have  been  able  to  stand  on  its  hind-legs  and  tail,  while  it  used  its  long 
35 


546 


CENOZOIC  ERA— AGE   OF  MAMMALS. 


free-moving  arms,  terminated  with  hands  a  yard  long,  to  tear  down 
branches  on  which  it  fed.  The  great  skeleton  represented  above  is 
eighteen  feet  long,  and  its  thigh-bones  are  three  times  as  thick  as  those 
of  an  elephant.  The  grinding  surface  of  its  molar  teeth  (it  had  no 


FIG.  885.— Claw-Core  of  a  Megalonyx,  x  J  (drawn  from  a  cast  of  the  original). 

others)  is  traversed  by  triangular  ridges  admirably  adapted  to  triturate 
its  coarse  food. 

Megalonyx  (big  claw)  is  the  name  of  another  genus  of  these  gigan- 
tic sloths,  and  Mylodon  of  a  third.  Both  of  these  genera  extended  into 
North  America.  In  fact,  the  Megalonyx  was  first  discovered  in  Green- 


FIG.  8S6.— Skeleton  of  Mylodon  robnstus,  Quaternary.  South  America. 


MAMMALIAN  FAUNA   OF   SOUTH  AMERICA  AND   AUSTRALIA.       547 


brier  County,  Virginia,  and  named  Megalonyx  by  Thomas  Jefferson. 
The  larger  species  of  Mylodon  and  Megalonyx  were  about  the  size  of  a 
buffalo,  or  larger. 

Of  the  Armadillos  or  mailed  Edentates,  there  were  several  of  gi- 
gantic size  belonging  to  the  genera  Glyptodon,  Chlamydotherium,  and 
Pachytherium.  The  accompanying  cut  represents  one  of  these  eight  feet 


FIG.  8SV. — Skeleton  of  Glyptodon  clavipes,  x   ^B,  Quaternary,  South  America. 

long,  with  an  invulnerable  coat-of-mail.  Some  species  of  the  genus 
Chlamydotherium  were  much  larger — one  as  big  as  a  rhinoceros,  and 
of  Pachytherium  as  big  as  an  ox  (Dana). 

Australia. — In  Australian  caves,  also,  great  abundance  of  remains 
has  been  found,  and  they  show  the  same  prevalence  of  gigantic  spe- 
cies. As  now,  so  then,  the  mammals  of  Australia  were  almost  all  Mar- 
supials, but  the  present  species  are  dwarfs  in  comparison.  The  largest 
of  these  was  the  Diprotodon  (two  front 
teeth),  a  pachydermoid  kangaroo  as  big 
as  a  rhinoceros.  A  reduced  figure  of 
the  skull,  which  was  three  feet  long,  is 
given  below. 

Among  other  remarkable  species  of 
marsupials  were  Macropus  (kangaroo) 
Titan  and  M.  Atlas,  of  great  size  ;  Noto- 
therium  Jfitchelli,  as  large  as  a  bullock, 
and  a  very  remarkable  species,  supposed  by  Owen  to  have  been  carniv- 
orous, and  therefore  called  Thylacoleo  (pouched  lion)  carnifex,  as  large 
as  a  lion.  The  striking  peculiarity  of  this  animal  was  the  existence  of 
a  broad  trenchant  premolar,  as  shown  in  Fig.  889. 

Geographical  Fauna  of  Quaternary  Times.— We  observe,  then,  that 

already  the  geographical  distribution  of  families  was  similar  to  that 
which  we  find  at  present.  Then,  as  now,  Herbivores  greatly  predomi- 
nated in  America,  while  Carnivores  were  very  abundant,  and  of  great 
size,  in  the  Eastern  Continent.  Then,  as  now,  sloths  and  armadillos 
and  llamas  characterized  the  fauna  of  South  America,  while  Marsupials 


FIG.  8S8.— Skull  of  Diprotodon  Australia, 
x   i'j,  Post-Pliocene,  Australia. 


548  CENOZOIC  ERA— AGE   OF  MAMMALS. 

characterized  that  of  Australia.     But  in  each  locality  the  animal  life 
seems  to  have  been  then  more  abundant,  and  the  species  gigantic. 


Fio.  889.— Thylacoleo,  skull  reduced  (after  Flower). 


Some  General  Observations  on  the  Whole  Quaternary. 

1.  Cause  Of  the  Climate. — This  is  confessedly  one  of  the  most  diffi- 
cult questions  in  geology.  There  seems  to  be  no  doubt  that,  coincident 
with  the  great  changes  of  climate,  there  were  also  great  oscillations  of 
the  earth's  crust  in  polar  regions ;  furthermore,  it  seems  certain  that 
the  intense  cold  was  attended  with  elevation,  and  the  subsequent  mod- 
eration of  climate  with  subsidence.  This  coincidence  is  itself  strong 
evidence  of  a  relation  of  cause  and  effect.  It  is  generally  admitted  that 
increase  in  the  area  and  height  of  polar  lands  would  increase  the  rigor 
of  the  climate,  and  decrease  of  area  and  height  of  polar  lands  would 
moderate  the  climate  of  northern  regions.  The  amount  of  this  effect 
it  is  impossible  to  estimate ;  but  the  effect  was  so  enormous  and  so 
wide-spread  that  the  cause,  even  when  supplemented,  as  it  has  been, 
by  changes  in  the  course  of  oceanic  currents  such  as  the  Gulf  Stream, 
has  seemed  to  most  physicists  and  geologists  to  be  insufficient.  They 
have  cast  about,  therefore,  for  some  other  possible  cause,  external  to 
the  earth  itself — i.  e.,  cosmical  cause — to  explain  it. 

The  only  theory  of  this  kind  which  seems  entitled  at  the  present 
time  to  serious  attention  is  that  of  Mr.  Croll,  embraced  by  Geikie  and  oth- 
er English  geologists,  which  attributes  it  to  the  combined  influence  of 
precession  of  the  equinoxes  and  secular  changes  in  the  eccentricity  of 
the  earth's  orbit.  By  the  former — precession — aphelion,  or  greatest  dis- 
tance of  the-  earth  from  the  sun,  which  is  now  in  summer  in  our  north- 


GENERAL   OBSERVATIONS  ON   THE   WHOLE   QUATERNARY.        549 

ern  hemisphere,  is  finally  brought  around  to  winter.  The  effect  of  this, 
it  is  claimed,  would  be  to  make  colder  winters  and  warmer  summers, 
like  those  now  in  the  southern  hemisphere.  By  the  latter,  viz.,  change  in 
the  degree  of  ellipticity  of  the  orbit,  these  effects,  viz.,  cold  winters  and 
warm  summers,  would  be  increased  or  decreased  according  as  the  ellip- 
ticity were  increased  or  decreased.  A  Glacial  epoch  is  the  result  of  a 
coincidence  of  an  aphelion  winter  with  a  time  of  greatest  eccentricity. 

This  theory  is  still  under  discussion,  and  is  yet  too  hypothetical  to 
justify  an  elaborate  presentation  in  an  elementary  work.  Suffice  it  to 
say  that,  if  true,  one  corollary  from  it  would  be  a  recurrence  of  glacial 
conditions  with  every  period  of  greatest  eccentricity.  Another  would 
be  the  alternation  of  extreme  glacial  conditions  between  the  two  poles 
during  each  Glacial  period  (period  of  greatest  eccentricity)  with  inter- 
vals of  26,000  years  (cycle  of  precession).  It  is  hoped  that  future  ob- 
servations will  test  these  conclusions.1 

In  any  case,  the  coincident  oscillations  of  the  earth's  crust  are  unac- 
counted for ;  and  in  these  oscillations  we  have,  if  not  a  sufficient  cause, 
at  least  a  true  cause,  as  far  as  it  goes,  of  the  climate. 

On  any  view  of  the  cause  of  the  climate  of  the  Quaternary  period,  it 
seems  hardly  probable  that  it  is  entirely  exceptional.  Recently,  many 
geologists,  especially  Ramsay,  have  looked  for  evidences  of  previous 
Glacial  periods.  Some  evidences  of  this  kind  have  been  collected,  but 
they  are  far  from  conclusive.  That  there  have  been  periods  of  great 
oscillations  of  the  earth's  crust,  and  consequent  changes  of  physical 
geography,  there  can  be  no  doubt.  That  these  were  attended  with 
corresponding  oscillations  of  local  climate  is  also  certain.  But  that 
glacial  conditions  were  ever  before  reached,  even  in  polar  regions,  seems 
more  than  doubtful.8  The  oscillating  temperature  of  the  earth  at  any 
one  place,  when  combined  with  the  gradual  cooling  of  the  earth  from 
early  incandescence,  might  well  reach  glacial  conditions  but  once.  We 
may  graphically  represent  this  by  the  diagram  on  page  550 :  Let  the 
absciss  a  b  represent  the  course  of  geological  time  from  early  Archaean 
times  till  now,  and  ordinates  upon  this,  degrees  of  cold,  at  any  time 
and  place  ;  then  the  curved  line  a  c  would  represent  the  cooling  of  the 
earth  through  all  time  if  the  rate  were  uniformly  decreasing  (as  it 
would  be  if  it  cooled  by  radiation  only),  and  the  undulating  line  would 
represent  the  actual  oscillating  changes  of  temperature.  If  g  g  repre- 

1  Among  other  cosmical  causes  which  have  been  suggested  ought  to  be  mentioned 
secular  variation  in  the  amount  of  heat  emitted  by  the  sun.  Langley  (American  Journal 
of  Science  and  Arts,  December,  1875)  has  modified  this  view  slightly  by  attributing  the 
changes  of  climate  to  variation  in  the  amount  of  heat  received  from  the  sun ;  the  variation 
being  due  to  change  in  the  absorptive  power  of  the  solar  atmosphere. 

a  See  "  Former  Climate  of  Polar  Regions  "  Nordemkiold,  Geological  Magazine,  Novem- 
ber, 1875,  p.  525. 


550 


CENOZOIC  ERA— AGE   OF   MAMMALS. 


sent  the  line  of  glacial  conditions  such  as  existed  in  Quaternary  times, 
then  it  is  seen  that  the  actual  temperature  touches  this  line  only  once. 


ff 


FIG.  890.— Diagram  showing  the  Secular  Increase  of  Cold,  and  the  Oscillations  of  Temperature  in  the 
Course  of  Geological  Times. 

2.  Time  involved  in  the  Quaternary  Period. — There  can  be  no  doubt 
that  the  changes  of  physical  geography  and  climate  spoken  of  were 
brought  about  gradually,  and  therefore  involved  long  periods  of  time — 
so  gradually  that  they  might  well  be  unremarked  by  contemporaneous 
man.     There  are  changes  by  elevation  and  depression  now  going  on 
in  various  parts  of  the  earth,  which  are  probably  as  rapid  as  those  of 
the  Glacial  and  Champlain  epochs.     The  shores  of  the  Baltic  and  of 
Norway  are  now  rising  at  an  average  rate  of  two  and  a  half  feet  per 
century  (p.  129).     Continue  this  process  for  800  centuries,  and  Norway 
would  attain  an  elevation  equal  to  that  of  the  Glacial  epoch  ;  and,  if 
such  elevation  produces  cold,  would  again  be  ice-sheeted.     Depression, 
at  similar  rate  for  the  same  time,  would  bring  about  the  condition  and 
climate  of  the  Champlain  epoch.     Yet  these  changes  are  unremarked, 
except  by  the  eye  of  Science.     The  only  difference,  if  any,  between 
what  is  in  progress  now,  and  what  took  place  in  Glacial  times,  is  the 
comparative  universality  of  the  oscillations  then,  and  therefore  the 
greatness  of  the  effect  upon  climate.     It  is,  of  course,  impossible  to 
estimate  in  years  the  time  of  the  Glacial  and  Champlain  epochs,  unless, 
indeed,  the  theory  of  Croll  be  admitted  ;  but  it  is  not  probable  that  the 
estimates  given  above  are  exaggerated. 

3.  The  Quaternary  a  Period  of  Revolution— a  Transition  between 
the  Cenozoic  and  the  Modern  Eras. — We  have  already  seen  (pp.  269 
and  280)  that  between  the  great  eras,  and  perhaps  also  at  other  times, 
there  have  been  periods  of  oscillation  of  the  earth's  crust,  and  there- 
fore of  changes  of  physical  geography,  marked  by  unconformity  of 
strata ;  and  changes  of  climate,  marked  by  apparently  abrupt  changes 
of  species.     These  have  been  the  critical  periods  of  the  earth's  history 
— periods  of  revolution  and  rapid  change.     But  for  that  very  reason 
they  are  also  periods  of  lost  records.     We  have  already  spoken  of  the 
lost  interval  at  the  end  of  the  Archaean,  evidently  the  greatest  of  all; 
again,  of  a  lost  interval  at  the  end  of  the  Palaeozoic,  partly  recovered 
in  the  Permian,  evidently  the  next  greatest ;  again,  of  a  lost  interval  at 
the  end  of  the  Cretaceous,  in  a  large  measure  recovered  in  the  Rocky 


GENERAL   OBSERVATIONS   ON  THE  WHOLE   QUATERNARY.         551 

Mountain  region.  There  are  doubtless  many  others  of  less  extent. 
These  periods  are  always  marked  by  unconformity  of  the  strata  and 
change  in  the  life-system.  The  old  geologists  regarded  these  changes 
as  sudden  and  cataclysmic.  All  geologists  now  regard  the  suddenness 
as  largely  apparent,  and  the  result  of  lost  record. 

Now,  the  Quaternary  is  also  a  critical  period.  It  corresponds 
with  one  of  the  lost  intervals ;  only,  in  this  case,  on  account  of  its  near- 
ness to  us,  the  record  has  been  recovered.  By  the  study  of  this  period, 
therefore,  we  may  hope  to  solve  many  problems  which  have  heretofore 
puzzled  us.  Here,  for  example,  we  have  oscillations  of  the  crust  on  a 
grand  scale,  producing  great  changes  of  physical  geography  and  cli- 
mate, and  therefore  of  fauna  and  flora.  Here  we  have  unconformity, 
now  being  produced  by  sedimentation  on  old  eroded  land-surfaces  in 
all  the  region  affected  by  the  oscillations — marine  sediments  in  fiords 
and  river  sediments  in  old  river-channels.  But  we  observe  that  in  this 
case  these  effects  have  been  produced  slowly,  and  that  the  fauna  and 
flora  have  not  been  suddenly  destroyed  and  suddenly  recreated,  but 
have  continued  to  live  throughout,  the  species  gradually  changing. 
But,  what  is  still  more  interesting,  much  light  is  thrown  also  on  the 
hitherto  insoluble  problem  of  the  mode  and  the  cause  of  the  compara- 
tively rapid  change  of  species  in  these  critical  periods.  The  attentive 
study  of  the  Quaternary  shows  that,  in  addition  to  the  direct  effect  of 
change  of  climate,  one  great  cause  of  change  of  species  has  been  migra- 
tion :  migration  north  and  south,  enforced  by  change  of  temperature  ; 
migration  in  any  direction,  permitted  by  change  of  physical  geography. 
This  point  is  so  important,  that  we  must  explain  it  somewhat  fully. 

It  will  be  remembered  (p.  481)  that  in  Miocene  times  Greenland, 
Iceland,  and  Spitzbergen,  were  covered  with  a  luxuriant  temperate 
vegetation.  The  congeners  of  their  vegetation  at  that  time  are  found 
now  in  California,  along  the  shores  of  the  Southern  Atlantic  States,  and 
in  Southern  Europe.  Evidently  at  that  time  there  was  no  polar  ice- 
cap, and  therefore  no  arctic  plants.  At  the  end  of  the  Pliocene,  the 
vegetation  shows  a  climate  not  greatly  differing  from  the  present.  It 
is  probable,  therefore,  that  the  cold  had  increased  until  an  ice-cap  had 
formed,  such  as  now  exists  in  polar  regions,  with  its  accompaniment  of 
arctic  species.  As  the  Glacial  epoch  came  on  and  culminated,  the 
polar-  ice-cap  slowly  extended — its  margin  crept  slowly  southward, 
until  it  reached  40°  in  America  and  50°  in  Europe  and  Asia,  with  local 
extensions  stretching  still  farther  southward,  in  the  form  of  separated 
glaciers.  The  southern  polar  regions  were  probably  similarly  affected, 
either  simultaneously  or  alternately. 

We  must  not  confound  this  movement  southward  of  the  southern 
limit  of  the  ice  with  the  current  motion  of  the  ice-sheet  itself.  The 
limit  of  the  ice-cap  is  like  the  lower  limit  of  a  glacier  (p.  44).  It  may 


552  CEXOZOIC  ERA— AGE   OF   MAMMALS. 

be  stationary,  or  advancing  or  retreating,  but  the  glacial  stream  flows 
ever  onward.  Again,  the  motion  of  a  glacial  current  is  slow — perhaps 
one  to  three  feet  per  day — but  the  extension  or  recession  of  the  glacial 
limit  is  far  slower,  perhaps  a  few  feet  per  annum.  We  may  thus  easily 
appreciate  the  immense  time  necessary  to  advance  this  limit  of  the  ice- 
cap to  40°  latitude. 

At  the  end  of  the  Glacial  and  the  commencement  of  the  Champlain 
epoch  a  movement  of  the  ice-limit  in  a  contrary  direction — a  retreat 
northward — commenced  and  continued,  with  perhaps  some  alternate 
progressions  and  regressions,  to  its  present  position. 

Now,  the  effect  of  this  advance  and  retreat  of  polar  ice  upon  plants 
and  animals  must  have  been  very  marked.  Temperate  plants,  inhabit- 
ing Greenland  in  the  Miocene,  were  pushed  to  the  shores  of  the  Gulf. 
Arctic  plants — i.  e.,  those  which  haunt  the  margin  of  perpetual  ice — 
were  pushed  to  Middle  United  States  and  to  Middle  Europe  ;  and  arc- 
tic shells  were  similarly  driven  southward,  slowly,  generation  after  gen- 
eration. We  say  slowly,  for  otherwise  they  must  have  been  destroyed. 
With  the  return  of  temperate  conditions,  and  the  retreat  of  the  ice- 
cap, these  species,  both  shells  and  plants,  again  went  northward  to  their 
appropriate  place.  But  the  plant  species,  and  some  land  invertebrate 
species,  such  as  insects,  had  an  alternative  which  the  shells  had  not, 
viz.,  to  seek  arctic  conditions  also  upward  on  mountains.  Many  did 
so  and  were  left  stranded  there.  Thus  is  explained  the  remarkable  fact 
that  Alpine  plant-species  in  Europe  are  similar  to  and  largely  identical 
with  those  in  America  ;  and  both  with  the  present  arctic  species.  This 
indicates  a  former  wide  distribution  of  identical  arctic  species  all  over 
Europe  and  America,  and  their  subsequent  retreat  northward  into 
polar  regions,  and  upward  into  Alpine  isolation.  Recently  Grote 
has  observed  a  similar  isolation  of  Labrador  insect-species  on  Mount 
Washington  and  on  the  Colorado  mountains.1 

There  was  probably  a  similar  movement,  to  a  less  extent,  of  temper- 
ate species.  In  the  Taxodium  of  the  Southern  Atlantic  and  Gulf 
swamps,  and  the  Sequoias  of  California,  we  doubtless  have  examples  of 
species  wide-spread  in  Miocene  times,  which  have  been  destroyed  by 
these  climatic  changes,  except  in  certain  limited  areas. 

But  plants  and  lower  species  of  animals  are  far  less  affected  by 
changes  in  physical  conditions  than  are  the  higher  species  of  animals. 
This  is  shown  by  the  wide  range  both  in  space  and  time  of  the  former 
as  compared  with  the  latter.  Under  these  great  changes  and  enforced 
migrations,  therefore,  plants  and  invertebrate  animal  species  maintained 
their  specific  characters  mostly  unchanged,  or  but  slightly  changed. 
But  in  the  case  of  mammals  destruction  or  change  was  inevitable.  Both 
took  place — destruction  of  some  and  change  of  the  remainder. 
1  American  Journal  of  Science,  1875,  vol.  x.,  p.  335. 


GENERAL   OBSERVATIONS  ON   THE  WHOLE   QUATERNARY.         553 

In  America  during  Quaternary  times  there  was  probably  a  broad 
land-connection  of  North  America  with  South  America  by  the  Carib- 
bean Sea  region ;  and  certain!}-,  as  shown  by  the  similarity  of  plants, 
with  Northern  Asia  by  the  region  between  the  Aleutian  Isles  and 
Behring  Straits.  Thus  migrations  were  not  only  enforced  by  climatic 
changes,  but  permitted  by  geographical  connections  with  adjacent  con- 
tinents. Also  the  great  Pliocene  lake  (p.  478)  which  separated  West- 
ern from  Eastern  North  America  was  abolished,  and  migrations  estab- 
lished between  the  East  and  West.  It  is  evident  that  from  all  these 
causes  mammalian  faunae  from  widely-different  regions  were  precipitated 
upon  each  other,  and  struggled  together  for  mastery.  Large  numbers 
of  species  were  destroyed,  and  the  fittest  only  survived,  and  these  only 
under  changed  forms.  It  is  quite  possible  that  man  came  to  America 
with  the  Asiatic  mammalian  invasion.  If  so,  his  earliest  remains  in 
America  may  be  looked  for  on  the  Pacific  coast. 

Of  course,  we  use  the  word  migrations  in  its  widest  sense,  as  change 
of  habitat  of  species  as  well  as  of  individuals.  In  the  case  of  plants 
and  many  lower  animals,  the  place  of  species  only  moved  slowly,  from 
generation  to  generation.  In  the  case  of  mammals  there  was  more  de- 
cided movement  of  individuals. 

This  very  important  subject  has  been  more  closely  studied  in  Europe 
than  here,  although  we  believe  that  America  is  the  simplest  and  best 
field  for  its  elucidation.  During  the  Quaternary  probably  at  least  four 
distinct  mammalian  faunas  struggled  together  for  mastery  on  European 
soil  :  1.  The  Pliocene  autochthones.  2.  Invasions  from  Africa,  per- 
mitted by  geographical  connection  opening  a  gatevvav  through  the 
Mediterranean,  since  closed.  3.  Invasions  from  Asia,  by  opening  of  a 
gateway  which  has  remained  open  ever  since  ;  with  this  invasion  prob- 
ably came  man.  4.  Invasions  from  arctic  regions.  Probably  more  than 
one  such  invasion  took  place ;  certainly  one  occurred  during  the  second 
Glacial  epoch,  called  on  that  account  the  Reindeer  period. 

The  final  result  of  all  this  struggle  was,  that  the  Pliocene  autoch- 
thones were  destroyed ;  the  southern  species  were  mostlv  destroyed  or 
driven  back,  with  changed  forms  and  diminished  size ;  the  northern  spe- 
cies, reindeer,  glutton,  etc.,  retreated  again  northward,  and  the  Asiatics 
remained  in  possession  of  the  field,  but  greatly  changed  by  the  strug- 
gle. Man  was  among  these,  and  certainly  one  of  the  principal  agents 
in  the  change.  Speaking  more  accurately,  the  present  fauna  of  Europe 
may  be  said  to  be  a  product  of  all  these  factors ;  but  the  Asiatic  inva- 
sion seems  to  be  the  largest  factor. 

Thus,  then,  the  gradual  progress  of  evolution,  and  the  causes  of  the 
phenomenon  of  rapid  change  of  species  at  critical  periods  of  the  earth's 
history,  may  be  briefly  summarized  as  follows  : 

1.  A  gradual,  extremely  slow  evolution  of  organic  forms  under  the 


554  CENOZOIC  ERA— AGE   OF  MAMMALS. 

operation  of  all  the  forces  and  factors  of  evolution  known  and  unknown, 
whatever  we  may  conceive  these  to  be.  This  cause  acting  alone  would 
produce  gradual  changes  in  time  (geological  fauna),  but  without  geo- 
graphical diversity. 

2.  This  slow  evolution  takes  different  directions  in  different  places 
and   under  different  physical  conditions,  and  thus  gives  rise  to  geo- 
graphical faunae  and  florce.     This  cause  acting  alone  would  produce 
extreme  geographical  diversity,  and  render  determination  of  synchron- 
ism impossible. 

3.  During  critical  periods  physical  changes  and  consequent  migra- 
tions^ partly  enforced  by  changes  of  climate,  partly  permitted  by  removal 
of  barriers,  and  the  precipitation  of  adjacent  faunas  and  floras  upon  each 
other,  and  the  consequent  severe  struggle  for  life,  give  rise  to  far  more 
rapid  changes  of  species,  but  at  the  same  time  to  greater  geographical 
uniformity.      This  more   rapid   change   of  organic  forms  is  produced 
partly  by  severer  pressure  of  external  conditions,  certainly  one  factor 
of  change ;  partly  by  severer  struggle  for  life,  certainly  another  factor 
of  change ;  and  doubtless  partly  also  by  the  mere  active  operation  of 
other  factors  of  change,  which  we  do  not  yet  understand.     This  last 
cause  tends  to  produce  not  only  more  rapid  general  evolution,  but  also 
to  destroy  extreme  geographical  diversity  ;  and  since  it  operates  on  ani- 
mals rather  more  than  plants,  plant  species  are  more  apt  to  be  local, 
and  are  less  certainly  carried  along  with  the  stream  of  general  evolu- 
tion, and  are,  therefore,  less  reliable  in  determining  geological  age  than 
animals. 

Thus,  then,  regarding  the  Cenozoic  and  the  Modern  as  consecutive 
eras,  and  the  Quaternary  as  the  transitional,  revolutionary,  or  critical 
period  between,  we  see  a  great,  and,  if  we  had  lost  the  Quaternary,  an 
apparently  sudden,  change  of  species.  Yet  this  change,  as  great  as  it 
is,  is  not  to  be  compared  in  magnitude  with  that  which  separates  the 
great  eras  or  even  ages  from  each  other.  Evidently,  therefore,  we  must 
regard  the  lost  interval  between  the  Archaean  and  Palaeozoic,  and  that 
between  the  Palaeozoic  and  Mesozoic,  yes,  even  that  between  the  Meso- 
zoic  and  Cenozoic  (as  small  as  this  latter  is  in  comparison  with  the 
others),  as  all  of  them  far  greater  than  the  whole  Quaternary  period ; 
or  else  the  forces  of  evolution  must  have  been  far  more  active  in  those 
earlier  times  than  more  recently. 

4.  Drift  'in  Relation  to  Gold. — We  have  already  stated  (p.  231)  that 
gold  occurs  in  two  positions,  either  in  quartz-veins  intersecting  meta- 
morphic  slates  (quartz-mines)  or  in  drift-gravels  (placer-mines).     The 
auriferous  slates  may  be  of  various  ages.     In  the  Appalachian  chain, 
and   in  the  Ural  Mountains,  and   in  Australia,  the  slate  or  schist  is 
metamorphic  Silurian.      In  California  it  is  Jura-Trias.      The  placer 
gold  deposits  are  everywhere  Quaternary  drift-gravels. 


GENERAL   OBSERVATIONS   OX   THE   WHOLE   QUATERNARY.        555 

There  has  been  throughout  all  geological  time  a  progressive  con- 
centration of  gold,  as  well  as  many  other  metals,  in  a  more  and  more 
available  form  :  1.  It  was  first  disseminated  in  excessively  small  quan- 
tities, too  small  to  be  detected,  through  the  slates,  derived  doubtless 
from  the  sea,  in  the  waters  of  which  it  is  detectable  in  very  small  quan- 
tities. 2.  After  the  upheaval,  crumpling,  metamorphism,  and  fissuring 
of  these  slates,  the  gold  was  dissolved,  and  accumulated,  along  with 
silica  and  metallic  sulphides,  in  these  fissures,  as  auriferous  veins.  3. 
Atmospheric  agencies  acting  on  these  outcropping  veins  dissolved  away 
the  sulphides,  and  left  the  gold  in  a  still  more  available  form  along  the 
backs  of  the  veins.  4.  Then  came  the  ice-sheet  and  the  glaciers 
of  the  Quaternary,  like  a  plough,  cutting  away  the  backs  of  the 
quartz-veins,  together  with  the  containing  slates,  and,  like  a  mill, 
grinding  all  to  gravel,  and  heaping  it  away  in  moraines.  Some  of  the 
placer-mines  are  in  these  moraines,  but  most  of  the  gold  has  been 
subjected  to  still  another  process  5.  Lastly,  in  the  Champlain  epoch, 
the  river-floods  washed  these  moraine-heaps  down  the  rivers,  sorting 
them  and  depositing  where  the  velocity  of  the  current  diminished. 
These  river-gravels,  thus  sorted,  cradled,  panned  by  the  action  of  cur- 
rents, and  therefore  with  the  coarse  gold  near  the  bottom  and  high 
up  the  gulches,  constitute  the  richest  placer-mines. 

The  placers  of  California,  however,  are  of  two  kinds,  viz.,  the 
ordinary  or  superficial  placers,  and  the  deep  placers.  The  superficial 
placers  are  gravel-drifts  in  the  present  river-beds.  The  deep  placers 
are  gravel-drifts  in  old  river-beds.  These  old  river-beds,  as  already 
stated  (pp.  238,  529),  are  in  many  cases  covered  up  with  lava.  In  some 
cases  the  general  direction  of  the  old  bed  coincides  with  that  of  the 
present  river-system,  but  more  commonly  the  present  river-system  cuts 
across  the  old  river-system.  In  all  cases,  however,  it  is  evident  that  the 
old  river-gravels  were  formed  before  the  lava-flow,  and  the  newer  grav- 
els after  the  lava-flow.  In  all  cases  also  the  present  river-system  has 
cut  down  far  below  the  old  beds,  in  this  respect  entirely  different  from 
the  old  river-beds  of  the  eastern  portion  of  the  continent. 

The  following  figures  are  ideal  sections  altered  a  little  from  Whit- 
ney's :  Fig.  891,  of  a  case  in  which  the  old  and  the  present  river-beds 
are  parallel  to  each  other ;  Fig.  892,  where  the  latter  cut  through  the 
former.  In  the  former  case  the  section  is  across  the  lava-flow,  as  well 
as  across  the  river-beds ;  in  the  latter  case  it  is  in  the  direction  of  the 
lava-flow,  and  therefore  of  the  old  river-bed,  but  across  the  present 
river-bed. 

In  Fig.  891,  which  is  a  section  across  Table  Mountain,  in  Tuolumne 
County,  California,  L  is  the  lava-cap,  140  feet  thick,  beneath  which  is  the 
old  river-bed,  R,  with  its  gravel,  6r,  now  worked  by  a  tunnel,  driven 
through  the  rim-slate  S.  More  recent  gravels,  G',  are  seen  in  the  pres- 


556 


CENOZOIC  ERA— AGE   OF  MAMMALS. 


ent  river-beds,  R'.     In  this  locality  G  represents  the  deep  placers,  and 
G'  the  superficial  placers. 

The   history  of  changes   shown  in  these   sections  is    sufficiently 
obvious.     In  the  time  of  the  old  river-system,  R  was  a  river-bed,  doubt- 


FIG.  891. — Section  across  Table  Mountain,  Tuolumne  County,  California :  L,  lava ;  G,  gravel ;  S,  slate  ; 
Ji,  old  river-bed  ;  Jt',  present  river-bed. 

less  with  a  ridge  on  either  side  represented  by  the  dotted  lines.  In 
this  bed  accumulated  gravel,  containing  gold.  Then  came  the  lava- 
flow,  which  of  course  ran  down  the  valley,  displacing  the  river  and 


FIG.  692.— Lava-Stream  cut  through  by  Elvers :  a,  a,  basalt;  6,  b,  volcanic  ashes ;  c,  c,  Tertiary;  rf,  rf, 
Cretaceous  rocks;  It  £,  direction  of  the  old  river-bed;  Ji\  R',  sections  of  the  present  river-beds  (from 

covering  up  the  gravels.  The  displaced  rivers  now  ran  on  either  side 
of  the  resistant  lava,  and  cut  out  new  valleys,  2,000  feet  deep,  in  the 
solid  slate,  leaving  the  old  lava-covered  river-beds  and  their  auriferous 
gravels  high  up  on  a  ridge.  In  other  cases  the  convulsion  which  ejected 
the  lava  also  changed  greatly  the  general  slope  of  the  country,  and 
therefore  the  direction  of  the  streams.  In  such  cases  of  course  the 
present  river-system  cuts  across  the  old  river-beds  and  gravels,  and 
their  covering  lavas,  as  shown  in  Fig.  892. 

Age  Of  the  River-Gravels. — The  age  of  the  old  river-gravels  is  still 
doubtful ;  that  of  the  newer  river-gravels  is  undoubtedly  Champlain 
or  early  Terrace.  Below  we  give  a  list,  taken  from  Whitney,  of  the 
remains  found  in  these  gravels  : 


Newer  placers. 


Mastodon. 

Elephant. 

Bison. 

Tapir. 

Horse,  modern. 

Man's  works, 
f  Mastodon.1 

Rhinoceros. 
Deep  placers.     -{  Hippopotamus  (ally). 

Camel  (ally). 
[  Horse,  extinct  species. 

Whitney  states  that  the  mastodon  is  not  found  here,  but  it  has  been  since  found. 


PSYCHOZOIC  ERA— AGE  OF  MAN— RECENT  EPOCH  55? 

It  will  be  seen  that  the  fauna  of  the  deep  placers  unite  Pliocene 
and  Quaternary  characters,  though  the  Pliocene  are  the  more  numer- 
ous. The  mastodon  is  distinctively  Quaternary,  but  all  the  others  are 
Pliocene.  Hence  Whitney  very  naturally  places  the  deep  placers  in 
the  uppermost  Pliocene,  and  makes  the  lava-flow  the  division-line  be- 
tween Tertiary  and  Quaternary.  It  is  not  at  all  impossible,  however, 
nor  even  improbable,  that  many  of  the  Pliocene  animals  may  have  lin- 
gered on  this  coast  into  the  Quaternary.  In  that  case  we  might  assign 
the  deep  placers  to  the  earlier  Quaternary,  and  the  newer  placers  to 
the  later  Quaternary.  Certain  it  is  that  the  deep  placer-gravels  are 
similar  in  all  respects  to  the  Quaternary  gravels  all  over  the  world, 
except  that,  by  percolating  alkaline  waters  containing  silica,  they  have 
been  cemented  in  some  cases  into  grits  and  conglomerates.  This  is 
because  they  are  covered  with  lava  which  yields  both  the  alkali  and 
the  soluble  silica,  as  already  explained  (p.  238). 

In  any  case,  we  have  here  an  admirable  illustration  of  the  immensity 
of  geological  times.  The  whole  work  of  cutting  the  hard  slate-rock 
2,000  feet  or  more  has  been  done  since  the  lava-flow,  and  therefore 
certainly  since  the  beginning  of  the  Quaternary. 


CHAPTER  VI. 
PSYCHOZOIC  ERA— AGE  OF  MAN— RECENT  EPOCH. 

Characteristics. — The  Quaternary,  and,  indeed,  all  previous  ages, 
were  reigns  of  brute  force  and  animal  ferocity.  A  condition  of  things 
prevailed  which  was  inconsistent  with  the  supremacy  of  man.  The  age 
of  man,  on  the  contrary,  is  characterized  by  the  reign  of  mind.  There- 
fore, as  was  necessary,  the  dangerous  animals  decreased  in  size  and 
number,  and  the  useful  animals  and  plants  were  introduced,  or  else 
preserved  by  man. 

Distinctness  of  this  Era. — In  regard  to  the  distinctness  and  impor- 
tance of  this  era,  there  are  two  views  which  will  probably  ever  divide 
geologists,  depending  on  the  two  views  regarding  the  relation  of  man 
to  Nature.  From  the  purely  structural  and  animal  point  of  view,  man 
is  very  closely  united  with  the  animal  kingdom.  He  has  no  department 
of  his  own,  but  belongs  to  the  vertebrate  department,  along  with  quad- 
rupeds, birds,  reptiles,  and  fishes.  He  has  no  class  of  his  own,  but  be- 
longs to  the  class  Mammalia,  along  with  quadrupeds.  Neither  has  he 
an  order  of  his  own,  but  belongs  to  the  order  of  Primates,  along  with 
monkeys,  lemurs,  etc.  Even  a  famity  of  his  own,  the  Hominidce,  is 
grudgingly  admitted  by  some.  But  from  the  psychical  point  of  view  it 


558  PSYCHOZOIC  ERA— AGE   OF   MAN— RECENT  EPOCH. 

is  simply  impossible  to  overestimate  the  space  which  separates  man 
from  all  lower  things.  Man  -must  be  set  off  not  only  against  the  animal 
kingdom,  but  against  the  whole  of  Nature  besides,  as  an  equivalent : 
Nature  the  book — the  revelation — and  man  the  interpreter. 

So  in  the  history  of  the  earth :  from  one  point  of  view  the  era  of 
man  is  not  equivalent  to  an  era,  nor  to  an  age,  nor  to  a  period,  nor  even 
to  an  epoch.  But  from  another  point  of  view  it  is  the  equivalent  of 
the  whole  geological  history  of  the  earth  besides.  For  the  history  of 
the  earth  finds  its  consummation,  and  its  interpreter,  and  its  signifi- 
cance, in  man. 

The  rocks  of  this  epoch  are  the  present  river-deposits,  lake-deposits, 
sea-deposits,  volcanic  ejections,  etc.,  already  treated  of  in  Part  I.  The 
fauna  and  flora  of  this  epoch  are  the  species  still  living  on  the  earth. 
These  are  different  from  those  of  the  Tertiary,  and  largely  from  those 
of  the  Quaternary,  times  ;  but  the  change,  as  we  have  already  shown, 
has  been  gradual,  not  sudden  ;  man  himself  being  one  of  the  chief  agents 
of  change. 

The  Change  still  in  Progress— Examples  of  Recent- 
ly-Extinct Species. — The  gradual  change  of  fauna  has 
been  going  on  through  many  ages,  and  is  still  going 
on  under  our  eyes.  Many  remarkable  Quaternary 


FIG.  893.— Dinornis  pig-ant  PUS.  x  ,V  (from  ft  photo-      FIG.  894. — Aptornis  didiformis,  x  ^  (from  a  pho- 
graph  of  a  skeleton  in  Christchurch  Museum,  toprraph  of  a  skeleton  in  Christchurcu  Muse- 

New  Zealand).  urn,  New  Zealand). 


PSYCHOZOIC   ERA— AGE   OF   MAN— RECENT  EPOCH. 


559 


species  have  lingered,  and  become  extinct  by  the  agency  of  man,  even 
in  historic  times.  Among  the  most  remarkable  of  these  are  the  huge 
wingless  birds,  the  remains  of  which  have  been  discovered  in  New  Zea- 
land and  Madagascar,  viz.,  the  Dinornis  (huge  bird),  JEpiornls  (tall 
bird),  Palapteryx  (old  wingless  bird),  the  Solitaire,  and  the  Dodo. 
Through  the  kindness  of  Mr.  C.  D.  Voy,  I  am  able  to  give  good  figures 
of  the  skeletons  of  several  of  these  extraordinary  extinct  birds,  taken 
from  photographs  (Figs.  893,  894). 


FIG.  895.— Dinornis  elephantopus,  r'5  (after  Owen). 

The  Dinornis  giganteus  of  New  Zealand,  and  the  JEpiornis  of 
Madagascar,  were  probably  twelve  feet  high.  The  tibia  of  the  former 
has  been  found  nearly  a  yard  long,  and  as  thick  as  the  tibia  of  a  horse, 
and  the  egg  of  the  latter,  well  preserved,  thirteen  inches  long  and  nine 
inches  in  diameter,  with  a  capacity  of  two  gallons.  The  toe-bones  of  the 
D.  elephantopus  (Fig.  895)  rivaled  in  size  those  of  an  elephant  (Owen). 
These  huge  birds  must  have  been  capable  of  making  tracks  nearly  as 
large  as  those  of  the  supposed  birds  of  the  Connecticut  Valley  sandstone 
(p.  444).  The  dodo,  a  heavy,  clumsy  bird,  of  fifty  pounds'  weight,  with 


560  P3YCHOZOIC  ERA— AGE   OF  MAN— RECENT  EPOCH. 

loose,  downy  feathers,  and  imperfect  wings,  like  a  new-born  chicken, 
became  extinct  only  about  150  or  200  years  ago.  The  Apteryx,  to 
which,  of  all  living  birds,  the  Dinornis,  Apatornis,  etc.,  are  most  nearly 
allied,  still  survives,  ready  to  disappear  (Fig.  896). 


FIG.  896.— Apteryx  Australia. 

The  -50s primigenius,  the  gigantic  ox  of  Quaternary  times,  is  sup- 
posed to  be  the  same  as  the  Urus  of  Caesar,  and  therefore  became  ex- 
tinct since  Roman  times.  The  aurochs,  another  Quaternary  ox,  would 
have  been  now  entirety  extinct  but  for  the  imperial  edict  which  pre- 
serves a  few  in  the  forests  of  Lithuania.  The  lion,  the  tiger,  the  bison, 
the  elephant,  and  the  rhinoceros,  and,  in  fact,  all  the  fiercer  and  larger 
animals,  are  even  now  disappearing  before  the  advance  of  civilized  man. 

Thus,  in  passing  from  geological  to  present  times,  we  trace  rocks 
into  sediments  and  soils ;  geological  agencies  into  chemical  and  physical 
agencies,  now  in  operation;  extinct  faunas  and  florae  into  the  living 
fauna  and  flora ;  in  a  word,  geology  into  chemistry  and  physics,  and 
paleontology  into  zoology  and  botany. 

Now,  in  this  gradual  change  of  fauna,  when  did  man  first  appear 
upon  the  scene,  and  what  was  the  character  of  primeval  man  ?  This 
introduces  us  to  two  very  important  but  very  difficult  and  obscure  sub- 
jects. 

I. — ANTIQUITY  OF  MAN. 

On  this  interesting  subject  the  three  sciences — History,  Archaeology, 
and  Geology — meet  and  cooperate ;  and  the  recent  rapid  advance  has 
been  the  result  of  this  union,  and  especially  of  the  application  of  geo- 
logical methods  of  research. 

Archaeologists  have  long  ago  divided  the  history  of  human  civiliza- 


ANTIQUITY   OF  MAN.  561 

tion  into  three  epochs  or  ages,  named,  from  the  materials  of  which 
weapons  and  tools  are  made,  respectively  the  Stone  age,  the  Bronze 
age,  and  the  Iron  age.  We  are  here  concerned  only  with  the  Stone 
age  ;  the  others  belong  to  history. 

Closer  study  has  again  divided  the  Stone  age  into  two,  viz.,  the 
Palaeolithic  (old  Stone  age)  and  the  Neolithic  (newer  Stone  age). 
During  the  former,  only  chipped  stone  implements  were  used ;  while  in 
the  latter  polished  stone  implements  were  also  used.  It  is  principally 
with  the  Palaeolithic  that  we  are  here  concerned. 

Still  closer  study,  in  connection  with  geology,  has  again  divided  the 
Palaeolithic  into  an  earlier  and  a  later.  The  earlier,  being  contempora- 
neous with  the  mammoth,  is  called  the  Mammoth  age;  and  the  latter, 
for  similar  reasons,  the  Reindeer  age.  The  mammoth,  however,  existed 
also  in  this  latter  age.  The  former  seems  to  correspond  with  the  Cham- 
plain  epoch  in  geology,  and  the  latter  with  the  Terrace  of  America,  or 
Second  Glacial  epoch  of  Europe.  The  Neolithic  commences  the  Psycho- 
zoic  era,  or  reign  of  man — the  period  when  man  had  established  his 
supremacy.  The  following  table  expresses  these  views  : 

3.  Iron  age \ 

2.  Bronze  age V  Psychozoic  era. 

{Neolithic — Domestic  animals.  ) 
Paleolithic  ^  Reindeer  age  =  Terrace  or  Second  Glacial  epoch. 
'  (  Mammoth  age  =  Champlain  epoch. 

These  divisions  and  their  relations  to  geological  epochs  have  been 
established  in  Europe.  They  would  probably  apply  also  to  some  parts 
of  Asia  and  Africa,  for  in  portions  of  these  old  countries  man  has  doubt- 
less passed,  successively  and  slowly,  through  all  these  stages.  But  all 
these  stages  are  not  represented  in  all  countries,  nor  do  they  necessarily 
correspond  to  the  geological  epochs  mentioned  above.  The  South-Sea- 
Islanders,  for  example,  are  still  in  the  Stone  age.  The  American  Indians 
were  in  the  Stone  age  only  three  centuries  ago. 

The  table  given  above  carries  man  back  to  the  Champlain  epoch. 
There  are  some  geologists  who  think  they  find  evidence  of  a  much  ear- 
lier existence  of  man.  We  will,  therefore,  very  rapidly  review  the  evi- 
dences of  the  antiquity  of  man.  In  doing  so,  however,  we  shall  accept 
none  but  thoroughly  reliable  evidence.  There  has  been  recently  far  too 
much  eagerness  to  find  facts  which  overthrow  accepted  beliefs,  and  to 
accept  them  on  this  account  alone.  We  will  take  up  European  locali- 
ties first,  because  the  subject  has  been  more  carefully  studied  there. 

Primeval  Man  in  Europe. 

Supposed  Miocene  Man— Evidence  unreliable.— The  earliest  period 
in  the  strata  of  which  any  supposed  evidences  of  the  existence  of  man 
36 


562  PSYCHOZOIC  ERA— AGE   OF  MAN— RECENT  EPOCH. 

have  been  found  is  the  Miocene.  These  evidences,  however,  are  con- 
fessedly meagre,  and  by  all  careful  investigators  considered  unreliable. 
Some  flint-flakes,  so  rough  that  they  may  be  the  result  of  physical  in- 
stead of  intelligent  agencies ;  some  bones  of  animals,  marked  with  par- 
allel scratches  as  if  scraped,  but  the  scratches  may  have  been  produced 
by  currents,  or,  as  Lyell  thinks,  by  the  teeth  of  Rodents;  some  more 
positive  evidences  of  man's  agency,  but  in  strata  of  more  doubtful  age, 
or  else  the  result  of  accidental  mixture  not  contemporaneous  with  the 
deposit  itself — such  is,  in  brief,  the  evidence.  The  Miocene  man  is  not 
acknowledged  by  a  single  careful  geologist. 

Supposed  Pliocene  Man. — The  evidence  of  the  existence  of  man 
during  the  Pliocene  period  is,  if  possible,  still  more  meagre  and  unre- 
liable. M.  Hamy  thinks  he  has  found  undoubted  evidence  of  human 
agency  in  flint  implements  in  Pliocene  strata  at  Savone ;  but  the  con- 
temporaneousness of  the  flints  and  the  deposit  is  regarded  as  doubtful. 
Again,  Palaeolithic  implements  have  been  found  in  Madras  in  strata  sup- 
posed by  Falconer  to  be  Pliocene ;  but  more  recent  investigations  make 
the  strata  Quaternary.1  Of  the  supposed  Pliocene  man  in  California  we 
will  speak  further  on.  Suffice  it  to  say  that  M.  Favre,  reviewing  the 
whole  subject  up  to  1870,"  and,  again,  Evans,  President  of  the  Geolog- 
ical Society  of  London,  reviewing  the  subject  up  to  1875,3  decide  that 
the  existence  of  Tertiary  man  is  yet  unproved. 

Quaternary  Man — Mammoth  Age. — But  of  the  existence  of  man  in 
Europe  and  America,  as  early  as  the  middle  of  the  Quaternary  period, 
there  seems  to  be  abundant  evidence.  We  shall  select  only  a  few 
striking  examples : 

a.  In  River-Terraces. — In  the  terraces  of  the  river  Spmme,  near  Abbe- 
ville, were  found,  nearly  twenty  years  ago,  by  M.  Boucher  de  Perthes, 
chipped  flint  implements,  associated  with  bones  of  the  mammoth,  rhi- 
noceros, hippopotamus,  hyena,  horse,  etc.  The  doubts  with  which  the 


FIG.  897.— Section  across  Valley  of  the  Somme:  1,  peat,  twenty  to  thirty  feet  thick,  resting  on 
gravel,  a  „•  2,  lower-level  pravels,  with  elephant-bones  and  flint  implements,  covered  wHh  river-loam 
twenty  to  forty  feet  thick ;  8,  upper-level  gravels,  with  similar  fossils  covered  with  loam,  in  all, 
thirty  feet  thick ;  4,  upland-loam,  five  to  six  feet  thick  ;  6,  Eocene-Tertiary. 

first  announcement  of  these  facts  was  received  have  been  entirely  re- 
moved by  careful  examination  of  the  locality  by  many  scientists,  both 
of  France  and  England. 

The  findings  were  in  undisturbed  gravels,  both  lower  (2)  and  upper 

1  American  Journal  of  Science,  1875,  vol.  x.,  p.  232. 

8  "  Bibliotheque  Universelle,"  "  Archives  des  Sciences,"  vol.  xsxvii.,  p.  97. 

8  American  Journal  of  Science,  vol.  x.,  p.  229. 


PRIMEVAL  MAN  IN  EUROPE.  5$3 

(3),  beneath  river-loam  twenty  to  thirty  feet  thick.  Supposing  that 
the  upper  loam  (4)  represents  the  full  Champlain  flood-deposit,  then 
3  and  2  represent  the  later  Champlain  or  early  Terrace  epoch. 

In  England,  also,  at  Hoxne,  similar  flint  implements,  associated  with 
bones  of  extinct  animals,  were  found  in  strata  underlying  the  higher- 
level  river-gravels,  but  overlying  the  bowlder-drift  or  true  glacial  de- 
posit. This  fixes  the  age  as  Champlain.  Many  other  examples  of  sim- 
ilar findings  might  be  cited. 

b.  Bone-Caves. — Engis  Skull. — In  the  caves  of  Belgium  and  Germany 
have  been  found  human  bones  associated  with  extinct  animals.  The 
best  example  is  that  of  the  skull  found  in  a  cave  at  Engis,  on  the  banks 
of  the  Meuse,  near  Liege.  Of  the  great  antiquity  of  this  skull  there 
seems  to  be  no  doubt.  It  was  found  in  bone  breccia,  associated  with 
bones  of  Quaternary,  extinct  species  and  living  species,  beneath  a  stal- 
agmitic  crust.  This  association  unmistakably  indicates  the  middle  or 
latter  part  of  the  Quaternary  period. 

Neanderthal  Skull. — In  a  cave  at  Neanderthal,  near  Dtisseldorf,  was 
found  a  very  remarkable  human  skeleton,which  has  greatly  excited  the  in- 
terest of  scientific  men.  The  limb-bones  are  large,  and  the  protuberances 
for  muscular  attachments  very  prominent ;  the  skull  very  thick,  very 
low  in  the  arch,  and  very  prominent  in  the  brows.  It  has  been  sup- 
posed by  some  to  be  an  intermediate  form  between  man  and  the  ape  ; 
but,  according  to  the  best  authority,  it  is  in  no  respect  intermediate,  but 
truly  human.  It  is  probably  the  skeleton  of  a  man  exceptionally  mus- 
cular in  body  and  low  in  intelligence.  The  evidences  of  antiquity  are 


FIG.  899. — Comparison  of  Forms  of  Skulls:  ft,  Euro- 
pean :   &,  the  Neanderthal  Man ;   c,  a  Chimpan- 
Fia.  80S.— Engis  Skull,  reduced  (after  Lyell).  nee  (after  Lyell). 

far  less  complete  than  in  the  case  of  the  Engis  skull,  though  it  proba- 
bly belongs  to  the  same  epoch.  The  Engis  skull,  on  the  other  hand,  is 
a  well-sliaped  average  human  skull.  A  figure  of  the  Engis  skull  is 
given  above  (Fig.  898),  and  a  comparison  in  outline  of  the  Neander- 
thal with  the  ape  and  European  (Fig.  899). 


564: 


PSYCHOZOIC  ERA— AGE   OF   MAN— RECENT   EPOCH. 


Mentone  Skeleton. — Only  a  few  years  ago  an  almost  perfect  skele- 
ton of  a  Palaeolithic  man  was  found  in  a  cave  at  Mentone,  near  Nice. 
It  is  that  of  a  tall,  well-formed  man,  with  average  or  more  than  aver- 
age-sized skull,  and  a  facial  angle  of  85°.  The  antiquity  of  this  man  is 
undoubted,  for  his  bones  are  associated  with  those  of  the  cave-lion, 
cave-bear,  rhinoceros,  reindeer,  together  with  living  species.  The  bones 
of  the  skeleton  are  all  in  place,  surrounded  with  the  implements  of  the 
chase  (flint  implements),  and  the  spoils  of  the  chase,  viz.,  the  bones  of 
reindeer,  perforated  teeth  of  stag,  etc.  Of  the  latter,  twenty-two  lay 
about  his  head.  These  are  supposed  to  have  been  worn  as  a  chaplet. 
This  Quaternary  man  seems  to  have  laid  himself  down  quietly  in  his 
cave-home  and  died. 

All  these,  and  many  more  which  might  be  mentioned,  belong  to 
the  early  Palaeolithic,  although  the  last  is  possibly  a  transition  to  the 
next  or  Reindeer  age.  They  were  contemporaneous  with  the  mam- 
moth, the  rhinoceros,  the  hippopotamus,  the  cave-bear,  the  cave-lion, 
the  cave-hyena,  and  other  extinct  animals  ;  but  the  reindeer  had  not 
yet,  to  any  extent,  invaded  Middle  Europe  from  the  north.  They  seem 
to  have  been  savages  of  the  lowest  type,  living  by  hunting  and  dwell- 
ing in  caves.  There  is  no  evidence  of  agriculture  or  of  domestic  ani- 
mals. In  many  cases  there  have  been  found  some  anatomical  charac- 
ters of  a  low  or  animal  type,  such  as  flattened  shin-bones,  very  promi- 
nent occipital  protuberance,  less  than  usual  separation  between  the 
temporal  ridges,  large  size  of  the  loisdom  teeth,  etc.  But  all  these 
characters  are  found  now  in  some  savage  races,  either  as  racial  or 
as  individual  peculiarities.  The  earliest  men  yet  found  are  in  no 
sense  connecting  links  between  man  and  ape.  They  are  distinctively 
and  perfectly  human,  and  even  in  many  cases 
of  far  better  conformation  than  the  lowest  types 
now  living. 

Reindeer  Age  or  Later  Palaeolithic.— Dur- 
ing this   age  man   was  still   associated  in 
Middle  Europe  with  Quaternary  ani- 
mals, but  also  now  with  arctic 
animals,     especially     the 
reindeer.       It    prob- 
ably   c  o  r  r  e  - 
sponds     with 
the      Second 
Glacial  epoch 
in    Europe, 
and    the   full 

beddedTc,  layerof "ashes  and  charcoal  eicrht  inches  thick,  with  broken,  burnt,  and    TVrrn r>e>  pnnn h 
gnawed  bones  of  extinct  and  livinir  mammals.  uNo  h.virth-stones  and  works  of     A 


Kn,.  '.mo. — A  Section  of  the  Auriirnac  Cave:  a.  vault  in  which  re-mains  of  seventee 
human  skeletons  were  found  ;  6,  made  ground,  two  feet  thick,  in  which  huma 
bones  and  entire  bones  of  extinct  and  livinp  mammals,  and  works  of  art,  were 


art;  d,  deposit  with  similar  contents  ;  e,  talus  washed  down  from  hill  above;  fg,    jn    Ampriffl 
slab  of  stone  which  closed  the  vault ;  /  «,  rabbit-burrow,  which  led  to  discovery 


PRIMEVAL  MAX  IN  EUROPE. 


565 


Aurignac  Cave. — This  sepulchral  cave  and  its  rich  contents  were 
accidentally  discovered  by  a  French  peasant.  Fig.  900,  page  564,  is  a 
diagram  section  of  the  cave,  taken  from  Lyell. 

On  removing  the  talus,  e,  a  slab  of  rock,/^,  was  exposed,  covering 
the  mouth  of  the  cave,  a.  In  this  cave  were  found  seventeen  human 
skeletons  of  both  sexes  and  of  all  sizes,  together  with  entire  bones  of 
extinct  animals  and  works  of  art.  Outside  of  the  cave  was  found  a 
deposit,  c  and  d,  consisting  of  ashes  and  cinders,  mingled  with  burnt 
and  split  and  gnawed  bones  of  recent  and  extinct  animals,  and  works 
of  art.  The  conclusion  reached  by  M.  Lartet  is,  that  this  was  a  family 
or  tribal  burial-place  ;  that  in  the  cave  along  with  the  bodies  were 
placed  funereal  gifts  in  the  form  of  trinkets  and  food  ;  and  that  the 
funereal  feast  was  cooked  and  eaten  on  the  level  space  in  front  of 
the  cave  ;  and,  finally,  that  carnivorous  beasts  gnawed  the  bones  left 
on  the  spot.  It  is  evident  that  the  Aurignac  men  practised  religious 
rites  which  indicated  a  belief  in  immortality. 

The  following  is  a  list  of  the  animals  the  remains  of  which  were 
found  in  and  about  the  cave  ;  those  marked  f  are  either  wholly  extinct 
or  extinct  in  this  locality  : 


FAUNA  OF  AURIGNAC  CAVE. 

CARNIVORES. 

HERBIVORES. 

•(•Cave-bear 

5  or  6 

f  Mammoth  
•(•Rhinoceros  
•(•Horse 

.  2  molars. 
.  1 
12-15 

Brown  bear 

1 

Badger.  .  . 

1  or  2 

Polecat  

1 

f  Ass 

1 

•(•Cave-lion  
Wild-cat  

1 
1 

Hog; 

.   1 

Sta<r                  .  .    . 

1 

f  Cave-hyena  
Wolf. 

5-6 
3 

f  Irish  elk  

.  1 

3-4 

Fox  

18-20 

f  Reindeer  
•(•Aurochs  

.10-12 
.12-15 

Perigord  Caves. — In  Southern  France,  along  the  course  of  the  river 
Vezere,  are  found  many  caves  in  which  are  preserved  many  interesting 
relics  of  man.  The  Palaeolithic  Aquitanians  seem  to  have  been  some- 
what more  advanced,  and  of  a  more  peaceful  temper,  than  the  early 
Palaeolithic  men  already  described.  Although  there  is  no  evidence  of 
agriculture,  they  lived  by  fishing  as  well  as  by  hunting.  This  is  shown 
by  the  number  of  fishing-hooks  of  bone  found  there.  They  seemed  also 
to  have  had  a  taste  and  some  skill  in  drawing,  for  they  have  left  some 
drawings  of  contemporaneous  but  now  extinct  animals,  especially  the 
mammoth,  the  reindeer,  and  the  horse.  Fig.  901  is  a  piece  of  reindeer- 
horn  on  which  is  a  rude  etching  of  a  mammoth. 


566       PSYCHOZOIC  ERA— AGE  OF  MAN— RECEXT  EPOCH. 


FIG.  901. — Drawing  of  a  Mammoth  by  Contemporaneous  Man. 

Conclusions. — It  seems  evident  that  in  Europe  the  earliest  men  were 
contemporaneous  with  a  large  number  of  now  extinct  animals,  and  were 
a  principal  agent  in  their  extinction  ;  that  they  saw  the  flooded  rivers 
of  the  Champlain  epoch,  and  the  great  glaciers  of  the  Second  Glacial 
epoch  ;  but  there  is  no  reliable  evidence  yet  of  their  existence  before  or 
even  during  the  true  Glacial  or  ice-sheeted  epoch.1 

Neolithic  Man  ;  Ref use- Heaps  ;  Shell- Mounds  ;  Kitchen- Middens. 
In  Northern  Europe,  especially  in  Denmark,  are  found  shell-mounds 
of  great  size,  1,000  feet  long,  200  feet  wide,  and  ten  feet  high.  They 
are  probably  the  accumulated  refuse  of  annual  tribal  feasts.  The  early 
races  of  men  in  all  countries  seem  to  have  had  the  custom  of  gathering 
in  large  numbers  at  stated  intervals,  and  feasting  on  shell-fish  and 
other  animals,  and  leaving  their  remains  in  large  heaps  to  mark  the 
spot  of  assembly.  The  evidences  of  a  very  marked  advance  are  found 
in  these  heaps.  The  implements  are  many  of  them  carefully  shaped  or 
else  polished  by  rubbing.  There  are  no  longer  any  remains  of  extinct 
animals,  but  only  of  living  animals  ;  and  there  are  now  found  remains 
of  at  least  one  domestic  animal,  viz.,  the  dog,  though  not  yet  any  evi- 
dence of  agriculture. 

Transition  to  the  Bronze  Age — Lake  Dwellings.— In  the  Swiss, 
Austrian,  and  Hungarian  lakes  are  found  abundant  evidences  of  a  more 
advanced  race  than  any  yet  mentioned,  which  had  the  singular  custom 
of  dwelling  in  houses  constructed  on  piles  in  the  lakes,  and  connected 
with  the  land  by  means  of  piers  or  bridges.  Similar  lake-dwellings  are 
found  now  in  New  Guinea  and  in  South  America,  and  very  recently,  by 
Lieutenant  Cameron,  in  Africa.8  By  means  of  dredging,  a  great  num- 
ber and  variety  of  implements  of  polished  stone  and  of  bronze  have 
been  obtained.  Some  of  these  were  evidently  used  for  ornament,  some 

1  Some  evidences  of  Glacial  man,  which  seem  somewhat  more  reliable,  have  recently 
been  found  in  England,  but  it  may  be  only  the  Second  Glacial  epoch. 

2  Nature,  vol.  xiii.,  p.  202,  January,  1876. 


PRIMEVAL  MAN  IN  AMERICA.  567 

for  domestic  purposes,  some  for  agriculture  ;  some  were  weapons  of  war, 
some  fishing-tackle.  Many  of  these  are  wrought  with  great  skill  and 
taste.  Domestic  animals — ox,  sheep,  goat,  and  dog;  cereal  grains — 
wheat  and  barley ;  fruits — wild  apples,  blackberry,  etc. ;  coarse  cloth, 
not  woven  but  plaited — have  also  been  found.  In  a  word,  we  have  here 
all  the  evidences  of  communities  far  .above  the  state  of  savagism. 

From  this  time  the  history  of  man  may  be  traced,  by  means  of  his 
remains,  through  the  time  of  Megalithic  structures,  through  'the  Ro- 
man age,  step  by  step,  to  the  present  time.  But  this  belongs  to  the 
archaeologist,  not  the  geologist.  The  Neolithic  may  be  regarded  as 
the  beginning  of  the  Psvchozoic  era — the  connecting  link  between  geol- 
ogy and  archaeology.  The  Bronze  age  and  all  that  follows  it  belong 
clearly  to  archaeology. 

Primeval  Man  in  America. 

The  facts  on  this  subject  are  far  less  numerous  and  well  attested  in 
America  than  in  Europe.  There  is,  however,  undoubtedly  a  very  rich 
field  for  investigation,  especially  on  the  Pacific  coast. 

Supposed  Pliocene  Man. — Several  cases  are  reported  of  human 
bones  and  works  of  art  having  been  found  in  the  sub-lava  drift  de- 
scribed on  page  555.  These  cases  are  none  of  them  thoroughly  well 
attested,  though  the  evidence  is  such  as  to  make  us  suspend  our  judg- 
ment. The  best-attested  cases  are  the  Calaveras  skull  mentioned  by 
Whitney,  and  the  Table  Mountain  skull  reported  by  C.  F.  Winslow. 
Besides  these  there  are  several  cases  reported  of  mortars  and  pestles 
found  in  the  sub-lava  deposit.  Many  claim  these  as  evidence  of  the 
existence  of  man  in  a  somewhat  advanced  stage  of  progress  (at  least 
as  much  so  as  the  Neolithic  man  of  Europe),  on  the  Pacific  coast,  dur- 
ing the  Pliocene  period.  The  doubt  in  regard  to  this  extreme  antiquity 
of  man  is  of  two  kinds — namely,  1.  Doubt  as  to  the  pre-lava  age  of  the 
remains  or  works  of  art,  no  scientist  having  seen  any  of  these  in  situ. 
2.  Doubt  as  to  the  age  of  the  sub-lava  drift.  It  may  be  not  older 
than  the  Champlain  (p.  556). 

In  any  case,  and  whatever  be  the  geological  age  of  the  sub-lava 
drift,  if  man  should  be'  undoubtedly  found  there,  it  would  show  an 
immense  antiquity  ;  for,  since  the  lava-flow,  canons  have  been  cut  by 
the  present  rivers  2,000  or  3,000  feet  deep  in  solid  slate-rock. 

Quaternary  Man. — Even  leaving  out  the  supposed  sub-lava-drift 
remains,  the  earliest  appearance  of  man  on  the  American  Continent 
seems  to  have  been  on  the  Pacific  coast,  probably  as  migrants  from 
Asia.  There  seems  to  be  no  doubt  that  the  works  of  man  have  been 
found,  associated  with  the  remains  of  animals,  both  recent  and  extinct, 
in  the  superficial  placer  deposits  (p.  556). *  Among  the  extinct  animals 
1  Whitney,  "  Geological  Survey  of  California,"  vol.  i.,  p.  252. 


568 


PSYCHOZOIC  ERA— AGE   OF  MAX— RECENT  EPOCH. 


may  be  mentioned  the  elephant,  the  mastodon,  and  the  horse.     This 
corresponds  with  the  period  of  primeval  man  in  Europe. 

No  well-attested  evidence  of  Quaternary  man  has  yet  been  found 
in  other  parts  of  the  United  States.1  Shell-mounds  are  abundant  on 
both  the  Atlantic  and  Pacific  coasts,  but  these  seem  to  be  of  later  date 
than  those  of  Europe. 

Quaternary  Man  in  Other  Countries. — In  India a  Palaeolithic  imple- 
ments, precisely  like  those  found  in  Europe  and  elsewhere,  were  found, 
in  1873,  associated  with  extinct  species  of  elephant  and  hippopotamus 
in  Quaternary  deposits.  In  the  South  American  bone-caverns  human 
remains  have  been  found  associated  with  Quaternary  animals. 

Man,  therefore,  has  been  traced  back  with  certainty  to  the  later 
Champlain  or  early  Terrace  epoch.  It  is  possible  that  he  may  be  here- 
after traced  farther  to  the  Glacial  or  pre-Glacial  period.  Some  confi- 
dently expect  that  he  will  be  traced  to  the  Miocene,  but  this  seems 
extremely  improbable,  for  the  following  reasons  : 

a.  He  has  been   diligently  searched  for,  without  success.    Now, 
while  negative  evidence  is  rightly  regarded  as  of  little  value  in  geol- 
ogy, yet,  in  this  instance,  it  is  undoubtedly  of  far  more  than  usual 
value,  because  man's  works  are  far  more  numerous  and  far  more  im- 
perishable than  his  bones. 

b.  Man  probably  came  in  with  the  present  mammalian  fauna.    We 
repeat  here  the  diagram  illustrating  the  law  of  extinction  and  appear- 


T-  .E     f?     T     I  .  .A     B       Y        |j  QUATERNARY     | RECENT 

CRETACEOUS     EOCENE     [MIOCENE^  [PLIOCENE  I&L/IC  I  CHAM  I  TER U  RECENT 


Fio.  902. 

ance  of  species.  It  is  seen  that  lower  species  are  far  less  rapidly 
changed  than  higher.  Living  foraminifers  may  be  traced  back  into 
the  Cretaceous  ;  living  shells  and  other  invertebrates  to  the  beginning 
of  the  Tertiary  :  but  living  mammals  pass  out  rapidly  and  disappear  in 
the  Middle  Quaternary.  Not  a  single  species  of  mammal  now  living  is 
found  in  the  Tertiary.  Shall  man,  the  highest  of  all,  be  the  only  ex- 
ception ?  Man  is  one  of  the  present  mammalian  fauna,  and  came  in 
with  it. 

But,  again,  several  distinct  mammalian  faunae  have  appeared  and 

1  Very  recently  such  evidence  has  been  found  by  Mr.  Abbot  in  New  Jersey. 

2  American  Journal  of  Science,  1875,  vol.  x.,  p.  232. 


PRIMEVAL  MAN  IN  AMERICA. 


569 


disappeared  since  the  beginning  of  the  Miocene.  The  Miocene  mam- 
malian fauna  is  totally  different  from  the  Eocene  ;  the  Pliocene  total- 
ly different  from  the  Miocene  ;  the  Quaternary  from  the  Pliocene  ;  and 
the  present  from  the  Quaternary.  This  is  graphically  represented  in 


FIG.  903. — Diagram  illustrating  the  Appearance  and  Extinction  of  Successive  Mammalian  Faunae. 

the  diagram,  Fig.  903,  in  which  the  alternate  shaded  and  white  spaces 
represent  five  consecutive  mammalian  faunae  (there  are  really  more 
than  five)  overlapping  each  other,  but  substantially  distinct.  It  seems 
in  the  highest  degree  improbable  that  man,  a  mammal,  should  survive 
the  appearance  and  disappearance  of  several  mammalian  faunae.  If, 
therefore,  man  should  ever  be  traced  to  the  Miocene,  it  would  probably 
be  a  different  species  of  man — the  genus  Homo,  but  not  the  species 
Sapiens. 

Time  since  Man  appeared.— Geology  reckons  her  time  in  periods, 
epochs,  etc.;  History  hers  in  years.  It  is  impossible  to  express  the 
one  chronology  in  terms  of  the  other  except  in  a  very  rough  approxi- 
mative way,  for  want  of  a  reliable  common  measure.  If  Mr.  Croll's 
theory  of  glacial  cold  should  indeed  prove  true,  then  we  might  hope 
to  measure  man's  time  on  the  earth  with  some  degree  of  accuracy. 
But  in  the  absence  of  confidence  in  this  theory,  our  only  resource  is  to 
use  the  measure  which  we  have  already  used  on  several  occasions,  viz., 
the  effects  of  causes  now  in  operation.  This  measure,  however,  can 
give  but  very  rough  approximate  results. 

There  is  no  doubt  that  very  great  changes,  both  in  physical  geog- 
raphy and  in  the  mammalian  fauna,  have  taken  place  since  man  ap- 
peared. Judging  by  the  rate  of  changes  still  in  progress,  we  are  natu- 
rally led  to  a  conviction  of  a  lapse  of  time  very  great  in  comparison 
with  that  recorded  in  history.  On  the  other  hand,  some  attempts  to 
estimate  more  accurately  by  means  of  the  growth  of  deltas  in  which 
have  been  found  implements  of  the  Roman  age,  the  Bronze  age,  and 
the  Stone  age  ;  and  by  the  progressive  erosion  of  lake-shores,  which  is 
supposed  to  have  commenced  after  the  Champlain  epoch,  have  led  to 
very  moderate  results,  viz.,  7,000  to  10,000  years.  While  these  results 
cannot  be  received  with  any  confidence,  yet  it  is  hoped  that  many  such 
will  continue  to  be  made. 

In  conclusion,  we  may  say  that  we  have  as  yet  no  certain  knowledge 


570  PSYCHOZOIC  ERA— AGE  OF   MAN— RECENT   EPOCH. 

of  man's  time  on  the  earth.     It  may  be  100,000  years,  or  it  may  be 
only  10,000  years,  but  more  probably  the  former  than  the  latter. 

II. — CHARACTER    or   PRIMEVAL   MAX. 

In  regard  to  the  second  question,  viz.,  the  character  of  primeval 
man,  we  will  make  but  one  remark.  We  have  seen  that  the  earliest 
men  yet  discovered  in  Europe  or  America,  though  low  in  the  scale  of 
civilization,  were  distinctively  and  perfectly  human,  as  much  so  as  any 
race  now  living,  and  were  not  in  any  sense  an  intermediate  link  between 
man  and  the  ape.  Nevertheless,  we  must  not  forget  that  the  cradle  of 
mankind  was  probably  in  Asia.,  Man  came  to  Europe  and  America 
by  migration.  The  intermediate  link,  if  there  be  any  such,  must  be 
looked  for  in  Asia.  This  question  can  only  be  settled  by  a  complete 
knowledge  of  the  Quaternary  of  that  country. 

In  any  case,  man  is  the  ruler  only  of  the  modern  era.  The  presence 
of  man  in  Quaternary  times  must  be  regarded  as  an  example  under  the 
law  of  anticipation  (p.  267).  He  only  fairly  established  his  supremacy 
in  the  Recent  epoch,  and  therefore  the  age  of  man  and  the  Psycho- 
zoic  era  ought  to  date  from  that  time. 


A  IIST  OF  THE  PRINCIPAL  AUTHORITIES  FOR  THE  ILLUSTRATIONS  IN  THIS  WORK, 
AND  THE  BOOKS  FROM  WHICH  THEY  HAVE  BEEN  TAKEN. 


AGASSIZ,  L.     "  Etudes  sur  les  Glaciers  " — "  Poissons  Fossiles." 

AUTHOR.  Am.  Jour,  of  Sci.,  III.,  vols.  x.  and  xi. ;  also  many  (about  150) 
diagrammatic  illustrations  throughout  this  work. 

BAILEY,  J.  TV.     Am.  Jour,  of  Sci.,  II.,  i. 

BRADLEY,  F.  II.     Geological  Chart  of  the  U.  S. 

BRONGNIART,  A.     "  Histoire  des  V6ge"taux  Fossiles." 

BUCKLAND,  W.     "  Bridgewater  Treatise." 

COXRAD,  T.  A.     Jour.  Acad.  Sci.,  Philadelphia. 

COPE,  E.  D.  Hayden's  "U.  S.  Geog.  and  Geol.  Surv.,"  vol.  ii.— "Creta- 
ceous Vertebrates  ;  "  Newberry's  "  Ohio  Survey  " — "Pal.  Reptiles  of  the  Coal." 

DADDOW,  S.  H.     "  Coal,  Iron,  and  Oil." 

DANA,  J.  D.  Wilkes's  "  U.  S.  Explor.  Exped." — vol.  on  "  Geology  ;  "  "  Man- 
ual of  Geol." 

DAWSON,  J.  W.     "  Acadian  Geology." 

DE  LA  BECHE,  H.  "  Geological  Observer  " — "  Sections  and  Views  of  Geo- 
logical Phenomena." 

D'ORBIGNY,  A.     "  Paleontologie  et  Geologic." 

EMMOXS.  E.     "  Eep.  Geol.  of  K  Y. ;  "  "  Rep.  Geol.  of  K  C." 

FORBES,  J.     "  Alps  of  Savoy." 

FOSTER  and  WHITNEY.     "  Rep.  on  Geol.  of  L.  Supr.  District." 

GABB,  W.  M.     Whitney's  "  Geol.  Surv.  of  California." 

GEIKIE,  A.     "  Great  Ice  Age." 

GUYOT,  A.     "  Physical  Geography." 

HALL,  J.  "Rep.  Palaeontology  of  N.  Y.;  "  "Rep.  Geol.  of  Iowa;  "  "Rep. 
on  State  Cabinet  of  K  Y." 

HAYDEN,  F.  V.  "  Rep.  Geog.  and  Geol.  Surv.  of  Terr.,  1871-'73,"  and  sev- 
eral figures  not  yet  published. 

HILGARD,  E.  W.     "  Rep.  Agric.  and  Geol.  of  Miss.,"  1860. 

HITCHCOCK,  E.     "  Ichnology  of  Mass.,"  1858. 

HITCHCOCK,  Jr.,  E.     Geol.  Map  of  U.  S.  in  Walker's  Statistics  of  U.  S. 

HOLMES,  W.  II.     Drawing  of  Geyser  Eruption. 

HOWELL,  E.  E.     "  U.  S.  Geog.  Surv.  by  Wheeler,"  vol.  iii.— "  Geology." 

HUXLEY,  T.  H.     "  Manual  of  Anat.  of  Vertebrates." 

JACKSON,  W.  H.     Photograph  of  Geyser  Eruption. 

JOHNSTONE,  A.  K.     "Phys.  Atlas,"  last  edition. 

LEIDY,  J.  "  Cretaceous  Reptiles  of  U.  S. ;  "  "Smithsonian  Contributions," 
1865  ;  "  Fossil  Vertebrates  of  U.  S. ;  "  Hayden's  "  Geol.  Surv.  of  Terr.,"  vol.  i. 


572  LIST   OF  AUTHORITIES. 

LESLEY,  J.  P.     "  Manual  of  Coal  and  its  Topography." 

LESQUEEEUS,  L.  Owen's  "  Rep.  Geol.  of  Ky. ;  "  Owen's  "  Rep.  Geol.  Ark. ;  " 
Hayden's  "  Geog.  and  Geol.  Surv.,"  vol.  vi. ;  "  Cret.  Flora  of  U.  S." 

LOGAN,  W.     "  Rep.  GeoL  Canada." 

LTELL,  C.  "Principles  of  Geology;"  "Elements  of  Geology;"  "An- 
tiquity of  Man." 

MANTELL,  G.  A.     "  Fossils  of  British  Museum." 

MAOUT  and  DECAISNE.     "  General  System  of  Botany." 

MAESH,  O.  C.  "Cretaceous  Reptiles  and  Birds,  and  Tertiary  Mammals ;" 
Am.  Jour,  of  ScL,  1871-'77. 

MEEK  and  WORTHED.     "  Geol.  Surv.  of  111.,"  vols.  ii.,  iii.,  iv.,  v.,  vi. 

MEEK,  F.  B.  Whitney's  "  Geol.  Surv.  California,"  vol.  i. ;  Newberry's 
"  Geol.  Surv.  of  Ohio  Pal.,"  vols.  i.  and  ii. ;  "  Palaeontology  of  Upper  Missouri ;  " 
"Smithsonian  Contributions,"  1864. 

MURCHISON,  R.  I.     "  Siluria." 

NEWBERRY,  J.  S.  "  Rep.  Geol.  Surv.  of  Ohio ;  "  "  Geol.  of  Macomb  Expe- 
dition; "  "  Pacific  R.  R.  Reports,"  vol.  vi. 

NICHOLSON,  H.  A.  "Manual  of  Palaeontology;"  "Manual  of  Zoology;" 
"Ancient  Life  History." 

OWEN,  D.  D.     "  Rep.  Geol.  of  Wisconsin,  Iowa,"  etc. 

OWEN,  R.     "  British  Fossil  Mammals ;  "  "  Palaeontology." 

PACKAED,  A.  S.     Life-Histories. 

PHILLIPS,  J.     "  Manual  of  Geology ;  "  "  Geology  of  Oxford." 

PICTET,  F.  J.     "  Traite"  de  Pale"ontologie." 

POWELL,  J.  W.  "  Exploration  of  Colorado  River ;  "  "  Geology  of  Uintah 
Mountains." 

ROGERS,  H.  D.     "  Rep.  Geol.  of  Pennsylvania." 

SAFFORD,  J.  M.     "  Rep.  Geol.  of  Tennessee." 

SCTTDDER,  S.  H.     Worthen's  "  Rep.  Geol.  of  111.,"  vol.  iii. 

SHAEPE,  D.     Quar.  Jour.  Geol.  Soc.,  vol.  iii.,  1847. 

STEVENSON,  J.  J.     "Wheeler's  "  U.  S.  Geog.  Survey,"  vol.  iii. — "  Geology." 

TAYLOR,  R.  C.     "  Statistics  of  Coal." 

TYNDALL,  J.     "  Glaciers  of  the  Alps." 

WAILES,  B.  L.  C.     "  Rep.  Agric.  and  Geol.  of  Miss.,"  1854. 

WARD,  H.     Illustrated  Catalogue  of  Casts. 

WHEELEB,  G.  M.     "  IT.  S.  Geog.  Surv.  west  of  100th  Meridian." 

WHITE,  C.  A.     Wheeler's  "  U.  S.  Geog.  Surv.,"  vol.  iv. — "Palaeontology." 

WHITNEY,  J.  D.  "  Geol.  Surv.  of  California ;  "  "  Geol.  of  L.  Superior  Dis- 
trict." 

WOODWAED,  S.  P.     "  Manual  of  Mollusca." 

WOETHEN.     "  Geol.  Surv.  of  Illinois." 


INDEX. 


PAGE 
A 

Acanthaspis 326 

Acanthotelson  Stimpsonl 387 

Acanthoteuthis  antiquus 424 

Acephals  or  Bivalves 301 

Acer  trilobatum 483 

Acervularia  Davidson! 319 

Acrodus  minimus 408 

nobilis 427 

Acrogens,  age  of 269,  280 

and  amphibians,  age  of 333 

Actinoceras 306 

^Eschna  eximia 426 

Agencies,  aqueous 9 

atmospheric 3 

igneous 76 

organic 

Age  of  acrogens 

amphibians 

fishes 

invertebrates 282 

mammals 475 

man 557 

mollusks 282 

reptiles 404 

Ages 269 

Agnostus  interstrictus 285 

Alecto  auloporoides 297 

Alethopteris  Massilonis 352 

Whitneyi 448 

Alorisma  pleuropistha 385 

ventricosa 335 

Alps 256 

Atnblypterus  macropterus 890 

Ambonychia  belli  striatus 304 

American  mud-fish 339 

Amia 329 

Ammonites  bifrons 422 

cerdatus 422 

Chicoensis 454 

Humphreysianus 422 

Jason 422 

Jurassic 421 

margaritanus 422 

Ammonite  tribe 306 

Amphibamus 394 

Amphibians,  age  of 269,  280 

Carboniferous 390 


133 


314 


Amphitherium. 

Prevostii. . 

Amygdaloid.... 


211 


Ancyloceras 

percostatus 

Andrias  Scheuchzeri , 

Andromeda  vaccinifolise  afflnis 

Auemopteris  oblongata 

Animals  of  Carboniferous 

Cretaceous 

DeTonian 319 

Jurassic 419 

Jura-Trias 442 

Permian 402 


464 


459 


Quaternary  .............................  536 


Silurian 290 

Tertiary 485 

Triassic 406 

Annelids,  Silurian 309 

Annularia  inflata 361 

Anomalocardia  Mississippiensis 486 

Anomodonts 410 

Anomoepus  minor 443 

Anoplothcrium  commune,  restored 498 

Anon  ra 390 

Antholites 349 

Anthophyllitis  Devonicus 318 

Anthracite 342 

region  of  Pennsylvania,  map  of 337 

section  of,  magnified 341 

Anthracosaurus 394 

Anthrapalasmon  gracilis 387 

Anticline 177 

Antiquity  of  man 560 

Apateon 394 

Apatornis 470 

Apiocrinua  Eoissianus 420 

Aporrhais  falciformis 463 

Appalachian  chain 254 

coal-field 33S 

— —  revolution 400 

Apteryx  Anstralis 560 

Aptornis 558 

Arancaria,  cone  of 419 

Araucariae 348 

Araucarites  gracilis 349 

Arch«an  or  Eozoic  age 269 

and  Palaeozoic  eras,  interval  between . .  280 


574: 


INDEX. 


Archaean  era,  time  represented  by 274 

times,  physical  geography  of 274 

Archseocidaris 383 

Archseopteryx  macron ra 470 

restored 436 

fore-limb  of 437 

tail,  vertebrae,  and  feather  of 437 

Archegosaurus 393 

Archimedes  Wortheni 381 

Arenicolites  didymus 286 

Armadillos 547 

Artesian  wells 69 

Arthrophycus  Harlani 289 

Articulates 308 

Artiodactyls 508 

Asaphus  gigas 310 

Aspidura  loricata 406 

Astarte  excavata 421 

Astartella  Newberry i 385 

Asteria  lombricalis 420 

Asteroids,  Silurian 300 

Asterophyllites  foliosus 361 

latifolia 317 

Atlantic  Ocean,  currents  of 39 

Atlantochelys  gigas 468 


PAGE 

Belemnites  unicanaliculatus 424 

Bellerophon  Newberryi 321 

sublsevis 386 

Belodon 494 

Carolinensis 447 

Beryx  Lewesiensis 466 

Big  Bone  Lick 542 

Bird,  fore-limb  of 437 

Birds,  Cretaceous 470 

Jurassic 436 

Tertiary 494 

gigantic  extinct,  of  New  Zealand 558 

Bitumen,  geological  relations  of. 376 

origin  of 376,  379 

Blastids,  Carboniferous 382 

Blatta  Maderse 388 

Blattina  vennsta 388 

Bog-iron  ore 136 

Bone-caverns 536 

origin  of 539 


small 143 

Auriferous  quartz-veins 231 

Aurignac  Cave 565 

Avicula  contorta 407 

socialis 407 

Trentoneneis 304 

Aviculopecten  parilis 321 

Axes,  anticlinal 177 

monoclinal 178 

synclinal —  177 


Baculites 463 

anceps 464 

Badlands 247,478,  479 

Bakervillia  parva 402 

Balanced  stones 52 

Baphetes 394 

Barren  Island,  section  of. 89 

Bars 30 

Basalt 205 

cause  of  columnar  structure  of 210 

columnar 209 

direction  of  columns  of 210 

Bat,  fore-limb  of 437 

Bathygnathns  borealis 447 

Batocrinus  Chrystii 382 

Beaches  and  terraces 539 

Belemnite,  animal,  restored 423 

Belemnites 423,424 

clavatus 424 

densns 449 

fossil  ink-bags  of. 424 

hastatus 424 

Belcmnite-shell,  restored 423 

Bclemuites  impresses 463 


ne-caves 542 

-  human  remains  in 563 

Bone-rubbish,  cave,  origin  of 538 

;  Bos  primigenius 560 

j  Botanical  temperature-regions,  in  latitude..  156 

vertical 156 

|  Bowlders 515 

of  disintegration 6 

!  Bracliiopod,  general  description  of. 301 

!  Brachiopods 301 

Carboniferous 384 

Cretaceous 461 

Devonian S20 


-  Jurassic  ............................  419,  421 

-  Permian  ...............................  403 

-  Silurian  ................................  303 

-  structure  of  ............................  302 

Brachiospongia  Roemerana  ..................  291 

Brachyphyllum,  branch  of.  ..................  448 

Brains  of  coryphodon,  dinoceras,  and  bron- 

totherium,  compared  ......................  507 

Breccia  ......................................  210 

Bridgerbeds  ..............  .................  502 

Brontotheridae  ..............................  505 

Brontotheriuin  ..................  ............  505 

-  skull  and  brain  of  ......................  507 

-  ingens,  skull  of  ...........  .............  505 

Brontozoum  giganteum 

Bronze  age 

-  transition  to 
Buprestidium.... 

Buthotrephis  graciliP  ........................  289 

-  succnlens  ..............................  239 

Buttes  ......................................    17 

Buttes  of  the  Cross  .........................    16 


Calamite,  restoration  of. 362 

Calamites  and  their  allies 361 

cannseformis 861 

California  Miocene  shells 487 

Caliipteris  Sullivanti 852 


561 


42T 


INDEX. 


575 


PAGE 

Camarophoria  globulina 403 

Canada  period 282 

Cancellaria  vetusta .' . .  487 

Canons,  how  formed 15 

Caprina  adversa 462 

Carboniferous  age,  echinoderms  of. 383 

age,  fauna  of 381 

bivalve  and  univalve  shells 385,  386 

brachiopods 384 

conifers,  affinities  of 349 

crustaceans 386, 387 

fishes 388-390 

fresh-water  shells 385 

goniatites 386 

insects 388 

period  proper,  rock-system  of 334 

plants,  structure  and  affinities  of. 346 

system 280,333 

system  and  age,  subdivisions  of 334 

vertebrates 388 

Carcharodon  augustidens 491 

Cardiocarpon 350 

Cardiocarpum  Bailey i 318 

Cardiocarpns 348 

Cardiola  interrupts 304 

Cardium  Hillannm 1S2 

Meekianum 487 

Rhaeticum 407 

Carpolithes  irregularis 482 

Caryocrinus  ornatus 299 

Casts  of  organic  remains 194 

Catskill  period 315 

Caulopteris  primeva 354 

Cave  bear 536 

Gailenreuth,  section  of. 537 

hyena 536 

Caves,  limestone 70 

Cenozoicera 269,475 

—  divisions  of 476 

general  characteristics  of 476 

Cephalaspis  Lyelli 323 

Cephalopods 305 

Carboniferous 386 

Cretaceous 464 

Devonian 320 

diagram  showing  distribution  in  time. .  425 

Jurassic 419 

Silurian 307,  308 

suture  and  siphon  of,  diagrams  showing.  422 

Ceratites 406 

nodosus 407 

Whitneyi 449 

Ceratodus 328 

—  altus,  dental  plate  of. 408 

Fosterii . .  329 

serratus,  dental  plate  of 408 

structure  of  limbs  of 327 

Cervus  Americanus 

megaceros,  skeleton  of. 

Cestracion  Phillips! 

Ceteosanrus,  femur  of 434 

Chalk ...  452 


PAGE 

Chalk  cliffs  with  flint  nodules 190 

continuity  of  the 473 

foraminifera  of 453 

origin  of. 453 


seas  of  Cretaceous  times,  extent  of,  in 

Europe 4E4 

—  seen  under  the  microscope 453 

Chamaerops  Helvetica 483 

Champlain 532 

epoch 513,  520 

Cheirotherinm 419 

Chemical  effects  of  subterranean  waters  —    70 

Chemung  period 315 

Chlamydotherium 547 

Chlorite-schist 214 

Chonetes  Dalmaniana 384 

Chrysalidina  gradata. 453 

Cinder-cone,  section  of 87 

Cinnamomum  Mississippieuse 482 

polymorphnm 483 

Cladodus  spiuosus 389 

Clay,  formation  of 7 

slate 214 


Cleavage,  association  with  contorted  laminae  183 

association  with  foldings  of  strata 182 

crystalline 180 


flag-stone . 180 

organic 180 

physical  theory  of 185 

planes 181 


planes  intersecting  strata 182 

planes  cutting  through  strata 180 

slaty 180 


5-42 


slaty,  Sharpe's  theory  of. 181 

Sorby's  theory  of 185 

structure 179 

theory,  geological  application  of. 187 

Tyndall's  theory  of 186 

Clepsysaurus  Pennsylvanicus 447 

Clinometer 176 

Clisiophyllum  Gabbi 381 

Club-moss  compared  with  Icpidodendron. . . .  357 

Clupea  alata 491 

Clypeus  Plotii 420 

Coal  areas  of  different  countries  compared.  339 

areas  of  the  United  States 338 

basins,  Jura-Trias 445 

basins,  Jura-Trias,  fossils  of 446 

catamites 347 

conifers 347 

Cretaceous 455 

estuary  or  raft  theory  of 363 

extra-carboniferous 339 

fat 343,  352 

ferns 347 

field,  Appalachian 338 

field,  central 339 

field,  Michigan 339 

field,  Nova  Scotia  and  New  Brunswick  339 

field,  Rhode  Island 339 

field,  Richmond  and  North  Carolina. . .  445 

field.  Western 339 

formation,  estimate  of  time  in 307 


576 


INDEX. 


PAGE  PAGE 

Coal,  formation  of 344     Conocoryphe  Kingii 265 

fusing 343     Continental  form,  laws  of 168 

highly-bituminous 343     Conularia  Trentonensis 305 

lepidodendrids 347     Copper,  veins  of 230 

measure  shale  weathering  into  spheroids  191     Coral,  compound,  orcorallum 138 

•  measures,  iron-ore  of. 373     forests, 138 

measures,  Jurassic 415     growth,  conditions  of 140 

measures,  period  of. 334     islands 139 

measures,  plication  and  denudation  of.  336     islands,  amount  of  vertical  subsidence.  146 

measures,  thickness  of  strata  of 335 islands,  area  of  land  lost 146 

metamorphic 345     islands,  crater  theory  of 143 

mode?  of  occurrence  of. 335     islands,  rate  of  subsidence 147 

origin  of 343     islands,  subsidence,  geological  applica- 

origin  of,  and  its  varieties 340        tion 148 

peat-bog  theory  of 363     islands,  subsidence,  theory  of. 144 

period,  cause  of  climate  of 370     islands,  subsidence,  time  revolved 148 

period,  physical  geography  and  climate  369     polyp 138 

plants,  fruits  of. 350     reef. 139 

plants,  living  congeners  of 348     reefs  and  islands 138 

plants,  principal  orders  of 347     reefs,  barrier  and  circular,  theories  of . .  143 

plants,  where  found 347     reefs,  barrier 141 

plants  of,  their  structure  and  affinities  346     reefs,  circular 142 

relative  production  of 340     reefs,  friniMng 140 

seams,  faults  in 337  j  reefs,  Florida 149 

seams,  number  and  thickness  of. 338  I  reefs.  Pacific 140 

seams,  thickness  of. 338  |  Corals,  Carboniferous 381 

sigillarids 347     cup 292 

steam 343  j  cyathophylloid 293 

theory  of  the  accumulation  of 363  |  Devonian 319 

typical 343     Favositid 293 

arieties  depending  on  degree  of  bitu-  Halysitid 293 

mi  ization 342     Jurassic 419,420 

arieties  depending  upon  proportion  Silurian 290 

of  fixed  and  volatile  matter 342     Cordaites 34! 

varieties  depending  upon  purity 341     Robbii 311 

arieties  of 341     Corniferous  period 315 

vegetable  structure  in 341     Cormilites  serpentarius 309 

Coas  Range  of  California 242,256     Coryphodon  beds 509 

of  California,  time  of  formation 512     hamatns,  head  and  feet  of. 502 

Coccolith? 452     Bkull  and  brain  of 607 

Coccospheres 452     Coryphodontid* 502 

Coccosteus   326  !  Crepidophyllum  Archiaci 319 

decipiens 324  |  Cretaceous 404,450 

Cochliodus  contortus 389     animals 451 

Comatnla  rosacea 298  |  area  of,  in  America 451 

Compsognathns,  restoration  of. 434     birds 470 

Colorado,  canon  of. 16     brachiopods 461 

Colossochelys  Atlas 493     coal 455 

Columbia  River,  falls  of. 14     echlnoderms 459.460 

Columnaria  alreolata 293     fishes 465,466 

Columnar  structure  of  rocks 209     gasteropods 46! 

Columns,  granitic 221  ;  lamellibranchs 461 

Concretions,  limestone 190     life-system  of. 45( 

Conformable  strata 179     mammals 47: 

Conifer,  branch  and  fruit  of  Jura-Trias 448     mollnsks 463 

Carboniferous  section  of  trunk  of 351     physical  geography  of,  in  America 

trunk  of  Carboniferous 351     plants 456-458 

Coniferous  wood,  fossil,  section  of 316     reptiles 46' 

wood,  recent,  section  of 316     rocks  of 45: 

Conifers,  Triassic 406 rock-system  of 45] 

Coniopteris  Mnrrayana 418     spomres 45! 

Connecticut  River  sandstone 440     subdivisions  of. 451 

Conocardium  trigonale 321     vertebrates 465 


INDEX. 


577 


Crinoids,  Carboniferous 382,  383 

Jurassic 420 

living 297 

Silurian 300 

Triassic 406 

Crioceras 463 

restored 464 

Cristellaria  subarcuatula 454 

Critical  periods ;  400,  551 

Crocodile 432 

Crossopterygians 328 

Crustacea,  Devonian 322 

Jurassic 425 

Silurian 309 

Crustaceans,  Carboniferous 386,387 

Crustacean  tracks 285 

Cruziana  bilobata 289 

Ctenacanthus 389 

vetustus 325 

Ctenopistha  antiqua 321 

Cuneolina  Pavonia 453 

Cupriferous  veins 230 

Cyathophylioid  corals £92 

Cycadeoidea  megalophylla 417 

Cyeads,  Triassic 406 

Cycas  circinalis 417 

—  cross-section  of  stem  of 360 

Cyclopteris  Jacksoni 218 

obtusa 317 

Cynod  racon 411 

canine  tooth  of. 410 

Cyprea  Matthewsonii 463 

Cypris 385 

Cyrtolites  compressus 305 

Dyeri 305 

Trentonensis ...  305 


Dadoxylon  Quangondianum 318 

Dalmania  liinulurus 312 

—  punctate 322 

Daonella  Lommellii. . .  .v 407 

Deltas 24 

formation  of 26 

—  rate  of  growth 27 

Dendrerpeton 391 

Acadeanum.  jaw  and  tooth  of 392 

Deridrograptus  Hallianus 296 

Denudation 260 

agents  of 260 

amount  of 261 

—  geological  time  estimated  by 264 

Deposits,  deep-sea  shell 155 

fresh-water  shell 154 

from  icebergs 66 

from  wave-action 36 

shell 153 

Depression  of  coast  of  South  Atlantic  States  130 

of  deltas  of  rivers 129 

of  earth-crust  in  Pacific  Ocean 130 

Devonian  animals 319 

area  of,  in  the  United  States 315 

37 


PAGE 

320 

320 

319 


Devonian  brachiopods 

cephalopoda 

corals 

Crustacea 322 

division  into  periods 315 

fishes 322-325 

fishes,  affinities  of. 327 

fishes,  general  characteristics  of. 328 

fishes,  nearest  living  allies  of 329 

fishes,  rank  of. 331 

gasteropods 321 

insects 323 

lamellibranchs 321 

land-plants,  general  remarks  on 316 


life-system  of 315 

plants 315-318 

physical  geography  of 315 

radiates 320 

system 280,  314 

trilobites 322 

Diamaguetism,  apparent,  of  cleaved  slates  . .  184 

Diatoms  of  Tertiary 483 

Diatryma  gigantea 494,  501 

Dictyopyge 445 

Dicynodon  lacerticeps 410 

Didymograptus  V-fractus 295 

Dike,  definition  of 207 

Dikes,  age  of,  how  determined 207 

effect  of,  on  intersected  strata 207 

radiating,  of  volcanoes  87 

Dinichthy s 325 

jaws  of 326 

Terrelli 326 

Dinoceras  mirabilis,  skull  and  feet  of. 503 

skull  and  brain  of. 507 

Dinornis  elephantopus 559 

giganteus 558 

Dinosaurs 429,  431,  432 

Dinotherium  giganteum,  head  of 498 

Diorite 205 

Dip,  definition  of 175 

overturn 176 

Diplacanlhus  gracilis 325 

Diplograptus 295 

Diprotodon  Australis,  skull  of 547 

Dirt-beds,  Jurassic 415 

Discoidea  cylindrica 460 

Dolerite 205 

Drift 514 

in  relation  to  gold 554 

—  theory  of  origin  of. 517 

timber 136 

Dromseus 432 

Dromatherium  sylvestre,  jaw  of. 446 


Earliest  reptiles,  general  observations  on. . .  395 

Earth,  constitution  of  the  interior  of 78 

crust  of  the,  definition  of 166 

crust  of  the,  solid  thickness  of V8 

— —  cruet  of  the,  gradual  oscillations  of. ....  127 
crust  of  the,  means  of  geological  obser- 
vation of. 166 


5T8 


INDEX. 


PAGE  PAGE 

Earth,  density  of  the 165     Encrinus  liliformis 406 

formofthe 164     Engis  skull 563 

general  surface  configuration  of  the 166     Entomostraca 312 

Earthquake  bridges 118     Eocene  basin  of  Paris 496 

determination  of  focus 124     epoch 477 

effect  of  moon  on 126     lower,  mammals  of 501 

epicentrum,  determination  of 124     marine,  of  Alabama 500 

focus,  depth  of. 122     middle,  mammals  of. 502 

fissures  produced  by 118     Tertiary  shells 4H6 

great  sea-wave 119     Eohippns  504 

phenomena,  explanation  of. Ill     Eosaurus 393 

relation  to  seasons  and  atmospheric  con-  Acadieusis 394 

ditions 126  j  Eoscorpius  carbonarius 388 

shocks  less  severe  in  mines 118  i  Eozofln  Canadense 275 

shocks  more  severely  felt  in  mines 117     Equus 510 

wave,  spherical,  determination  of Ill  |  Eras 269 

waves,  definition  of  terms 107  I  prehistoric 271 

waves,  their  kinds  and  properties 106  |  Erosion,  average 10,  263 

Earthquakes 104     by  glaciers 51 

— —  connection  with  other  forms  of  igneous  —  general 2GO 

agency 104     glacial,  some  general  results  of. 534 

circle  of  principal  destruction  of 117     examples  of  great 12 

elevation  or  depression  during 127  ;  of  continents,  rate  of 10 

frequency  of. 104  \  of  rain  and  rivers 9 

minor  phenomena  of,  explanation 116     Erosive  power  of  water,  law  of  variation  of   11 

motion  of 116     Eruption  of  volcanoes 82 

originating  beneath  ocean 119     Eryon  arctiformis 426 

—  proximate  cause  of. 106     Barrovensis 426 

soundsof 116  j  Estuaries 29 

ultimate  cause  of. 106     deposits  in 30 

vorticose 114     mode  of  formation  of 29 

Earth's  interior,  constitution  of 78     Etna,  volcano  of. 81 

Echinoderms,  Carboniferous 382,  383  j  Euomphalus  subquadratus 386 

Cretaceous 459,460     Euphoberia  armigera 388 

Jurassic 420     EnproOps  Danse 386 

Silurian 296     Eurypterids,  Devonian 322 

Triassic 406     Silurian 313 

Echinoids  and  asteroids,  Carboniferous 383     Eurypterus  remipes 313 

Echinorachnis  Breweranus 487     Evolution  of  organic  kingdom,  illustrated  by 

Edestosauras  (clidastes)  restored 468        Devonian  fishes 332 

jaw  of 469     Coal  plants 362 

Edestusvorax 389  I  early  reptiles 395 

Elephas  Americanus 542,543  j  early  birds 436 

antiquus 539  |  early  mammals 438,506 

Falconeri 539     the  central  idea  in  geology 396 

Melitensi? 539     Exogyra  costata 401 

mcridionalis 639 

primigenins 5:59,  542  p 

primigenius,  molar  tooth  of 544 

primigenius,  skeleton  of 541     Fagus  ferruginea 482 

tooth  of 543     polyclada 458 

Elevation  and  depression  of  earth's  crust,  Fault,  definition  of. 174 

gradual 127     law  of  slip  in 224 

theoriesof 131     Faults 222 

Babbage's  theory 131     Fauna  and  flora,  geographical  definition  of. .  155 

general  theory 132     and  flora,  continental 159 

Herschel's  theory 132     and  flora,  geological 196 

gradual,  of  earth's  crust,  Greenland... .  129     and  flora,  cases  of  local 161 

gradual,  of  earth's  crust,  Italy 12"     first  distinct,  general  remarks  on 287 

gradual,  of  earth's  crust,  Scandinavia..  129     marine,  distribution  in  latitude 162 

gradual,  of  earth's  crust.  South  America  127     marine,  in  longitude 162 

Elk,  Irish 636     marine,  special  cases 162 

Enaliosanrs 428,  429     marine,  variation  with  depth  and  bottom  162 


INDEX. 


579 


PAGE   I  PAGE 

Fauna,  Quaternary  mammalian,  in  North         j  Ganoids,  Carboniferous 390 

America 542     Devonian 323 

Quaternary  mammalian,  of  England....  541 Jurassic 428 

Favosites  hemispherica 319     Gar-fish 327 

Ficus  pyrifonnis 487     Gas,  smoke  and  flame  from  volcanoes 85 

Felisatrox 542     Gasteropoda,  Carboniferous 385,386 

Felstone 205     Devonian 321 

Fenestella  elegans 297  j  Cretaceous 463 

Ferns,  Coal 352-354     Silurian 304,  305 

Fingal's  Cave 209  j  Tertiary 486 

Fiord,  ideal  section  through 535  j  Gault 415 

Fiords,  how  formed 534  <  Geographical  distribution  of  organisms 155 

Fire-clay 335     Fauna  of  Quaternary  times 547 

Fishes,  age  of 269,  280,  314  j  Geological  chronology,  manner  of  construct- 

Cretaceous 465,466  I      ing 200 

Carboniferous 388-390  (  fauna  and  flora  differ  more  than  geo- 

Devonian 322-324        graphical 197 

Jurassic 425,427,428     period,  tested  by  life-system 196 

of  Tertiary 490,  491  j  period,  tested  by  rock-system 196 

Triassic 407,  408  j  Geology,  definition  of 1 

Fissures, 221     departments  of 2 

cause  of 221     dynamical 3 

Flabellina  rugosa 453     historical 266 

Flood-plain  deposits 22.  522     structural 164 

Flora,  definition  of. 155     German  Ocean,  tides  of. 41 

Florida  reefs 149     Geyser-eruption,  theories  of 99 

compared  with  other  reefs 152     Geysers,  Bunsen's  investigations  of 100 

formation  of. 150  |    definition  of 94 

history  of  changes  of. 150     description  of. 94 

Fold,  monoclinal, 178  j  Mackenzie's  theory  of 99 

Foliation-structure 214  '  phenomena  of  eruption  of 94 

Forarainifera  of  Chalk 453     of  the  Yellowstone 9« 

shells  of  living 454     Glacial  epoch 513,  514 

Foraminifers 155     epoch,  first 531 

Forbesiocrinus  Wortheni 382  j  epoch,  drift-materials  of. 514 

Forest,  fossil,  ground-plan  of  Carboniferous  365     epoch,  second 534 

Forest-grounds,  fossil,  Jurassic 416  •  erosion,  some  general  results  of. 534 

Formation,  geological  definition  of. ...  179,  196  '  lakes "54,  535 

Formica  lignitum 488  j  scorings 53,  516 

Fossils,  definition  of 191     times  in  America,  probable  condition 

degrees  of  preservation  of. 191        during 519 

distribution  in  strata 195  j  valley,  section  across 54 

nature  of,  determined  by  age  196     Glaciation 52,  516 

nature    of,    determined    by    country  Glacier,  great  RhGne 532 

where  found 195  i  Glaciers  as  a  geological  agent 51 

—  nature  of,  determined  by  kind  of  rock  195  !  conditions  necessary  for 43 

primordial,  American 285  |  definition  of 43 

primordial,  foreign 286  }  earth  and  stones  on  surface  of. 49 

stratified  rocks  classified  by  means  of  199 evidences  of  former  extension  of 53 

their  origin  and  distribution 190     fissures  in 62 

Fractures 221     general  description  of 47 

Free  crinoid,  liviag 298     graphic  illustration  of 46 

Frost,  action  of 8  |  line  oflower  limit  of. 46 

Frozen  soils  and  ice-cliffs,  mammoth  in. ...  539     motion  of 44 

Fusing-point,  not  the  same  for  all  depths  in  motion  of,  and  its  laws 54 

earth 79  j  motion  of,  theories  of 57 

i  physical  theory  of  veins  of. 63 

Q                                           ramifications  of 44 

j  structure  of 60 

Gailenrenth  Cave,  section  of 537     transporting  power  of. 52 

Galerites  albogalerus 460     veined  structure  of 62 

Ganges,  average  erosion  by 10     Globigerina  bulioides 454 

Ganocephala 393  I  ooze 155 


580 


INDEX. 


Glyptocrinus  decadactylns 299 

Glyptodon 547 

claripes 547 

Glyptolemus  Kinairdii 325 

Giant's  Causeway 209 

Gigantitherium  caudatu  m 443 

Gneiss 214 

decay  of 7 

Gold,  auriferous  veins 231,  237 

drift  in  relation  to 554 

Goniatites,  Carboniferous 386 

crenistria 386 

lamellosus 321 

Lyoni 386 

Goniopygus  major 

Gorges,  how  formed 

Granite,  decay  of 

graphic 

origin  of 

syenitic 

veins 205 

Granitic  rocks,  chemical  composition  and 
kinds  of 2C4 

rocks,  mode  of  occurrence  of 204 

Graptolites 293,  296 

Clintonensis 296 

Graptolithus  Logani 295 

Green  River  Basin,  Wahsatch  beds 501 

River  Basin,  Bridger  beds 502 

Greensand 456 

Green-stones ...  206 

Gryphaea  calceola 449 

speciosa 449 

Gulf  Stream,  probable  agency  in  formation 

of  Florida  reefs 152 

Gymnophiona 390 

Gypsum  deposited  from  springs 73 

Gyracaiithus 389 


PAGE 

Huronia 306 

Hyalithis  primordialis 285 

Hybcdus  apicalis 408 

reticulatus,  spine  and  tooth 427 

Hyrachyus 502 

Hydrographical  basin 10 

Hydrozoa,  living 294 

Silurian 293 

Hyena  spelaea,  skull  of. 538 

Hylseoeaur 434 

Hylerpeton 394 

j  Hylonomus 394 

Hymenocaris  vermicauda 286 

;  Hymenophyllites  alatus 353 

|  splendens 354 


Ice,  agency  of. 

floating 

Tcuberss  as  a  geological  agent 

deposits  from 

formation  of 

number  of 

period  of 

Ice-pillars,  formation  of 

Ichthyocrinus  sublajvis 

Ichthyornis 

dis  par 

Ichthyosaurus 

communi? 


Hadrosaurus,  restored 468 

teeth  of 467 

Haly sites  catenulata : 293 

Hamilton  period 315 

Hamites 463 

Helioceras 463 

Robertianus 464 

Hemerobioides  giganteas 427 

Hemicidaris  crennlaris 420 

Hemitelites  Brownii 418 

Hesperornis  regalis,  jaw,  vertebra,  and  tooth, 

of 471 

Heterocrinus  simplex 300 

Hipparion 509 

Hippnrites  Toucasiatia 462 

Holoptychins  Hibberti,  tooth  of 390 

nobilissimus 324 

Homocrinus  scoparius 300 

Hornblende  schist 214 

Horse  family,  diagram  of  gradual  changes  of  510 

genesis  of. 509 

Horizon,  geological 197 


paddle,  web  of 

tooth  of.. 

vertebrae  of. 

Igneous  agencies 

Iguana,  section  of  jaw  of,  showing  teeth.. 

Iguanodon 

pelvis  of. 


tooth  of 

Inachus  Kaempferi 

Indusial  limestone. . .' 

Infusorial  earth,  Richmond.. 

earths,  origin  of 

Inoceramus  dimidins 

Insects,  Carboniferous 

Devonian 

Jurassic  — 


43 
64 
lit1, 
66 
64 
65 
513 


470 
471 
429 
428 
429 
428 
428 
76 


432 
314 


Tertiary 

Interior  heat  of  the  earth 

of  earth,  increasing  temperature  of — 

of  earth,  rate  of  increase  of  temperature 

not  uniform 

Invariable  temperature,  stratum  of 

Invertebrates,  age  of 

Irish  elk,  skeleton  of. 

Iron  age 

hat  of  copper  vein 

ore  of  the  coal-measures 

springs,  deposits  in 

theory  of  the  accumulation  of 

Islands,  coral 

mangrove 

Isopods 


INDEX. 


581 


PAGE 

Jointing,  regular,  of  limestone 220 

Joints 181,220 

Jupiter  Serapis,  temple  of 128 

Jura  Mountains,  section  of 414 

Jurassic  animals 419 

birds 436 

—  cephalopods,  ammonites 422 

coal-measures 415 

corals 420 

Crustacea 425 

crustaceans  and  insects 426,  427 

echiuoderms 420 

fishes 425,  427,  428 

fossils  of  Utah 449 

insects 425 

lamellibranchs  and  brachiopods 421 

mammals 438 

—  period 404,  414 

plants 417,418 

plants,  cycads,  and  ferns 418,  419 

reptile? 428 

Jura-Trias,  bird-tracks  in 443 

distribution  of  strata  of 439 

disturbances  which  closed  it 450 

ephemera,  larva  of 442 

in  America 439 

life-system  of. 440 

of  Connecticut  Valley 441 

on  interior  plains  and  Pacific  coast. ...  447 

—  plants  of 448 

tracks,  reptilian 442,  443 


Kaolin,  formation  of 7 

Kilauea,  volcano  of 81 

Kimmeridge  clay 415 

King-crabs,  larva  of 312 

Kitchen-middens    560 


Labyrinthodon,  tooth  of 

Labyrinthodont,  section  of  tooth  of 

Labyrinthodonts 

Lagoonless  islands 

Lake-dwellings 

Lake-margins 

Lakes,  alkaline 

chemical  deposits  in 

flooded 

glacial 

salt 

Lake  Superior,  effect  of  waves  on  shore  of. . . 

Tahoe,  map  of  southern  end  of 

Lamellibranchs 

Carboniferous 

Cretaceous 

Devonian 

Jurassic 419, 


Tertiary. 


Lamellibranchs,  Triassic 407 

Lamination,  oblique  or  cross 173 

Lamna  elegans 

Land-surfaces  and  sea-bottoms,  cause  of. . 

Laurentian  rocks,  evidences  of  life  in 

system,  area  of,  in  North  America  . . . 

system,  rocks  of 

system  of  rocks 

Laurus  Nebrascensis 

Lava 83 

hardened 85 

sheets 207 

Lead,  veins  of. 231 

Lebias  cepbalotes 492 

Lepadocrinus  Gebhardii 299 

Lepidodendrids 355,  356 

Lepidodendron 355 

—  compared  with  club-moss 357 

—  corrugatum 356 

diplotigioides 356 

Gaspianum 317 

ideal  section  of. 357 

modulatum 356 

politum 356 

rigen? 35d 

Lepidoganoids 327 

Devonian 324,325 

Lepidophloios  Acadianus 356 

Lepidosiren 327,  329 

jaws  of 326 

Lepidosteus 327,  329 

Lestornis  crassipes 471 

Lestosaurus,  paddle  of 469 

Levees,  artificial 23 

natural 23 

Lias ' 414 

Libellula 427 

Westwoodii 427 

Life-system  a  test  of  formation 196 

Lime-accumulations 138 

Limestone  caves 70 

concretions 190 

decay  of — 7 

indusial 489,490 

Limuohyus  (Palaeosyops) 502,  504 

Limuloids 313 

Limulus  Moluccanus 314 

trilobite  stage  of 312 

young  of 312 

Lingula  acuminata 285 

anatina 301 


antiqua 285 

Credneri 403 

Lingulella  ferrnginea 286 

Liquidamber  integrifolium 457 

Lisbon  earthquake 120 

Lithostrotion  Californiense 881 

Litnites  cornu-arietis 308 

Graftonensis 308 

Lituola  nautiloides 453 

Londsdalia  floriformis 292 

Lookout  Mountain 247 

Lower  Helderberg  period 282 


582 


INDEX. 


LucinaObioensis.... 
Lycosaurus , 


PAGE 

....  321 
410,  411 


Machaeracanthus  major 325 

Machairodus  cultridene 500 

latidens 536 

neogsens,  tooth  of 544 

Macrocheilus  Newberryi 386 

Macropetalichthys 326 

Sullivanti 327 

Macropus  Atlas 547 

Titan 547 

Maelstrom 34 

Malacostraca 312 

Mammals,  age  of 269,  475 

Cretaceous 472 

first,  affinities  of 438 

Jurassic 438 

of  Tertiary,  general  remarks  on  ...  495,  506 

Triassic 411,447 

Mammoth 539 

age 561,562 

—  drawing  of,  by  contemporaneous  man. .  566 

molar  tooth  of. 544 

skeleton  of 541 

Man,  age  of. 269,  557 

—  antiquity  of 560 

Miocene,  supposed 561 

Neolithic 566 

Pliocene,  supposed 561,  567 

primeval 538 

Quaternary 562 

Mangrove  Islands 151 

Marble 214 

Mariacrinns  nobilissimus 300 

Marly  soil,  formation  of 7 

Marshes  and   bogs,  Quaternary  mammals 

in 539,542 

Mastodon  Americanus 542 

Americanus,  tooth  of. 543 

Mastodonsaurus  Jsegeri 408 

Manna  Loa,  volcano  of 81 

Mauvaises  Torres 247,  478,  479 

—  Torres  of  Nebraska 505 

Mechanical  agencies  of  water 9 

—  theory  of  slaty  cleavage,  Sharpens 181 

Megalonyx,  claw-core  of. 546 

Megalosaurus,  head  of. 433 

tooth  of 434 

Megaphyton a^l 

— —  leaf-scar  of 354 

Megatherium  Cnvieri 545 

jaw  of 545 

mirabilis 545 

Melaphyr 205 

Menodus 505 

Mentone  skeleton 564 

Mesas 17 

Mesozoic  animals 406 

era 265,404 

era,  disturbance  which  closed  the 475 


PAGE 

Mesozoic  era,  general  characteristics  of 404 

era,  subdivisions  of 404 

general  observations  on  the 474 

Metamorphism,  agents  of. 215 

alkali  as  agent  of 216 

crushing  as  cause  of  heat  in 216 

explanation  of  phenomena  associated 

with 217 

genera] 215 

215 
SIB 
816 
215 
215 
51 
3SS 
211 
411 
14 
225 


heat  as  agent  of 

local 

pressure  as  agent  of 

theory  of 

water  as  agent  of. 

Mer  de  Glace 

Miamia  Danae 

Mica-schist 

Microlestes  antiquus 

Minehaha,  falls  of. 

Mineral  veins 

Mines,  placer 

Miocene  epoch 

man,  supposed 

of  Nebraska 

shells,  California 

Miohippus 

Mississippi,  delta  of 

erosion  by 

flood-plain  of. 

River,  history  of 

Modern  epoch 

Modiolopsis  solvensis 

Mollueks,  age  of 269, 

Monograptus  priodon 

Mont  Blanc  glacier  region 

Monticules 

Moraine  profonde 

terminal,  material  of 

Moraines 

in  Colorado 

lateral 

median 

Mosasaure 


Mosasanrns,  tooth  of 

Mountain-chains,  age  of 

along  borders  of  continents 

fissure-eruptions  in 

general  form  of,  and  how  produced 

metamorphism  of 

—  occurrence  of  fissures,  slips,  and  earth- 
quakes in 

their  structure  and  origin 

theory  of  origin  of 

thick  sediments  of 

volcanoes  in 

Mountain-formation,  rate  of. 

Mountain-forms  resulting  from  erosion 

Mountain-origin 

Mountain-ranges,  parallel 

Mountain-sediments,  thickness  of 

Mountain-sculpture 

Mountain-structure 

Mountains,  folding  and  metamorphism  in... 


INDEX. 


583 


Mnrchisonia  gracilis 304 

Myalina  peratennata 402 

Myolodon,  skeleton  of 546 

Myophoris  lineata 407 

Myrmecobius  fasciatus 411 


N 


Naiadites 

Nautilus,  pearly 

pompilius 

tribe 

Neanderthal  skull 

Neolithic  age 

man 

Neuropteris 

flexuosa 352, 

hirsuta 

—  linsefolia 

polymorpha 

Niagara,  falls  of,  description  of 

gorge,  time  necessary  to  form 

period 

Nile,  flood-plain  of 

delta  of. 

Nodular  or  concretionary  structure 


Nodules,  flattened  by  pressure  ..............  183 


flint,  in  chalk-cliffs  ..................... 

form  of.  ................................ 


190 
189 
-  kinds  of,  found  in  different  strata  ......  189 

Norfolk  cliffs,  effect  of  waves  on  ............    34 

Norway,  effect  of  waves  on  the  coast  of  .....    35 

Notidamus  primigenius  .....................  491 

Nototherium  Mitchelli.  .  .   ..................  547 

Nuggets  .....................................  239 

Nummulina  laevigata  .......................  485 


o 


Obolella  sagittalis 

Obsidian 

Oceanic  agencies,  land  formed  by 

currents,  geological  agency  of 

theory  of 

Ocean-waves,  effect  of 

Odontolcse 

Odontopteris  gracillima 

Wortheni 

Odontopteryx 

toliapicus,  restored 

Odontornithes 

Odontotormae 

Oil-bearing  strata  of  the  Eastern  United 

States,  area  of 

Oil-formations 

Oil  -  horizons,  principal,  of  the  United 

States 

Oldhamia  antiqua 

Olenus  macrurus 

Oligoporus  nobilis 

Ondenodon  Bainii 

Onychaster  flexilis — 

Onychoclus  sigmoides  


PAGE 

OOlite 414 

OOlitic  limestones,  origin  of 415 

Ooze,  deep-sea 453 

globigerina 454 

Ophiderpeton 394 

Ophileta  compacta 285 

Ophiomorpha 390 

Orange  sand,  Mississippi,  section  of 515 

Orbulina  universa 454 

Oreodon 505 

major 506 


Ores 228 

metallic,  formation  of 236 

Organic  agencies 133 

remains,  decomposition  of,  prevented  191 

Oganisms,  geographical  distribution  of....  155 

—  progressive  changes  in 397 

Oriskany  period 282 

Ormoceras 306 

tenufllum 307 

Ornithoscelida 431 

Orodus  mammilare 389 

Orohippus 504,  510 

Orthis  Davidsonii 303 

Hicksii 286 

—  Livia 320 

Orthoceras 285 

Duseri 307 

medullare 307 

multicameratum 307 

restoration  of 309 

vertebrale 307 

Orthoceratite 306 

Orthonema  Newberryii 321 

Orthonota  parallela 304 

Osmeroides  Mautelli 466 

Osteolepis 324 

Ostrea  Caroliniensis 486 

Georgiana 485,  486 

Idriaensis 461 


Marshii 421 

selteformis 486 

Sowerbyi 421 

Titan 486,  487 

Otodus 465 

Otozamites  Macombii 448 

Otozoum  Moodii 442,  443 

Oxford  clay 415 


Pachyderms 495,  508 

Pachypteris  lanceolata 418 

Pachytherium 547 

Palamster  Shrefferi 300 

PalfEOcarus  typus 387 

Paleolithic  age 561 

mammoth  age 562 

eer  age 564 


Pateoniscus,  restoration  of. 403 

Palseopteris,  leaf-scars  of 354 

Pateosyops 502 

Pateotherium  magnum 497 


584 


INDEX. 


PAGE 

Palseotherium  magnum,  restored  497 

Placoderms  

PAGE 

324,  327 

era,  chemical  changes  daring  
era,  fauna  of,  general  comparison  of 
with  that  of  Neozoic  times  
era,  general  observations  on  

396 

397 
396 

30U 

Placoids,  Carboniferous  

389 

465  466 

Devonian  
Jurassic  

323,825 
427 

491  492 

era,  physical  geography  of,  on  Ameri- 

279 

280 

Plagiaulax  

438 
346 

456 

rocks,  area  of,  in  the  United  State?  

277 
276 

Devonian  

.  315,317,318 
417  418 

278 

448 

times,  general  picture  of  

398 

Silurian  . 

289 
481-483 
483 

transition  from,  to  Mesozoic  
Paradoxides  Bohemicus  

400 
287 
287 
496 
136 
133 
133 
135 
134 
135 
446 
353 
352 
487 
4-21 
486 
407 
407 

Tertiary  
Platanus  aceroides  

Plat6'    em°ra.^tlqua  

403 

Peat,  alternation  of,  with  pediments  

Plesiosaurus  
dolichodeirus,  restored  

430 
428 

299 

402 

composition  and  properties  of.  
conditions  of  growth  of.  
mode  of  growth  of  

Pleurocystites  squamosns  

Pleurotomaria  agave  

304 
304 

Pecopteris  falcatus  
lonchitica  

3S6 

Pliocene  epoch  
man,  supposed  

477 
561 

510 

Pecten  cerrocensis  

430 

head  and  tooth  of.  

430 

.   ..430 

Peneroplis  planatug  

454 
297 
303 

382 
382 
382 

565 
508 
384 
400 
403 
402 
389 
192 
192 
376 
378 
377 
379 
380 
322 
438 
387 
205 
490 
348 
295 
313 
419 
.  B55 

Plumbiferous  veins  
Podogonium  Knorrii  

231 
483,488 
448 
406,446 
327  329 

Pentacrinus  cap 

Pentremites  Burlingtoniensis  

lanceolatus  

Polyzoa,  living  
Silurian                     

2% 
296,297 
205 

pyriformis  
Perigord  caves  

Portheus  molossus,  tooth  of.  
restored  
Primeval  man,  character  of  
in  America  

465 
4C6 
570 
567 
561 
283 
282 

h 

—  shells  

in  Europe  
Primordial  beach  and  its  fossils.. 
period  

Prionastrea  oblongata  
Proboscidian  .. 

420 

..  508 

Petroleum,  geological  relations  of.  
kinds  of  rocks  which  bear  
laws  of  interior  distribution  of.  

403 

mcsialis                              •   • 

384 

384 

origin  of  varieties  of.  
Phacops  latifrons  

300 

Protester  Sedgw  c  - 

316 

Protohippns  
parvulus  
Protophyllum  quadratum  
Protozoa,  Cretaceous  
Silurian  
Pseudocrinus  bifasciatus  
Pseudomonotis  Hawnii  

510 
506 
458 
459 
290,291 
2?9 
402 
317 

PhillipsaaLodiensis...     .             

Phonolite  

Phyllocladus  

Phyllopods  

Psvchozoic  era...  .                   

...  2C9,  557 

INDEX. 


585 


Psychozoic  era,  characteristics  of 

era,  distinctness  of. 

Pteraspis 

Pterichthys  cornutus 

Pterodactyl,  fore-limb  of 

Pterodactylus  crassirostris 

Pteropliyllum  comptum 

Jasgeri 

Pteropods,  Silurian 

Pterosaurs 

Pterygotus  Anglicus 

Gigas 

Ptychodus  Mortoni 

Ptyonius 

Pumice 

Pupa  vetusta 

Pnzzuoli,  temple  of  J'ipiter  Serapis  near. . . 
Pythanomorpha 


Quaternary,  a  period  of  revolution  ..........  550 

-  mammalian  fauna  of  England  ..........  .  541 

-  mammalian  fauna  in  North  America.  .  ."  542 

-  man  ................................  562,  567 

-  period  ..................................  513 

-  period,  cause  of  climate  of  .............  548 

--  period,  characteristics  of.  ..............  513 

-  period  in  Eastern  North  America  .....  .'  514 

-  period  in  Europe  .......................  530 

-  period  in  South  America  ...............  544 

-  period,  mammals  of  ....................  536 

-  period,  life  of  the  .......................  595 

-  period  on  the  western  side  of  the  conti- 
nent ......................................  526 

-  period,  plants  and  invertebrates  of..  ..  535 

-  period,  subdivisions  of  .................  513 

-  period,  time  involved  in  ................  550 

--  period,  general  observations  on  ........  548 

-  times,  geographical  fauna  of 
Quercus  crassinervis 

-  primordialis 

-  Saffordi  .................................  482 

Quartzite  ...................................  214 

Quartz  veins,  auriferous  ....................  231 


Radiates,  Devonian 319 

Silurian 290 

Radiolites  cylindriasus,  section  of 462 

mammelaris 462 

Raniceps 394 

Ravines,  how  formed 15 

Recent  epoch 557 

Recently  extinct  species,  examples  of 558 

Receptaculites  formosus 291 

Reefs,  coral,  of  Florida 149 

coral,  of  Pacific 140 

Refuse-heaps 566 

Regelation  theory  of  glaciers,  of  Tyndall ...    58 

Reindeer  age 561, 564 

Reptiles,  age  of. 289,404 


PAGE 

Reptiles,  Carboniferous 390 

Cretaceous 467 

ganoids  allied  to 331 

Jurassic 428 

of  Tertiary 492 

Triassic 408-410 

Reptilian  footprints,  Carboniferous 390 

footprints  of  Jura-Trias 442 

Rhabdocarpon 350 

Rhabdocarpus 348 

Rhamphorhynchus  Bucklandi,  restored 435 

Rhizocrinus  Lofotensis 297 

Rhombus  minimus 492 

RhOne,  delta  of 27 

Ripple-marks,  how  formed 36 

River-deposits,  age  of 27 

River-gravels 544 


547 

482 
457 


age  of 556 

Rivers  during  the  Quaternary  period 529 

winding  course  of. 21 

River-swamp 22 

Roches  moutonnees 52,517 

Rock  disintegration 6 

Rocking-stones '. 6,  52 

Rock-salt,  age  of 412 

mode  of  occurrence  of 412 

origin  of 412 

theory  of  accumulation  of. 413 

Rock-system,  as  a  test  of  a  formation 196 

Rocks,  classes  of. 170 

consolidation  of,  cause  of 173 

definition  of 169 

extent  and  thickness  of 170 

fissure-eruption 205 

granitic 203 

igneous 202 

igneous,  classification  of. 203 

igneous,  different  modes  of  classifica- 
tion of 211 

Laurentian  system  of 272 

metamorphic 213 

metamorphic.  extent  of.  on  earth-surface  213 

metamorphic,  origin  of 213 

metamorphic,  position  of. 213 

metamorphic,  principal  kinds  of 214 

stratified,  classification  of 197 

stratified,  outline  of  classification  adopt- 
ed in  this  work 201 

stratified,  comparison  of  fossils  of 199 

stratified,  have  been  gradually  deposited  172 

stratified,  kinds  of 171 

stratified,  lithological  characters  of. ...  198 

stratified,  more  or  less  consolidated  sedi- 
ment s  171 

stratified,  originally  nearly  horizontal.  173 

stratified,  or  sedimentary..^. 179 

stratified,  order  of  superposition  of.. . .  198 

...  170 


n  of. 


structure  common  to  all 220 

trap,  mode  of  occurrence  of 203 

trappean 205 

trappean,  general  characteristics  of .. .  205 

trappean,  varieties  of. 805 


586 


INDEX. 


PAGE 

Rocks,  unstratified 202     Silurian  age,  general  life-system  of. ... 

unstratitied,  extent  on  the  surface 203     age,  physical  geography  of 

unstratified,  mode  of  occurrence  of. ....  202     system 

unstratified,  origin  of 202     system,  area  of,  in  America 282 

volcanic 207     system,  character  of  rocks  of 282 

Rotalia  concamerata 454     system,  lower 282 

Ruminants 508     system,  rocks  of 282 

Rush  Creek,  section  on 514     system,  subdivisions  of 282 

system,  upper 282 

g  Siphouia  ficns 459 

Sivatherium 498 

Sabal  major 483     Siwalik  Hills,  India,  Miocene  of 498 

Saccocoma  pectiuata 420     Slate-rocks,  decay  of 7 

Sal iferous  group 401     Slip,  law  of,  in  faults 224 

Salina  period 262  |  Soils,  depth  of 5 

Salisburia 348  |  how  formed 4 

Salix  proteaefolia 458  j  Solenomya  anodoutoides 385 

Salt-lake,  formation  of. 74     Sphenophyllum  erosum 361 

Salt  Lake,  Utah 73     Sphenothallus  angustifolius 2S9 

Salt-lakes,  deposits  in 75     Spirifer 406 

Sandstone,  Connecticut  River 440     Cumberlandise 303 

Sandstones,  decay  of 7     fornacula 320 

San  Pedro,  California,  sea-terraces  at 530     hysterica 302 

Sassafras  araliopsis 458     — -perextensus 320 

Mudgei 457     plenns S84 

Scalaria  Sillimani 463     striatus 302 

Scaphites 463     Spirorbis 385 

sequalis 464     Arkonensis 321 

Schists  ..  214     'omphalodes 321 

Scolithns  linearis 285     Sponges,  Cretaceous 459 

Scotland,  effect  of  waves  on  the  coast  of. ...    35  j  Springs 6f 

Scaphiocriims  scalaris 382  j  carbonated 72 

Seas,  chemical  deposits  in 76  |  chemical  deposits  in 71 

during  the  Quaternary  period 530  j  fissure 6. 

Sediments,  transportation  and  distribution         I  Squatina  acanthoderma 427 

of..  ]8  !  Stagonolepis 494 

Seismome'ters. :::.'...; 122  :  St.  Anthony,  falls  of 13 

Sepia   living  423  I  Stegocephali 39- 

Serapis,  temple  of. 128     Stigmaria  ficoides 359 

Serpentine 214     Stone  age W 

Shale,  black,  of  coal-measures 335     Btonefield  slate *S 

Shasta,  volcano  of. 81     Strata,  contorted 17< 

Shell-deposits 153     elevated 174 

Shell-mounds 536     eroded   l.t 

Shells,  bivalve    and   univalve,  Carbonifer-  folded 17^ 

ous 385,386     inclined 1'- 

distorted 181     outcrop  of,  definition  of 177 

fresh-water,  Carboniferous 385     overturned 171 

microscopic,  as  a  rock-forming  asrent..  154     undulating 1« 

molluscous,  as  a  rock-forming  agent...  153     vertical 

Permian 402     Stratification 20,1.0 

Shore-ice f7     planes 18: 

Sierra  Nevada 256     Strike,  definition  of 175 

Sigillaria  elegans,  leaf  of 358     Stromatopora  conceutrica 29: 

Grseseri 358     ruaroea 29( 

Isevigata ' 358     Strombodes  pentagonus 295 

obovata....' 358     Strophomena 4<X 

restoration  of 360     rhomboidalis 32( 

reticulata 358     Structural  geology 164 

Sigillflria-stem,  section  of 360     Subangnlar  stones 51< 

Sigillarids 358  I  Sub-Carboniferous  period 334 

Silica  deposited  from  geysers 94-96  j  Submarine  banks 4( 

deposited  from  springs 72  |  Submergence,  epoch  of 532 


INDEX. 


587 


PAGE  PAGE 

Subsidence  during  the  Quaternary,  evidence  Time  eince  man  appeared 569 

of. 521  Times,  geological,  estimate  of. 264 

Sulphur  deposited  from  springs 73  Tinoceras 503 

Surface-rock  underlying  drift 515  Titanotherium 505 

Syncliiie 177  Tooth,  ganoid,  structure  of 331 

Syringopora  vesticilata 293  Torreya 348 

System,  Carboniferous 280  Toxoceras 463 

Devonian 280  Trachyte 206 

Silurian 280,  282  Transition  period 400 

Tree-fern,  living 351 

,n  Tree-ferns  of  Coal  period 349 

Trees,  erect  fossil,  in  coal-measures 364 

Table  Mountain  of  California 246  Trematosaurus 408 

section  of 529,  556  I  Trenton  period 282 

Tseniopteris  elegans 448  Triassic  conifers  and  cycaas 406 

Tail-fin,  heterocercal 330  fishes 407,408 

homocercal 330  mammals 411 

Talcose  schist 214  period 404,405 

Tapirus  Indicus 496  period,  subdivisions  of 405 

Teleosaurus  brevidens 431  reptiles 408-410 

Teleosts,  Cretaceous 465  Triconodon 438 

Tellenomya  curta 304  Trigonia  clavellata 421 

Terebratula  Astieriana 461  longa,  shell  and  cast  of 194 

elonsrata 403  pandicosta 449 

flavescens 302  Trigonocarpon,  or  Trigonocarpus 350 

Terrace  Canon 250  Trigonocarpus,  or  Trigonocarpon 348 

epoch 513,524  1  Trilobite,  larva  of 312 

epoch  in  Europe 534  Trilobites,  affinities  of 312 

Terraces,  river 522 Devonian 322 

sea,  at  San  Pedro,  California 530  \  Silurian 309 

Tertiary,  animals  of. 485  i  Trocholites  Ammonius 308 

American  localities  of. 500  j  Tuditanns  radiatus 395 

birds  of 494  I  Turritella  alveata 486 

coal 480  !  Turrulites 463 

diatoms  of. 483  |  catenatus 464 

fishes  of 490,491  Tylosaurus  micromus ...  469 

life-system  of. 480 

mammalian  fauna. 495  U 

—  mammalian  fauna,  general  observations  Umtacrinus  eocialis. ...  . .  460 

TJintatherium 503 

-caro Cso SSK^:;:  :::  S 

—  period,  general  observations  on 511  |  __  how  produced '. 281 

—  period,  physical  geography  of 478  ;  Undercla    of  coa].seam 335 

=££.°*::::::::::::::::::: 4811S 

T^SS ^  riore^nd  8caies::::::::  S 

rextulariavariabilis 454  |  Ul»«liidta:!.""!'.!".";".'."'.'."!'.;!".'!;  542 

Theca  Davidn 286     pristinus.... 

Theory  of  coal  formation  applied  to  Amer-  epelseus  536 

ican  coai-fieids 366  _ spe]Knsi's^{ 'oL:^[ :::::::::::::::::  538 

Theriodonts 410i  411 

Thin-crust  theory  of  earth 80  y 

Thylacoleo  carnifex 547 

skull  of. 548     Vanessa  Pluto  488 

Tides,  effect  of. 32     Vein-stuffs 228,  235 

Tiger,  sabre-toothed 536     Veins,  age  of,  how  determined 229 

Tigrisuchus 411     auriferous  quartz 231 

Tillndonlia 504     auriferous,  of  California 237 

Tillotherium  fodiens,  skull  and  brain  of. ....  504     cupriferous 230 

Time,  great  divisions  and  subdivisions  of.. .  268     fissure 226 

involved  in  the  Quaternary  period 550     metalliferous 227 


588 


INDEX. 


Veins,  metalliferous,  contents  of 227 

metalliferous,  important  laws  affecting 

occurrence  andrichness  of. 232 

metalliferous,  ribboned  structure  of 228 

metalliferous,  theory  of 234 

metallifreous,  vein-stuffs  of 

mineral 

characteristics  of. 

irregularities  of 

of  infiltration 

of  segregation 

plumbiferous 

pockets  in 228 

surface-changes  of 230 

Ventriculites  simplex 459 

Venus  pertenuis 4g7 

Vesuvius,  section  of 88 

Viscosity  theory  of  glaciers,  of  Forbes 

Volcanic  cone,  comparison  of,  with  exoge 

nous  tree 

cone,  formation  of 

cones,  kinds  of 

Volcanic  conglomerate 

phenomena,  subordinate 

rocks,  decay  of 

Volcanoes,  aqueo-i^neous  theory  of 

chemical  theory  of 

definition  of 

estimate  of  a«e  of 

internal  fluidity  theory  of. 

mechanical  theory  of 

size,  number,  and  distribution 

superheated  gas  theory  of 

theory  of 

Volutalithes  dumosa 

symmetrica 

Voltziaheterophylla... 


57 


..    93 


406 


Vulture,  tail  of,  compared  with  archseopteryx  437 


W 


Wahsatch  beds 

Walchia  diffusus 

—  piniformis 

Water,  chemical  agencies  of. 

mechanical  agenies  of 

—  comparison  of  different  forms  of.. 

Water-shed 

Waters,  subterranean 

Waves,  transporting  power  of 

and  tides,  examples  of  action  of.. 

Wealden 

Wells,  artesian  . . . 


PAGE 

..  502 
..  446 


..    C7 
..     10 


..  414 
..    69 


Welwitschia 

White  River  Basin 

Winds,  action  of 

Wood,  petrified 

Worm,  marine,  trail  of 

Worms,  tracks  and  borings,  Silurian.. 


Yellowstone  Park,  springs  in 

geysers  of 

Yosemite,  falls  of 


Zamia  spiralis 

Zamites  occidentalis 

Zaphrentis  bilateralis 

Wortheni 

Zeacrinus  elegans 

Zermatt  glacier 

Zenglodon  cetoides.  tooth  of. 

cetoides,  vertebrae  and  tooth  of. 

hydrarchus,  skull  of 

Zoological  temperature-regions 

Zylobius  sigillarire 


416 
448 
292 
319 
383 
50 
500 
501 
501 
158 


THE     END. 


A    HAND-BOOK 


CHEMICAL   TECHNOLOGY, 

By  RUDOLF  WAGNER,  Ph.  D., 

Professor  of  Chemical  Technology  at  the  University  of  Wnrzbnrg. 

Translated  and  edited,  from  the  Eighth  German  Edition,  with  Extensive  Additions,  by 
WILLIAM  CROOKES,  F.  R.  S. 

1  Vol.,  8vo,  76O  pages,  336  Engravings.    Price,  $S.OO. 


"  His  book  is,  indeed,  excellent  in  the  department  which  above  all  others 
bears,  and  properly  bears,  the  name  of  Chemical  Technology,  that  is  to  say,  the 
production  of  the  crude  materials  used  in  industrial  chemistry;  and,  second,  the 
methods  of  that  industry." — Engineering  and  Mining  Journal. 

"  The  work  has  eight  divisions.  The  first  treats  of  chemical  metallurgy,  al- 
loys, and  preparations — iron,  steel,  cobalt,  nickel,  copper,  lead,  tin,  zinc,  arsenic, 
silver,  gold,  platinum,  etc. ;  the  second,  of  crude  materials  and  products — car- 
bonate of  potassa,  saltpetre,  nitric  acid,  gun-cotton,  common  salt,  sulphur,  etc. ; 
the  third,  of  glass,  ceramic  ware,  gypsum,  lime,  and  mortar;  the  fourth,  of  vege- 
table fibres  and  their  technical  application— hemp,  cotton,  paper,  starch,  sugar, 
wine,  beer,  vinegar,  oils,  cements,  etc. ;  the  fifth,  of  animal  substances  and  their 
industrial  application ;  the  sixth,  of  dyeing  and  calico-printing ;  the  seventh,  of 
methods  of  producing  artificial  light ;  and  the  last,  of  fuel  and  heating  apparatus. 

There  is  no  one  of  fair  intelligence  employed  in  any  of  all  the  industrial 
pursuits,  whether  as  owner  or  employ6,  who  will  not  gain  by  Prof.  Wagner's 
labor." — North  American  and  Gazette. 

"  This  is  a  valuable  treatise  on  the  materials  and  products  of  chemical  indus- 
try, with  all  the  modern  discoveries  and  appliances  accurately  described.  It  is 
of  great  value  to  those  employed  in  the  various  arts  and  manufactures,  as  it 
presents  in  compact  form  what  chemists  require  for  the  successful  pursuit  of 
their  various  avocations.  The  chapters  on  heat,  light,  and  bread-making,  might 
be  profitably  studied  by  householders." — Troy  Daily  Times. 

"  If  there  be  any  one  who  lacks  a  full  appreciation  of  the  importance  of  sci- 
entific knowledge  and  skill,  to  the  success  of  practical  industry,  he  will  do  well 
to  study  this  book  very  carefully.  No  other  work  with  which  we  are  acquainted 
treats  of  chemical  technology  so  extensively  and  thoroughly,  or  with  such  full- 
ness of  scientific  knowledge.  It  discusses  and  explains  all  the  processes  of  every 
kind  of  manufactures,  in  which  '  the  nature  of  raw  materials  is  usually  altered/  " 
— American  Union. 

NEW  YORK  :  D.  APPLETON  &  CO.,  PUBLISHERS,  549  &  551  BROADWAY. 


THE  INTERNATIONAL  SCIENTIFIC  SERIES, 


NO.      I.     FORMS    OF    WATER,  in  Clouds,  Rain,  Rivers,  Ice,  and  Glaciers.      By  Prof.  JOHN  TYNDALL,  LL.  D., 

F.R.S.     1vol.    Cloth.    Price,  $1.50. 
"        2.     PHYSICS  AND  POLITICS;  or,  Thoughts  on  the  Application  of  the  Principles  of  "Natural  Selection  » 

and  "  Inheritance  "  to  Political  Society.    By  WALTER  BAGEHOT,  Esq.,  author  of  "  The  English  Constitution." 

1  vol.    Cloth.    Price,  *1.50. 

"        3.     FOODS.     By  EDWARD  SMITH,  M.  D.,  LL.  B.,  F.  R.  S.     1vol.     Cloth.  •  Price,  $1.75. 
"        4.     MIND  AND  BODY.     The  Theories  of  their  Relations.     By  ALEX.  BAIN,  LL.  D.,  Professor  of  Logic  in  the 

"        5.     THE  STUDY  OF  SOCIOLOGY.    By  HERBERT  SPENCER.   1  vol.,  12mo.  Cloth.    Price,$1.50. 

"        6.     THE  NEW  CHEMISTRY.    By  Prof.  JOSIAH  P.  COOKE,  Jr.,  of  Harvard  University.   1  vol.,  12mo.  Cloth. 

Price,  $2.00. 
"        7.     THE     CONSERVATION    OF    ENERGY.      By  Prot  BALFOL-B  STEWABT,  LL.  D.,F.R.S.    1vol., 

12mo.    Cloth.    Price,  $1.50. 
"        8.     ANIMAL  LOCOMOTION  ;  or,  Walking,  Swimming,and  Flying,  with  a  Dissertation  on  Aeronautics.  By 

J.  BELL  PEITIGREW,  M.  D.,  F.  R.  S.,  K.  R.  S.  E.,  F.  R.  C.  P.  E.    1  vol.,  12mo.    Fully  illustrated.    Price,  $1.75. 
"        9.     RESPONSIBILITY  IN   MENTAL  DISEASE.   By  HENRY  MAUDSLEY,  M.  D.  1  vol.,  12mo.  Cloth. 

Price,  $1.50. 

"      10.     THE  SCIENCE  OF  LAW.     By  Prof.  SHELDON  AMOS.     1  vol.,  12mo.    Cloth.    Price,  $1.75. 
"      II.     ANIMAL    MECHANISM.     A  Treatise  on  Terrestrial  and  Aerial  Locomotion.    By  E.  J.  MAREY.    With 

117  Illustrations.    Price,  $1.75. 

"     12.    THE  HISTORY  OF  THE  CONFLICT  BETWEEN  RELIGION  AND  SCIENCE. 

By  JOHN  WM.  DBAPEB,  M.  D.,  LL.  D.,  author  of  "  The  Intellectual  Development  of  Europe."    Price,  $1.75. 
"      13.     THE    DOCTRINE    OF  DESCENT,  AND  DARWINISM.     By  Prof.  OSCAR  SCHMIDT,  Stras- 

burg  University.    Price,  $1.50. 
"      14.     THE  CHEMISTRY  OF  LIGHT  AND  PHOTOGRAPHY.    In  its  Application  to  Art,  Science, 

and  Industry.     By  Dr.  HERMANN  VOSEL.     100  Illustrations.     Price,  $2.00. 
"      15.     FUNGI}  their  Nature,  Influence,  and  Uses.     By  M.  C.  COOKE,  M.  A.,  LL.  D.    Edited  by  Rev.  M.  J.  BERKELEY, 

M.  A.,  F.  L.  S.    With  109  Illustrations.     Price,  $1.50. 
"      16.    THE   LIFE   AND  GROWTH   OF    LANGUAGE.     By  Prof.  W.  D.  WHITNEY,  of  Yale  College. 

"       17.     MONEY  AND   THE    MECHANISM    OF    EXCHANGE.      By  W.   STANLEY   JEVONS,  M.A., 

F.  R.  S.,  Professor  of  Logic  and  Political  Economy  in  the  Owens  College,  Manchester.    Price,  $1.75. 
"      18.    THE    NATURE  OF  LIGHT,  with  a  General  Account  of  Physical  Optics.    By  Dr.  EUOENE  LOMMEL, 

Professor  of  Physics  in  the  University  of  Erlangen.     With  188  Illustrations  and  a  Plate  of  Spectra  in  Chromo- 

iitbography.     Price,  $2.00. 
"      19.     ANIMAL    PARASITES    AND    MESSMATES.      By  Monsieur  VAN  BENEDEN,  Professor  of  the 

University  of  Louvain,  Correspondent  of  the  Institute  of  France.    With  83  Illustrations.    Price.  $1.50. 
"     2O.     ON    FERMENTATIONS.     By  P.  SCHUTZENBEBGEB,  Director  at  the  Chemical   Laboratory  at  the  Sor- 

bonne     With  28  Illustrations.     Price,  $1.50. 
"     21.     THE    FIVE   SENSES   OF    MAN.      By  JULICS  BEBNSTEIN,  O.  6.  Professor  of  Physiology  in  the  Uni- 

"    22.    TH'E   THEORY   OF  SOUND   IN^ITS   RELATION   TO   MUSIC.     By  Prof.  P,KTRO 

BLASKBNA,  of  the  Royal  University  of  Rome.     With  numerous  Woodcuts.     1  vol.,  12mo.    Cloth.    Price,  $1.50. 


The  character  and  scope  of  this  series  will  be  best  indicated  bv  a  reference  to  the  i 


,nd  subjects  included  in  the 

list,  from  which  it  will  be' seen  that  the  cooperation  of  the  most  dTrtlnjuillwd  professors  in  England,  Ger 
United  States,  has  been  Mound,  HII.I  if  rot&kmi  :tiv  pending  for  contributions  from  other 
The  works  will  be  Issued  simultaneously  in  New  York,  London,  Paris,  Leipsic,  Mil 


aneously  in  New  York,  Lon 

•me  INTERNATIONAL  SCIENTIFIC  SERIES  is  entirely  an    / 
Youmans,  who  spent  the  greater  part  of  a  year  in  Europe, 

Prof.  W.  KINGDON  CLIFFORD,  M.  A.,  The.  Fir*  Principle,  of  the 

Eract  Science,  a/Jain- it  t;  il,f  S  ,,i-. Mathematical. 
Prof.  T.  H.  HUXLEY,  LL.  D.,  F.  R.  S.,  Bodily  Motion  and  Con- 


Dr.  W.  B.  CARPENTER,  LL.  D.,  F.  R.  S.,  The  Phytical  Geogra- 
phy of  tie  Sea. 

Prof.  WM.  ODLING,  F.  R.  S.,  The  Old  Chemistry  meaed  from 
the  New  Stand-fmnt. 

W.  LAUDER  LINDSAY,  M.  D.,  F.  R.  S.  E.,  Mind  in  the  Lower 


Slr  JOHN  LUBBOCK,  Bart.,  F.  R.  S..  On  Ant,  and  Kee,. 

Prot  VV.T.  THISELTON  DYER,  B.  A.,  B.  Sc.,  Form  and  Habit  in 

Flowering  Plant,. 

Mr.  J.  N.  LOCKYER,  F.R.  S.,  Sfiectrum  Annl^h. 
Prof.  MICHAEL  FOSTER,  M.  I).,  /V.A.,,/,™,,;  „  '„<!  tkr  rW7  Theory. 
H.    CHABLTON    BASTIAN,  M.  D.,  F.  R.  S.,    The  Brain  at  an 

Protj 

Prof.  H.  SAINT-CLAIRE 'DKVILLE^>I 

Chemirtry. 

Prof.  WURTZ,  Atom*  and  the  Atomic  Theory. 
Prot  D«  QuATBKFtoES,  The  Human  Race. 
Prof.  LACAZE-DUTHIFRR.  ZoSbffytblM  Cumer. 
Prot  BEHTHELOT,  Chemical  Syniht,!,. 


d  St.  Petersburg. 

originated  and  organized   bv  Dr.  E.   L. 
thors  and  publishers.    The  forthcoming  volumes 


Prof.  C.  A.  YOUNG.  Ph.  D.  (of  Dartmouth  College),  The  Sun. 
Prof.  OC.DEN  N.  ROOD  (Columbia  College,  New  York),  Mod- 

ern  Chromatic,  ami  itx  /,'.  l,it;,,u*  t:.  A  rt  and  Indiulry. 
Prof.   J.  RORKNTHAL,  (rmrral  /'/^/.t/o/o*/v  t>1  jfn'cfe*  titnt  Xtrrr*. 
Prof.  JAM«S  D.  DANA,  M. A.,  I.I..  I >.,'<>„   i:,,haliiatta»;    or, 

Head-Character,  i,,  Hie  (in.dation  and  Proyre,,  ,,f  Life. 
Prof.  S.  W.  JOHNSON.  M.  A.,  On  <!„  2VWHHMO/'  I'/anfe. 
Prof.   FERDINAND     C'.HN    (Breslau    Universitv),  Tkall^ytet 

(Aloa,  Lichen,,  Fm,oi\ 

Prof.  HERMANN   (University  of  Zurich),  On  Jltyiration. 
Prot  LEUCKART  (University  of  Leipsic),   Outline,  of  Animal 


Organization. 
Prof.  L'IEBREICH  (Un 

sitv  of  Str 
ty  'of  Krlantienl.  i 
(Universitv  of  IVrli.O,  Out/inn  of  the 


. 

IEBREICH  (University  of  Berlin),  Outline,  of  Toxicology. 
r\i>T  (Unhersitv  of  Strasburg),  On  Sound. 
EER  (University  'of  Krlantienl.  ihi  Paratitic  Ptantl. 


y,  Paris),  Form,  of  Life  and 


1'rof.  K 
Prof.  R 
Pn.f.  STKIN 

of  Lan 
P.  BtRT  ( 

other  Comical  Condition,. 
P.  LORAIN  (l'r..fi";«..r  of  M.-.lirine,  Pnris),  .Viiiern  Endemic,. 
Mnns.  FKKIDEL,  The  Favf^nn  '•/'  On/nnic  Chemitiry. 
Mons.  DEBRAY,  Prtcim,  Mela/,. 
Prof.  COHFIELD,  M.  A.,  M.  D.  (Oion.),  ^ir  in  it.  Relation  to 

Health. 
Prof.  A.  GIARD,  General  Embryology. 


D.  APPLETON  &•  CO.,  Publishers,  549  &  551  Broadway,  N.  Y. 


THE  POPULAR  SCIENCE  MONTHLY. 


CONDUCTED    BY 


E.     DL,_ 


This  periodical  was  started  {in  1872)  to  promote  the  diffusion  of  valuable  scientific 

knowledge,  in  a  readable  and  attractive  form,  among  all  classes  of  the 

community,  and  has  thus  far  met  a  want  supplied  by  no 

other  magazine  in  the  United  States. 


Eight  volumes  have  now  appeared,  which  are  filled  with  instructive  J,nd  interesting  articles  and  ab- 
stracts of  articles,  original,  selected,  translated,  and  illustrated,  from  the  pens  of  the  leading  scientific  men 
of  different  countries.  Accounts  of  important  scientific  discoveries,  the  application  of  science  to  the  prac- 
tical arts,  and  the  latest  views  put  forth  concerning  natural  phenomena,  have  been  given  by  savants  of  the 
highest  authority.  Prominent  attention  has  been  also  devoted  to  those  various  sciences  which  help  to  a 
better  understanding  of  the  nature  of  man,  to  the  bearings  of  science  upon  the  questions  of  society  and 
government  to  scientific  education,  and  to  the  conflicts  which  spring  from  the  progressive  nature  of  scien- 
tific knowledge. 

THE  POPULAR  SCIENCE  MONTHLY  has  long  since  ceased  to  be  an  experiment.  It  has  passed  into  a 
circulation  far  beyond  the  most  sanguine  hopes  at  first  entertained,  and  the  cordial  and  intelligent  approval 
which  it  has  everywhere  met,  shows  that  its  close  and  instructive  discussions  have  been  well  appreciated 
by  the  reading  portion  of  the  American  people.  It  has  not  been  its  policy  to  make  boastful  promises  of 
great  things  to  be  done  in  the  future,  but  rather  to  appeal  to  what  it  has  already  accomplished  as  giving 
it  a  claim  upon  popular  patronage.  But  no  pains  will  be  spared  to  improve  it  and  make  it  still  more  worthy 
of  liberal  support,  and  still  more  a  necessity  to  the  cultivated  classes  of  the  country. 

The  following  quotations  illustrate  the  way  it  has  been  habitually  spoken  of  by  the  press: 


"  That  there  is  a  place  for  THE  POPULAR  SCIENCE 
MONTHLY,  no  one  can  doubt  who  has  watched  the 
steady  increase  of  interest  in  scientific  investigation 
manifested  in  this  country,  not  only  by  a  select  class, 
but  by  the  entire  community."— New  York  Times. 

"  We  think  it  is  not  too  much  to  say  that  this  is 
thd  best  first  number  of  any  magazine  ever  pub- 
lished in  America."— New  York  World. 

"  A  journal  which  promises  to  be  of  eminent 
value  to  the  cause  of  popular  education  in  this 
country." — New  York  Tribune. 

"It  is,  beyond  comparison,  the  best  attempt  at 
journalism  of  the  kind  ever  made  in  this  country." 
— Home  Journal. 

•llt  is  just  what  is  wanted  by  the  curious  and 
progressive  mind  of  this  country,  and  ought  to  be 
widely  circulated."— New  York  Evening  Post. 

11  It  is  the  first  successful  attempt  in  this  country 
to  popularize  science  in  the  pages  of  a  monthly." — 
JV.  Y.  Softool  Journal. 

"THE  MONTHLY  has  more  than  fulfilled  all  the 
promises  which  the  publishers  made  in  the  pro- 
spectus of  publication."— Niagara  Falls  Gazette. 


"  This  is  a  highly-auspicious  beginning  of  a  use- 
ful and  much-needed  enterprise  in  the  way  of  pub- 
lication, for  which  the  public  owe  a  special  debt  of 
obligation  to  Messrs.  D.  Appleton  &  Go:"1— Boston, 
Gazette. 

"  This  new  magazine,  in  our  estimation,  has  more 
merit  than  the  whole  brood  which  have  preceded 
it."—  Oswego  Press. 

"In  our  opinion,  the  right  idea  has  been  happily 
hit  in  the  plan  of  this  new  monthly."— Buffalo 
Courier. 

"  This  is  one  of  the  very  best  periodicals  of  its 
kind  published  in  the  world.  Its  corps  of  contribu- 
tors comprise  many  of  the  ablest  minds  known  to 
science  and  literature.  It  is  doing  a  great  and  noble 
work  in  popularizing  science,  promoting  the  growth 
of  reason,  and  leveling  the  battlements  of  old  su- 
perstitions reared  in  the  childhood  of  our  race  be- 
fore it  was  capable  of  reasoning." — Tlie  American 
Medical  Journal,  St.  Louis,  Mo. 

"  This  magazine  is  worth  its  weight  in  gold,  for 
its  service  in  ed  ucating  the  people."—  The  American 
Journal  of  Education,  St.  Louis,  Mo. 


THE  POPULAR  SCIENCE  MONTHLY  is  published  in  a  large  octavo,  handsomely  printed  on  clear  type, 
and,  when  the  subjects  admit,  fully  illustrated.    Each  number  contains  128  pages. 

Terms:  $5  per  Annum,  or  Fifty  Cents  per  Number. 

POSTAGE  FKEE  TO  ALL  SUBSCRIBERS  IN  THE  UNITED  STATES,  FROM  JANUARY  1,  18T5. 

A  new  volume  of  the  POPULAR  SCIENCE  begins  with  the  numbers  for  May  and  November  each  year 
Subscriptions  may  commence  from  any  date.     Back  numbers  supplied. 


JVbtP  Heady,  rol/t.  I.,  IT.,  III.,  IV.,  V.,  VI.,  VII.,  VIII.,  and  IX.,  of  TJie  Pop- 
ular  Scicnre.  Montlili/,  embracing  the  Numbers  from  1  to  54  (May,  18T2,  to  October,  18TC).  9 
vols.,  Svo.  Cloth,  |3.50  per  vol.  Half  Morocco,  $6.50  per  vol. 

For  Sale,  Binding  Cases  for  Vols.  I.,  II.,  III.,  IV.,  V.,  VI.,  VII.,  VIII.,  and 

IX.,  of  The  Popular  Science  Monthly.  These  covers  are  prepared  expressly  for  binding  the  vol- 
umes of  THE  POPULAR  SCIENCE  MONTHLY  as  they  appear,  and  will  be  sent  to  Subscribers  on  receipt  of 
price.  Any  binder  can  attach  the  covers  at  a  trifling  expense.  Price,  50  cents  each. 


AGE 

ADDRESS 

I 


AGENTS   WANTED. 


D.  APPLETON  8?  CO.,  Publishers, 

549  &  55 1  Broadway,  New  York. 


APPLETONS' 

AMERICAN  CYCLOPEDIA 


Entirely  rewritten  by  the  ablest  writers  on  every  subject.      Printed  from  new  type, 
and  illustrated  -with  Several  Thousand  Engravings  and  Maps. 

The  work  originally  published  under  the  title  of  THE  NEW  AMERICAN  CYCLOPAEDIA  was 
completed  in  1803,  since  which  time  the  wide  circulation  which  it  has  attained  in  all  parts  of 
the  United  States,  and  the  signal  developments  which  have  taken  place  in  every  branch  of 
science,  literature,  and  art,  have  induced  the  editors  and  publishers  to  submit  it  to  an  exact 
and  thorough  revision,  and  to  issue  a  new  edition  entitled  THE  AMERICAN  CYCLOPAEDIA. 

Within  the  last  ten  years  the  progress  of  discovery  in  every  department  of  knowledge 
has  made  a  new  work  of  "reference  an  imperative  want. 

The  movement  of  political  aftairs  has  kept  pace  with  the  discoveries  of  science,  and  their 
fruitful  application  to  the  industrial  and  useful  arts  and  the  convenience  and  refinement  of 
social  life.  Great  wars  and  consequent  revolutions  have  occurred,  hi  volving  national  changes 
of  peculiar  moment.  The  civil  war  of  our  own  country,  which  was  at  its  height  when  the 
last  volume  of  the  old  work  appeared,  has  happily  Been  ended,  and  a  new  course  of  com- 
mercial and  industrial  activity  has  been  commenced. 

Large  accessions  to  our  geographical  knowledge  have  been  made  by  the  indefatigable 
explorers  of  Africa. 

The  great  political  revolutions  of  the  last  decade,  with  the  natural  result  of  the  lapse  of 
time,  have  brought  into  public  view  a  multitude  of  new  men,  whose  names  are  in  every 
one's  mouth,  and  of  whose  lives  every  one  is  curious  to  know  the  particulars.  Great  bat- 
tles have  been  fought,  and  important  sieges  maintained,  of  which  the  details  are  as  yet 
preserved  only  in  the  newspapers,  or  in  the  transient  publications  of  the  day,  but  which 
ought  now  to  take  their  place  in  permanent  aud  authentic  history. 

In  preparing  the  present  edition  for  the  press,  it  has  accordingly  been  the  aim  of  the 
editors  to  bring  down  the  information  to  the  latest  possible  dates,  aha  to  furnish  an  accurate 
account  of  the  most  recent  discoveries  in  science,  of  every  fresh  production  in  literature,  and 
the  newest  inventions  in  the  practical  arts,  as  well  as  to  give  a  succinct  and  original  record 
of  the  progress  of  political  and  historical  events. 

The  work  has  been  begun  after  long  and  careful  preliminary  labor,  and  with  the  most 
ample  resources  for  carrying  it  on  to  a  successful  termination. 

None  of  the  original  stereotype  plates  have  been  used,  but  every  page  has  been  printed 
on  new  type,  forming  in  fact  a  new  Cyclopaedia,  with  the  same  plan  and  compass  as  its 
predecessor,  but  with  a  far  greater  pecuniary  expenditure,  and  with  such  improvements  in 
its  composition  as  have  been  suggested  by  longer  experience  and  enlarged  knowledge. 

The  illustrations,  which  are  introduced  for  the  first  time  in  the  present  edition,  have 
been  added  not  for  the  sake  of  pictorial  effect,  but  to  give  greater  lucidity  and  force  to  the 
explanations  in  the  text.  They  embrace  all  branches  of  science  and  of  natural  history,  and 
depict  the  most  famous  and  remarkable  features  of  scenery,  architecture,  and  art,  as  well  as 
the  various  processes  of  mechanics  and  manufactures.  Although  intended  for  instruction 
rather  than  embellishment,  no  pains  have  been  spared  to  insure  their  artistic  excellence  ; 
the  cost  of  their  execution  is  enormous,  and  it  is  believed  that  they  will  find  a  welcome  re- 
ception as  an  admirable  feature  of  the  Cyclopedia,  and  worthy  of  it's  high  character. 

This  work  is  sold  to  subscribers  only,  payable  on  delivery  of  each  volume.  It  is 

completed  in  sixteen  large  octavo  volumes,  each  containing  about  800  pages,  fully  illustrated 
with  several  thousand  Wood  Engravings,  and  with  numerous  colored  Lithographic  Maps. 

PRICE    AND    STYLE    OF    BINDING. 


In  extra  cloth,  per  vol.  ....  $s.c 
In  library  leather,  per  vol.  .  .  .  6.c 
in  half  turkey  morocco,  per  -vol.  .  .  7.c 


In  half  russia,  extra  gilt,  per  vol.  .      $8.00 

In  full  morocco  antique,  gilt  edges,  per  -vol.      10  oo 
In  full  russia,  per  vol.      .        .        .  io.o<? 


.  ***  Specimen  pages  of  the  AMERICAN  CYCLOPAEDIA,  showing  type,  illustrations,  etc., 
will  be  sent  gratis,  on  application. 

D.   APPLETON   &   CO.,  PUBLISHERS, 

549  &  551  Broadway,  New  York. 


t 


University  of  California 

SOUTHERN  REGIONAL  LIBRARY  FACILITY 

405  Hilgard  Avenue,  Los  Angeles,  CA  90024-1388 

Return  this  material  to  the  library 

from  which  it  was  borrowed. 


'  '000 


S6     2 


